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The laser increased the intensity of light that can be generated by orders of magnitude and thus brought about nonlinear optical interactions with matter. Chirped pulse amplification, also known as CPA, changed the intensity level by a few more orders of magnitude and helped usher in a new type of laser-matter interaction that is referred to as high-intensity laser physics. In this talk, I will discuss the differences between nonlinear optics and high-intensity laser physics. The development of CPA and why short, intense laser pulses can cut transparent material will also be included. I will also discuss future applications.
Le laser a augmenté de plusieurs ordres de grandeur l’intensité de la lumière qu’on peut générer et il a donc donné des interactions optiques non linéaires avec la matière. L’amplification des impulsions comprimées, ou AIC, a modifié le niveau d’intensité de quelques ordres de grandeur de plus et contribué à l’avènement d’un nouveau type d’interactions laser-matière appelé physique des lasers à haute intensité. Mon entretien portera sur les différences entre l’optique non linéaire et la physique des lasers à haute intensité. Je parlerai aussi de l’essor de l’AIC et j’expliquerai pourquoi les impulsions laser courtes et intenses coupent des matériaux transparents. J’aborderai également de futures applications.
Speakers:
Physics has long employed optical methods to probe and manipulate matter on scales from the infinitesimal to the immense. To understand the brain, we need to monitor physiological processes of single synapses as well as neural activity of a large number of networked neurons. Optical microscopy has emerged as an ideal tool in this quest, as it is capable of imaging neurons distributed over millimeter dimensions with sub-micron spatial resolution. Using concepts developed in astronomy and optics, my laboratory develops next-generation microscopy methods for imaging the brain at higher resolution, greater depth, and faster speed. By shaping the wavefront of the light, we have achieved synapse-level spatial resolution through the entire depth of the primary visual cortex, optimized microendoscopes for imaging deeply buried nuclei, and developed high-speed volumetric imaging methods. I will discuss our recent advances as well as their applications to understanding neural circuits.
La physique a longtemps employé des méthodes optiques pour sonder et manipuler de la matière à toutes les échelles, et ce, de l’infinitésimal à l’immense. Pour comprendre le cerveau, nous devons surveiller les processus physiologiques de simples synapses ainsi que l’activité neurale de neurones en réseau. La microscopie optique s’est révélée comme un outil idéal dans cette quête car elle est en mesure de produire des images de neurones répartis sur des dimensions supérieures au millimètre, avec une résolution spatiale inférieure au micron. En utilisant des concepts développés en astronomie et en optique, mon laboratoire conçoit des méthodes de microscopie de nouvelle génération pour l’imagerie cérébrale à des résolutions et des profondeurs plus élevées, et à des vitesses plus rapides. En façonnant le front d’onde de la lumière, nous avons atteint une résolution spatiale à l’échelle de la synapse pour toute la profondeur du cortex visuel primaire, nous avons optimisé des micro-endoscopes ou l’imagerie de noyaux profondément enfouis, de même que développé des méthodes d’imagerie volumétrique à haute vitesse. Je discuterai de nos avancées récentes, ainsi que de leurs applications pour la compréhension des circuits neuronaux.
We have measured the $n$=2 Lamb shift in atomic hydrogen. The measurement uses a direct microwave transition between the 2S and 2P states, and employs the new frequency-offset separated-oscillatory-fields (FOSOF) technique. The FOSOF technique is a variation of the Ramsey separated-oscillatory field technique in which the two separated fields have their frequencies slightly offset from each other. Our Lamb shift measurement, along with existing high-precision quantum-electrodynamics theory, leads to a precise measurement of the rms proton charge radius, and therefore helps to resolve the eight-year-old puzzle created by contradictory determinations of the proton size. In particular, this measurement is the direct analog of the very precise 2S-2P muonic-hydrogen determination of the proton size. Our recent work in determining the helium 2$^3$P fine structure and our new program to determine the electron electric dipole moment will also be discussed briefly.
Antihydrogen, as the simplest purely antimatter atomic system, is a natural candidate for testing fundamental symmetries between matter and antimatter. For example, CPT symmetry predicts that the spectra of hydrogen and antihydrogen should be identical. By making precise measurements of antihydrogen’s transition frequencies, and making comparisons to high precision measurements in hydrogen, we have an excellent test of CPT symmetry in the atomic sector. The primary transitions of interest are the 1S – 2S and ground-state hyperfine transitions, which are known to 4.2 parts in $10^{15}$ and 7 parts in $10^{13}$ in hydrogen, respectively. In addition, the fact that antihydrogen is electrically neutral means it can be used as a probe of the gravitational interaction between matter and antimatter. If the weak equivalence principle holds then the gravitational mass of antimatter should be identical to that of matter but so far there have been no direct free-fall style experiments to test this.
The ALPHA collaboration at CERN aims to test these principles using antihydrogen atoms confined in a magnetic trap. The ALPHA experiment has developed antihydrogen trapping techniques that allow the simultaneous confinement of more than 1000 antihydrogen atoms. This has led to detailed measurements of antihydrogen’s 1S – 2S, 1S – 2P, and ground-state hyperfine transitions. In addition, ALPHA has recently built a new vertical antihydrogen trapping apparatus for a direct measurement of the gravitational free-fall of antihydrogen.
In this talk, I will present a general overview of ALPHA’s apparatus, techniques, and results. Particular focus will be placed on ground-state hyperfine spectroscopy and magnetometry for precision spectroscopy and gravity measurements on trapped antihydrogen.
One method to improve our understanding of nuclear physics, for example the nuclear structures within atoms, is performing high-precision mass measurements of ions. Penning traps are widely used for mass spectroscopy with the lowest uncertainty and they can reach a precision of 𝛿m/m~1*10$^{-9}$ with radioactive ion beams. This precision can be further improved by using highly charged ions (HCI) of the isotope of interest because 𝛿m/m directly depends on the ions’ charge state q. These HCIs can be created with charge breeding inside an Electron Beam Ion Trap (EBIT) where high electron current densities are used to knock out electrons of trapped ions via electron impact ionization. This boost in measurement precision has been successfully demonstrated at the TITAN facility at TRIUMF.
At TITAN, the EBIT high voltage has recently been upgraded to allow 65 keV electron beams. To better take advantage of the higher energies, we are upgrading the electron gun of the EBIT to achieve maximum current densities and therefore the shortest breeding times and the highest charge states.
In order to get the optimized setup, the electron beam properties of the electron gun were simulated using the Field Precision Trak software. With modifications to the electromagnetic optics, a compression factor of 45 for the beam radius and thereby current densities of 1800 A/cm$^{2}$ were achieved in the trapping region of the EBIT for a 1.5 A cathode at 30 keV. For maximal experimental flexibility, three cathode sizes were simulated at different beam energies. Furthermore the geometry and design will also simplify routine maintenance. We will present the results of our simulations and the new design. The new electron gun will enable us to better perform high-precision mass measurements of nuclides with short half lives.
The hydrogen ground state hyperfine splitting is known to seven parts in 1013 [1]. The Antihydrogen Laser Physics Apparatus (ALPHA) Collaboration seeks to perform precision tests of symmetries between matter and antimatter by measuring properties of antihydrogen and comparing them to its matter counterpart, hydrogen. This pursuit leads us to probe the ground state hyperfine splitting in antihydrogen; if charge parity time (CPT) symmetry holds then it should be identical to that of hydrogen.
We measure two positron spin resonance (PSR) frequencies in the same magnetic field, from which we extract the hyperfine splitting. In 2017, we reported a 200-fold improvement in the precision to which the hyperfine splitting had been measured in antihydrogen, resolving the frequency to four parts in 104 [2]. I will describe methods that have enabled us to further increase the precision to which we are able to measure this quantity by a substantial margin. I will also discuss implications of these new methods for matter/antimatter comparisons in precision tests of fundamental symmetries.
[1] Petit, P., Desaintfuscien, M. & Audoin, C. Temperature dependence of the hydrogen maser wall shift in the temperature range 295–395 K. Metrologia 16, 7–14 (1980).
[2] Ahmadi, M., et al. Observation of the hyperfine spectrum of antihydrogen. Nature 548, 66 – 69 (2017).
Electronic , Optical and Magnetic Properties of low concentration Ni doped CdSe by First Principle Method
M. Yaseen,1, M. Dilawar1, H. Ambreen1, U. Shahid1, Misbah2, M. K. Butt1, A. Ghaffar1, and W. Ren3
1Department of Physics, University of Agriculture, Faisalabad 38040, Pakistan
2Department of Chemistry, University of Agriculture , Faisalabad 38040, Pakistan
3Electronic Material Research Laboratory, Key Laboratory of the Ministry of Education and
International Center for Dielectric Research, Xi’an Jiaotong University, Xi’an 710049, China.
Corresponding Author: myaseen_taha@yahoo.com
In this work, electronic, optical and magnetic properties of low concentration Ni doped CdSe was studied systematically by Wien 2K code. Calculations were performed for the band structure in the spin up and down channels, total density of states (TDOS) and partial density of states (PDOS). The Spin-polarized band structure showed that Cd0.9375 Ni0.0625 Se, Cd0.875 Ni0.125 Se and Cd0.75 Ni0.25Se are half metallic ferromagnetic in nature with direct band gap, when the concentration of the transition metal increases the band gap is also increased. In optical properties, we discuss about absorption coefficient, optical conductivity, reflectivity, refractive index, real and imaginary part of dielectric function. Analyzed magnetic properties of Ni doped CdSe compound offers the magnetic moment which indicates the magnetic properties due to d orbital of Ni atoms. Obtained results revealed that Cd1-xNix Se is a suitable and potential candidate for the spintronics and optoelectronics devices.
Key word: Density functional theory; Optical properties, Dilute magnetic semiconductor
Plasmonically enhanced absorption or scattering of radiation on the mesoscale forms the basis of promising applications in a wide variety of fields such as: biosensing, photothermal therapy, photocatalysis, solvothermal chemistry, energy harvesting, magnetic recording for data storage, control of radiative heat transfer and so on. In a majority of the applications based on plasmonics, the noble metals - gold (Au) and silver (Ag) - have been the materials of choice. However, it is also now widely acknowledged that these materials suffer from problems of poor thermal and chemical stability accompanied by significant dissipative losses under high-temperature conditions. These issues have thus prompted a quest for materials with better thermoplasmonic properties. In this regard, semiconductor particles have lately attracted a lot of attention because they exhibit low ohmic losses, are thermochemically more stable, and exhibit highly tunable plasmonic resonances through bandgap engineering, control over dopant concentration and dielectric environment. Here, we will present results from our recent work on the multiscale modeling of plasmonically enhanced control of heat radiation using semiconductor inclusions [1, 2]. Furthermore, a comparison of the size-dependent thermoplasmonic behavior of indirect and direct bandgap semiconductor particles of undoped silicon (Si) and gallium arsenide (GaAs), respectively, with the metallic (Au) particles that are characterized by a complete absence of the bandgap will also be presented [3].
References
It is shown that energy transported entirely by conduction electrons is totally responsible for the heat generated by a constant electric current in a wire and that possible energy flows associated solely with the electric and magnetic fields make no contribution whatsoever to the heat generation.
Spin and charge correlations are particularly pronounced in low-dimensional materials and enable new technologies that harness quantum behavior. Accessing these correlations on their intrinsic length and time scales is an important step towards a microscopic understanding of correlated-electron physics.
We combine scanning tunneling microscopy with pump probe schemes to achieve ultrafast spectroscopy of spin and charge dynamics with atomic spatial resolution. Using electronic pulse generation [1] and optical excitation with THz pulses [2] it is possible to achieve time resolution between milliseconds and femtoseconds thereby matching the instrument to the dynamics of the investigated system. At nanosecond time resolution, we can track the spin dynamics of magnetic atoms on surfaces and identify miniscule magnetic interactions between few-atom spin chains [3]. At femtosecond time resolution, we can detect electron dynamics and follow the evolution of collective modes in a correlated-electron state at individual atomic defects.
These experiments access the microscopic dynamics of quantum materials and highlight pathways to design and control matter at the single atom level.
[1] S. Loth, M. Etzkorn, C. P. Lutz, D. M. Eigler, A. J. Heinrich, Science 329, 1628 (2010).
[2] T. L. Cocker, V. Jelic, M. Gupta, S. J. Molesky, J. A. J. Burgess, G. de los Reyes, L. V. Titova, Y. Y. Tsui, M. R. Freeman, F: A. Hegmann, Nature Photon. 7 620 (2013).
[3] S. Yan, L. Malavolti, J. A. J. Burgess, S. Loth, Science Adv. 3 e1603137 (2017).
We study the pump-probe response of three insulating cuprates as a function of both pump and probe photon energies, time, and fluence. The fluence dependence follows a simple and universal analytical form that includes a characteristic volume scale, which we associate with the effective interaction volume for a photoexcitation. This characteristic size varies strongly with pump photon energy, with a maximum just above the charge-transfer absorption peak. We assign this behavior to ultrafast many-body recombination in the photocarrier kinetics, characterized by an anomalously large Auger coefficient.
We will present time-domain terahertz spectroscopy measurements of the optical conductivity of MnSi thin films. Measurements cover a temperature range T = 5−300 K and a frequency range ν = 0.1−4 THz. We find that at low temperatures and frequencies, the scattering rate is proportional to both the square of the temperature and the square of the frequency, as predicted Fermi liquid theory. As the temperature increases further, the system loses quasiparticle coherence, while the plasma frequency inferred from a Drude fit decreases dramatically.
Owing to their large spin-orbit coupling [1], the lead halide hybrid perovskites are of interest for applications in semiconductor spintronics and spin-optoelectronics. While the photophysical properties of these materials have been studied extensively in recent years due to their potential for solution-processed, high-efficiency photovoltaic applications [2], much less is known about their spin-related properties [3-6]. Our studies of the spin-dependent carrier kinetics in butylammonium methylammonium lead iodide perovskite [7] indicate dominant precessional spin relaxation tied to the Rashba effect. Our recent measurements of the coherent carrier kinetics in 3D CH3NH3PbI3also suggest a role played by the Rashba splitting on the rate of interband dephasing. These findings point to the need for further studies of the influence of the strong spin-orbit coupling on the charge and spin dynamics in this family of materials.
[1] M. Kepenekian and J. Even, J. Phys. Chem. Lett. 8, 3362 (2017).
[2] https://www.nrel.gov/pv/assets/images/efficiency_chart.jpg.
[3] D. Giovanni et al. Nano Lett. 15, 1553 (2015).
[4] C. Zhang et al. Nat. Phys. 11, 427 (2015).
[5] P. Odenthal et al. Nat. Phys. 13, 894 (2017).
[6] D. Niesner et al. Phys. Rev. Lett. 117, 126401 (2016).
[7] S. B. Todd et al. arXiv:1807.10803 (2018).
With the successful completion of Run-2 at the Large Hadron Collider, the ATLAS experiment recorded more than 140 fb-1 of integrated luminosity of pp collision data at 13 TeV centre-of-mass energy. This talk will review the latest results based on this wealth of new data in the area of the Brout-Englert-Higgs sector, important Standard Model processes and searches for physics phenomena beyond the Standard Model.
The performance of missing transverse momentum (MET) determination using a new algorithm is presented. The peak luminosity of the LHC in 2017 data-taking reached a new record of $1.90 \cdot 10^{34}$ cm$^{-2}$ s$^{-1}$ and it is aiming to achieve higher luminosity after up-coming upgrades of the Large Hadron Collider (LHC) and produce a larger number of interactions per bunch crossing $<\mu>$. Such increase in pile-up interactions will pose a challenge to the existing MET algorithm. The new pile-up fit (PUfit) algorithm is designed to reduce the sensitivity of the MET to pile-up collisions. The algorithm was tested in di-muon decays of the Z boson in 13 TeV ATLAS data. The Z boson was used as an imaginary source of MET to test and calibrate the algorithm. The estimated resolution and scale were then compared to that produced by the standard offline algorithm. Results of these comparisons will be presented and discussed in this talk.
Scale factors (SF) are powerful tools for any particle physics analysis. They provide a clean way of estimating particle identification uncertainty. A SF is defined as the ratio of particle identification efficiency in data to Monte Carlo (MC) simulation. From the SF definition, correlations between systematic uncertainties in data to MC may be cancelled, thereby reducing the systematic uncertainty on a measurement. Because the SF is applied directly to the measurement of a particle production cross-section, understanding the particle identification efficiencies and minimizing the systematic uncertainty are of primary importance. In this study, a W or Z vector boson is identified by its hadronic decay products using tagging techniques on jet substructure observables from the ATLAS detector. The observables implemented are mass and energy correlation fractions, which are functions based on transverse momenta and pair-wise angles between decay particles. After tagging a particle as a W or Z boson, fits with defined functional forms are done on the signal and background distributions in MC. The fits are then applied to data and the tagging efficiencies are extracted. SF measurements and results from the ATLAS detector at CERN using 2015-2018 data will be presented.
After running for over three years at a world-record collision energy of 13 TeV, the Large Hadron Collider (LHC) completed its Run-2 dataset last December. This new dataset offers an unprecedented opportunity to test a wide range of theories which, if true, would address questions that remain unanswered in the otherwise well-established Standard Model of particle physics. However, such gains in explanatory power hinge entirely on the existence of new fundamental quantum fields and their associated particles whose decay signatures can be recorded in the ATLAS detector. In particular, a decay signature involving a pair of high energy electrons or muons (dilepton) is theoretically and even historically well motivated, and has recently been investigated using the leverage of the full Run-2 dataset. In my presentation I will touch on these theoretical motivations and then detail the procedure and results of the latest search for exotic particles producing dilepton signatures in data collected with the ATLAS detector. I will also explain what the findings represent in the wider context of searches at the LHC and how they help guide theoretical developments toward a better description of particle physics.
The concept of a magnetic charge, analogous to electric charge, has been around for many centuries. Paul Dirac formally introduced the concept into electromagnetism and quantum mechanics concluding that, If observed, Magnetic Monopoles would potentially explain the quantization of the electric charge and complete the symmetry between electricity and magnetism in Maxwells’ equations. Since then there have been many searches for Magnetic Monopoles through different experimental techniques, none of which have given conclusive results on the matter. We present a search for Dirac Magnetic Monopoles and High Electric Charge Objects in the ATLAS detector from 13TeV pp collisions at the LHC. Detection is based on the particles’ characteristic high ionization, penetration distance and lack of shower. A data driven method called the ABCD method is used to estimate the background in the signal region. Model independent efficiency maps are implemented to reduce the need for computationally intensive simulations.
Systems based on lattices of triangular or tetrahedral units with nearest neighbour antiferromagnetic exchange interactions are the archetypal examples of geometric magnetic frustration, offering a means of accessing novel ground states. Rich phenomenology has similarly been observed in extensive investigations of Ce (4f1) and Yb (4f13) based intermetallic compounds, including heavy fermion behaviour, unconventional superconductivity, or non-Fermi liquid responses in the proximity of a quantum critical point. The ground states of these systems are considered to be mainly governed by the competition between the Kondo and the Ruderman - Kittel - Kasuya - Yosida (RKKY) exchange interaction, both mediated by the conduction electrons. Combining these two arenas, whereas earlier investigations concentrated on insulators, there are an increasing number of studies of frustrated magnetism in metallic systems.
For example, CeCd3As3 is one member of a family of rare earth ternary compounds which crystallise into a hexagonal ScAl3C3-type structure, where the magnetic Ce-ions form a quasi two dimensional anisotropic triangular lattice. Such model systems have been proposed as candidates for quantum spin liquid behaviour [1]. I report a specific heat and muon spin relaxation investigation of the low temperature ordered phase and associated spin fluctuations of this material.
Magnetic systems based on 5d transition metal ions on the pyrochlore lattice are another example, offering a unique opportunity to explore the exotic ground states which potentially arise when the electron-electron Coulomb interaction, electronic bandwidth and spin orbit coupling are all of comparable magnitude. I will compare magnetisation and muon spin relaxation investigations in a series of 4d Mo4+ and Ru4+, as well as 5d Ir4+ based pyrochlores, which may be controllably tuned through a metal insulator transition, providing insight into the magnetic excitations as these finely balanced systems evolve from a localised to itinerant spin character.
[1] P. Fazekas and P. Anderson, Philos. Mag. 30, 423 (1974)
For Ce- and Yb-based metallic systems, the competition between Kondo and RKKY interactions results in a great variety of ground states. When these rare-earth ions are located on the geometrically frustrated lattices, the magnetic order may be tuned by the introduction of magnetic frustration. Although the lattices are geometrically frustrated for the short range, nearest neighbor spin-spin interaction, often the conduction electron degree of freedom in metals takes over the longer-ranged interactions. Due to this reason, the effect of geometrical frustration in metals is not obvious. In this talk, I will present the frustration effect on two-dimensional, triangular lattice, metallic antiferromagnets.
Cubic compounds with Yb3Rh4Sn13-type structure have drawn attention because of their rich ground states such as heavy fermion behavior, intermediate valence behavior, charge density wave, and superconductivity. In this talk, we will present the structural, magnetic, and electrical properties of cubic R3Ru4Ge13 (R = Yb and Lu) compounds. Single crystals of R3Ru4Ge13 were characterized by magnetization, specific heat and electrical resistivity measurements. The resistivity measurement of Yb3Ru4Ge13 compound exhibits a metallic behavior, whereas Lu3Ru4Sn13 compound shows a semiconductor-like behavior with a superconducting transition at ~2.2 K. Low-temperature specific heat measurement indicates Yb3Ru4Ge13 is a heavy fermion.
The rare earth pyrochlores family of materials that are renowned for the diversity and novelty of their magnetic ground states. However, despite more than 20 years of effort, a consensus on the nature of the magnetic ground states of the terbium pyrochlores has remained elusive. Indeed, there are numerous confounding factors that come into play in these materials, including: a low-lying crystal electric field level, multipolar interactions, and magnetoelastic coupling. In my talk, I will introduce one member of this family, Tb2Ge2O7, and elucidate its phase behavior using heat capacity and neutron scattering measurements. We find that the magnetic state of Tb2Ge2O7 evolves through a complex series of transitions, starting from a correlated paramagnet, passing through a short-range ordered state that I will discuss in the context of quantum spin ice, and culminating in a fully long-range ordered spin ice state.
In general relativity producing a regular bounce entails violation of Null Energy Condition for a dynamical source in the model.
That generically indicates existence of ghosts or other instabilities.
However, in cuscuton modification of gravity, the correspondence between a background bounce and violation of Null Energy Condition for dynamical sources is broken.
Cuscuton action, modifies equations of motion in Infra Red limit allowing the background to go through a regular bounce phase.
At the same time, since it does not contain any dynamical degrees of freedom, it does not lead to ghosts or other instabilities.
Here, I present a toy scenario of a regular bouncing cosmology and prove this claim
As is well-known, qubits are the fundamental building blocks of quantum computers, and more generally, of quantum information. A major challenge in the development of quantum devices arises because the information content in any quantum state is rather fragile, as no system is completely isolated from its environment; generally, such interactions degrade the quantum state, resulting in a loss of information.
Topological states are promising in this regard because they are in ways more robust against noise and decoherence. But creating and detecting edge states can be challenging. We describe a composite system consisting of a two-state system (the qubit) interacting with an SSH chain (a hopping model with alternating hopping parameters). In this model, the dynamics of the qubit changes dramatically depending on whether or not an edge state exists. Thus, the qubit can be used to determine whether or not an edge state exists in this model.
The direct detection of gravitational waves from pulsars is a much anticipated discovery in the emerging field of multi-messenger gravitational wave (GW) astronomy. In this context we develop an analytic approach to study the Fourier Transform (FT) of the GW pulsar signal including spindown effects. To incorporate the spindown effects, we incorporate an extended model of Alvarez and Carraminana (2004), which includes the quadrupole term that accounts for the emission of gravitational radiation and assumes an inverse linear law of magnetic field decay of the pulsar (Chishtie et al, 2018). With this approach we are able to extract an all-order formula for the spindown parameters using the Taylor series representation of Jaranowski et al.(1998). We then include these spindown corrections in the overall FT expression of the GW pulsar signal and then analyze these corrections for the cases of the Crab, PSR B1509-58, PSR B0540-69 and Vela pulsars.
We establish the unitary representation theory of quantum group gravity.
(i) A dual pair of nonlinear diffusion equations are presented, first describes the growth phase transition process (as universe expansion); second describes the descent process (as Hall effect). These exactly analytical solutions explain the explicate and implicate physical causality, structure and law of dual quantum groups under nature gravity effects. We prove that nonlinear and linear operators are isomorphism under complex manifolds, then are connect, then some nonlinear diffusion equations are exactly solvable.
(ii) Using the Gelfand Mapping, we construct an exactly quantitative double spectrum digraph (DSD) of gravity phase evolution and coherent process, which reveal the essential detail of the phase transition (coherent) as: spectrums of graviton operator, three levels of gravity energies, three phase transition orbits. This model discovers the relationships between the mass, spin and graviton, determines the phase change limitations for particle and cosmology scales.
(iii) This DSD model just is a modern mathematical physics version of an ancient mysterious FuXi Hexagram, by which we exactly predict some fundamental physics constants, such as, Hubble parameter, Planck constant, dark energy density, Newtonian gravitational constant. These results deepen the understand of gravity and Nature.
The challenge of identifying the astrophysical site(s) of the rapid neutron capture ($r$) process, which is responsible for the creation of half of the heavy elements found today, is a multifaceted one which requires input from a gamut of disciplines including observation, theory, and experiment. More experimental data, particularly of nuclear masses, on the neutron-rich side of stability is needed to alleviate uncertainties in current $r$-process calculations. The Canadian Penning Trap (CPT) mass spectrometer is situated at Argonne National Laboratory in the CARIBU facility where intense beams of neutron-rich nuclei are created from the spontaneous fission of a $^{252}$Cf source. To take advantage of the unique beams available at CARIBU, an upgrade to a phase-imaging mass measurement technique (PI-ICR) has been completed at the CPT. This modern technique offers several benefits which cumulatively increase the experimental sensitivity of the CPT, enabling mass measurements of nuclei further from stability than was previously possible. I will describe the implementation of PI-ICR at the CPT and highlight recent mass measurements of rare-earth nuclei near $N = 100$ which may be used to impose constraints on the astrophysical site of the $r$ process.
About half of the elements in the universe heavier than iron ($^{56}$Fe) are believed to be produced via the $r$ process (rapid neutron capture process), which is thought to occur during explosive astrophysical events. Due to the short-lived nature of these nuclides and the unique conditions necessary for the occurrence of the $r$ process, our knowledge of this topic is limited. Observations from the recent gravitational wave event (GW170817) and its electromagnetic counterpart at LIGO and VIRGO have reported evidences of neutron star mergers being possible $r$-process sites. But exactly where the $r$ process occurs in the merging neutron star environment, and whether merging neutron stars can alone account for all the $r$-process elements are still open questions. Building theoretical models and making reasonable predictions about this $r$ process rely on the availability of mass and other nuclear data for nuclides near the expected $r$-process path, which is currently available in limited capacity, due to the challenges in producing the rare isotope beams (RIB) necessary for such experiments.
With the recent development of a number of advanced RIB facilities, the situation has improved. One such facility is the CAlifornium Rare Isotope Breeder Upgrade (CARIBU), at the Argonne National Laboratory (ANL), which uses the spontaneous fission of a $^{252}$Cf source to produce beams of neutron-rich isotopes. The fission fragments are collected, thermalized and extracted as an ion beam, which undergoes multiple stages of purification before being sent to the Canadian Penning Trap (CPT) mass spectrometer. An upgrade to the CPT detection system allowed for the implementation of the novel detection technique, Phase-Imaging Ion-Cyclotron-Resonance (PI-ICR), which permits us to make fast and precise mass measurements of the exotic neutron-rich isotopes nearer to the $r$-process path.
Recently, the masses of a number of rare-earth, neutron-rich nuclides have been measured, which are consistent with the masses needed to reproduce the solar abundance pattern in the rare-earth region for a hot, neutron star merger wind scenario, as determined by reverse engineered Markov Chain Monte Carlo simulations.
In asymptotic giant branch (AGB) stars, 22Ne plays an important role in several nucleosynthesis processes, with its production competing with the synthesis of 19F through the so called ‘poisoning reaction’, and the following transfer into 25Mg acting as the main neutron sources for the heavy element s-process, affecting the reaction rates of numerous isotopes.
In this contribution, we discuss a recent neutron transfer experiment done at TRIUMF in November 2018, directly populating 22Ne, allowing for high resolution measurements of the resonance energies with the SHARC silicon detector, coupled to the HPGe detector array TIGRESS for accurate measurement of the characteristic gamma rays. We will then present the method of using the angular distribution of these newly measured gamma rays to determine the spins of the resonance states, allowing for further constraint on the reaction cross-section.
Nuclei around doubly magic $^{132}$Sn are of particular interest in nuclear structure as well as nuclear astrophysics. The evolving shell structure near the shell closure is ideal for testing the current nuclear models far from stability. Additionally, the extra binding energy observed around $^{132}$Sn has direct implications in astrophysical models, leading to the second r-process abundance peak at $A\sim130$.
While the decays of neutron-rich Cd isotopes around the $N=82$ shell closure have been previously investigated, the information on some of the daughter In isotopes such as $^{128}$In is still limited. For $^{129}$In, the two reported level schemes have large discrepancies [1, 2]. It is also worthwhile to verify the recent results on the decay of $^{130\mbox{-}131}$Cd [3, 4, 5].
Detailed $\beta$-$\gamma$-spectroscopy of $^{128\mbox{-}131}$Cd was performed at the ISAC facility of TRIUMF, Canada. The data was collected with the GRIFFIN spectrometer, along with the $\beta$-particle detector SCEPTAR. The half-lives of $^{128\mbox{-}130}$Cd have been measured and reported [6]. In $^{128}$In, 32 new transitions and 11 new states have been observed in addition to the four previously observed excited states. The $^{128}$Cd half-life has also been re-measured with a higher precision via the time distribution of the strongest gamma rays observed in the decay. For $^{129}$In, 29 new transitions have been observed and 5 new excited states have been established. The log$ft$ values obtained suggest the dominant $\beta$-decay mode is the $\nu0g7/2 \rightarrow \pi0g9/2$ Gamow-Teller transition, which is consistent with the known characteristics of the $\beta$-decays in the $Z < 50$, $N \leq 82$ region. The new results for the decay of $^{128\mbox{-}131}$Cd will be presented and compared with previous studies as well as with shell model and IM-SRG calculations.
[1] O. Arndt et al., Acta Phys. Pol. B 40, 437 (2009).
[2] J. Taprogge et al., Phys. Rev. C 91, 054324 (2015).
[3] J. Taprogge et al., Phys. Rev. Lett. 112, 132501 (2014).
[4] A. Jungclaus et al., Phys. Rev. C 94, 024303 (2016).
[5] J. Taprogge et al., Eur. Phys. J. A 52, 347 (2016).
[6] R. Dunlop et al., Phys. Rev. C 93, 062801(R) (2016).
In the region where the expected N=82 shell closure approaches the proton drip-line, we are confronted with our poor knowledge of the nuclear binding energies. Precision experimental data is critical if we are to gain an understanding of how this neutron shell evolves for the heaviest N=82 isotones. The binding energies of neutron deficient nuclei can also reveal the exact location of the proton drip-line. However, the current lack of precision mass measurements in this region makes these nuclei stand out as a clear target for mass spectrometry studies, which allow for the determination of these binding energies. Furthermore, masses in this region can provide an anchor for chains of alpha decays whose origins extend up to A$\approx$170, and for which only relative masses are presently known.
The Isotope Separator and ACcelerator (ISAC) facility at TRIUMF produces intense beams of exotic isotopes for nuclear science. I will present the results of a series of atomic mass measurements of neutron-deficient Yb and Tm isotopes around the N=82 shell closure. Several of these masses were measured directly for the first time. These measurements were carried out using the recently commissioned Multiple Reflection Time-Of-Flight Mass Spectrometer (MR-TOF-MS) at TRIUMF’s Ion Trap for Atomic and Nuclear science (TITAN). The results enable us to reduce the uncertainty in the nuclear binding energies, thus pushing towards the proton drip-line and providing new insight into the behaviour of the N=82 shell closure far from stability.
Since its invention, cyclotrons have a history for applications in medicine. This can be either indirectly via the production of medical isotopes in nuclear medicine as well as directly for external beam therapy for the treatment of cancer. At TRIUMF, we have been involved in both over the year, by treating cancer with beams of pions and then with protons, as well as a long history of medical isotopes – gamma and positron emitters for diagnostics, and beta and alpha emitters for therapy.
In recent years Targeted Alpha Therapy (TAT) has shown impressive results in the treatment of incurable cancer. In TAT, alpha emitters like Ac-225 are attached to a targeting vector, a biomolecule, which accumulates in the cancer to be treated. As alpha emitters have high linear energy transfer (LET) and typically a short range in tissue, and together with its specific delivery system, they hold the promise to cure cancer that can not be treated with other therapies – surgery, chemotherapy or external beam therapy. Currently, there is a world-wide shortage of alpha emitters, which greatly hinders ongoing clinical trials. TRIUMF with its 520 MeV cyclotron, has the infrastructure in place to solve this shortage.
In this presentation, the highlights over the years as well as the current research and developments are discussed.
Optical microscopy has long been limited by the diffraction limit of light, but in recent decades many advancements have been made to push beyond this limit to achieve super-resolution microscopy. Breakthroughs in optics, mathematics, chemistry, and physics have led to the development of different families of super-resolution techniques, each taking a different approach to dealing with the diffraction limit. This talk will highlight the different types of super-resolution microscopy techniques currently employed by biologists and discuss some of the advantages and disadvantages to consider when deciding between these techniques.
The success and persistence of women and other under-represented minorities in physics post-secondary education has been studied from numerous perspectives. Recently, researchers have suggested that success and persistence in physics depends on the development of a strong physics-identity. However, to form a complex picture of how students access resources that contribute to the development of physics identities, it is necessary to understand the formal and informal structures and relationships at the university and beyond which support them. We know that under-represented students, especially women, tend to persist in physics if they become involved in initiatives such as equity, diversity and inclusion (EDI) in physics campus groups. Despite evidence that participation in these initiatives can contribute to persistence, we know relatively little about how students access and participate in these initiatives, or how these spaces can facilitate graduate students’ identity work in physics. This presentation explores the usefulness of social network analysis to understand the degree to which these initiatives may create networks of support for students’ identity work in physics. At the same time, I will discuss the potential of studying identity work paired with social network analysis, to capture the social and dynamic aspects of alternative spaces for physics learning and persistence. This theoretical and analytical pairing can to capture the kinds of identity-forming resources that flow through networks, and the social interactions that facilitate identity work in physics. These outcomes and novel framings are significant for faculty and administrators involved in EDI initiatives to improve the success and persistence of under-represented students in physics.
Across several years of Physics 100 and Physics 101, two large introductory physics courses at UBC, a consistent pattern has emerged: On exams and diagnostic tests for nearly 3000 students, female students underperform relative to their male peers. The existence of this performance gap is consistent with the science education literature. However, a focus on performance may be missing important details. Recent research has shown that affective outcomes—such as self-efficacy and belonging—may also suffer gender gaps and that differences in these may mediate learning. Here, we explore the gender inequalities that exist in our introductory physics course in both the cognitive and affective domains. Incoming knowledge was measured with the Force Concept Inventory (FCI), outgoing performance was measured with the FCI and the course exam, and we adapted survey tools to measure student self-efficacy and level of test anxiety. We report the inequalities that exist and analyze correlations between them to understand what the causes within our courses might be. The ultimate goal is to suggest interventions that reduce the gender gap and create a more equitable classroom for all.
In 2014 the Physics Department at Simon Fraser University (SFU) began a peer tutoring program to support the students enrolled in our two “Physics for the Life Sciences” first-year courses. Four undergraduate students who earned high grades in these courses were recruited to be volunteer peer tutors and run weekly drop-in tutoring sessions. We specifically chose students majoring in the life sciences and/or planning to attend medical school so that they would be true peers to the students in these courses. In 2016 this program was expanded to encompass three additional subject areas: Biology, Chemistry and Mathematics. In the same year with support from the SFU Institute for the Study of Teaching and Learning in the Disciplines, we conducted a study to determine the impact of this peer tutoring program on both the students being tutored (the tutees) and the tutors themselves. We administered surveys and conducted focus groups and interviews with both the tutors and the tutees in order to determine the influence their experiences with peer tutoring had in three arenas: enhanced learning, social experience and sense of agency. In addition we looked at the effect of peer tutoring on the academic performance of students who attended a significant number of tutoring sessions. The results of these investigations will be presented. We will also discuss how this program continues to grow at SFU beyond its humble beginnings and our plans for the future.
NSERC's Senior Policy Advisor on EDI will provide an overview of the agency's EDI-related initiatives, with a specific focus on the made-in-Canada version of the UK Athena SWAN program being launched to encourage and recognize commitments made by post-secondary institutions towards advancing equity, diversity and inclusion in the research community. This program is broader in scope than what has been done so far in other countries. All post-secondary institutions will be invited to participate (universities, colleges, CÉGEPs, polytechnics), the program will encompass all academic disciplines, and it is inclusive of all underrepresented and disadvantaged groups (including, but not limited to women, Indigenous Peoples, persons with disabilities, visible minority/racialized groups and members of LGBT2Q+ communities). Following a short presentation the speaker will welcome questions from the audience.
The K$^-$ beam intensity at the hadron experimental hall in J-PARC is steadily increasing for conducting strangeness nuclear physics program. The present production target in the hadron hall can accept up to ~50 kW proton beam from the 30-GeV main ring. In the summer of 2019, a new production target which can operate more than 80 kW is going to be installed, and the beam intensity will be increased more than 1 M/spill.
In this talk, recent topics obtained in the following several experiments at J-PARC will be presented:
1) Hypernuclear gamma-ray spectroscopy in $^4_\Lambda$He and $^{19}_Λ$F (E13),
2) Hybrid emulsion experiment to look for double-Λ hypernuclei (E07),
3) Search for Ξ-hypernucleus in the 12C(K$^-$, K$^+$) reaction (E05), and
4) Search for K-pp bound state in $^3$He(K$^-$, Λp)n reaction (E15).
Further, future prospect of the strangeness nuclear physics program will be mentioned.
The SNO+ experiment is located in SNOLAB (Sudbury, Canada), 2 kilometers underground in the Creighton mine. It is a large scale, low-background, neutrino detector whose main purpose is to search for neutrinoless double-beta decay. The outcome of the discovery would be a measurement of the half-life for such a process, which is inversely proportional to the effective neutrino mass squared. With 780 tons of liquid scintillator loaded with tellurium, SNO+ is going to explore the Majorana neutrino mass parameter space down to the inverted mass hierarchy region.
Designed as a multi-purpose detector, the SNO+ can also detect solar and reactor neutrinos, geo and supernovae neutrinos and perform nucleon decay searches. It is currently filled with ultrapure water and taking data since May 2017 to measure the 8B neutrino flux and constrain the lifetime of nucleon decay through invisible modes in 16O. The results have been published and they will be presented during this talk.
The experiment is now moving to the next phase, where the detector will be filled with liquid scintillator and subsequently loaded with tellurium.
The nEXO experiment is a proposed neutrino-less double beta decay (0$\nu\beta\beta$) search in the isotope Xe-136. Anticipated to be located at SNOLAB, nEXO aims to observe the Majorana nature of neutrinos with a sensitivity that will probe the entire inverted mass hierarchy parameter space. nEXO’s stringent low-background requirements necessitate a water shield in order to reduce contributions from external radiation. Photomultiplier tubes inside the water will also measure Cherenkov light of passing muons; this active shield is referred to as the Outer Detector. We present the status of Monte Carlo simulations, the instrumentation plan, and muon veto capabilities of the Outer Detector. We also discuss the Outer Detector’s potential as a supernova neutrino observatory with a focus on the inverse beta decay interaction channel on hydrogen in the water.
Barium tagging is a future upgrade for nEXO (next-generation Enriched Xenon Observatory), a planned experiment that will search for neutrinoless double beta decay (0νββ) in ¹³⁶Xe. If detected, 0νββ would confirm that neutrinos are their own anti-particles and reveal their absolute masses. Due to the long half-life (>10²⁶ years) of 0νββ, a low background is critical to achieve the necessary detection sensitivity. Barium tagging is the ultimate method for rejecting 0νββ-like background events by confirming the production of ¹³⁶Ba as the daughter isotope of this decay. This can be done by extracting ions from the xenon detector, then identify barium through laser spectroscopy in a Linear Quadrupole Ion Trap (LQIT). The ions will then be sent to a Multi-reflection Time of Flight (MR-TOF) mass spectrometer, to distinguish ¹³⁶Ba from other naturally occurring barium isotopes as well as to identify other contaminants. The LQIT has been simulated and optimized to achieve high efficiencies in ion trapping and transmission. The development of the LQIT, from simulations to most recent experimental results of a prototype, will be presented.
The equilibrium phase behaviour of a semi-dilute, liquid crystalline (LC) polymer brush is investigated, using self-consistent field theory with wormlike chains and implicit solvent. For good solvent conditions, the isotropic interactions favor a stretched brush while the anisotropic LC interactions favor collapsing into a high-density nematic state. By allowing the nematic director to orient freely, we find that the polymers collapse by tilting rather than backfolding as previously thought. Within the implicit solvent model the transition to a tilted state is an instability; however, the transition becomes continuous once the semi-dilute approximation is relaxed.
Micro-capsules are increasingly being studied as models for cells, as drug delivery vehicles, and even as devices for enhanced oil recovery form porous substrates. As such, understanding of their mechanical properties is critical for rational design and optimal performance. Here we describe experiments using a Cantilevered-Capillary Force Apparatus to probe the dynamic (or complex) modulus of micro-capsules under axial compression. The micro-capsules used in this study are approximately 200 microns in diameter and are composed of a sunflower-oil core surrounded by a gellan-gum shell.
Electrokinetic transport phenomena, predominantly realized in charged polymeric and porous media, offer possibilities for applications in nanofluidic systems, energy harvesting and biosensing. In this work, we present a theoretical and numerical study of nonlinear coupling between wall deformation and the flows of water and ions in a charged, deformable nanochannel. The classical treatment of mass and momentum conservation in the solid-liquid coupled system is based on the Stokes-Poisson–Nernst–Planck equations. For elastic but non-viscous walls in the limit of small deformation, analytically solvable differential equations were obtained in one dimension. The response of the walls’ relaxation dynamics and the channel’s electrokinetic transport was investigated at different charging regimes. Within the framework of nonequilibrium thermodynamics, compact formulae in terms of Onsager's phenomenological coefficients were derived for the electrokinetic transport parameters and energy conversion efficiency. Furthermore, an extension of the model is presented for electroactuator modelling which operates through a coupling of electrical and mechanical interactions for closed nanochannels. In this scenario, we explore numerically the transient dynamics and steady-state solutions for closed, finite nanopores. A full theoretical account, along with numerical results, of the effect of membrane charging and mechanical response on the differential capacitance is presented.
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Most phenomena or interactions whether they be physical, chemical or biological in nature take place at the nanoscale. Hence, materials under study should be characterized for various properties such as structural, mechanical, optical and chemical at high resolution. Not many techniques are capable of nanoscale measurements. Development of novel techniques for this purpose is possible by employing recent advances in technology. One such advancement is the development of Multifrequency Lock-in Amplifier (MLA) that outperforms conventional lock-in amplifiers in many ways. This MLA is able to simultaneously detect more than 20 close by frequencies with no interference from each other. Thus, making it possible for incorporating MLA to develop new techniques.
In this research program, a novel method has been developed in order to measure the elastic property in combination with structural property at the nanoscale which is achieved by intermodulation Atomic Force Microscopy (ImAFM) while driving the tip at two frequencies. This results in generation of other frequencies from which the force curve can be reconstructed to be fitted with various force models in order to extract the elastic values. A very important consideration to be made is that the handling of force curves and data processing not only depends on the surface properties of the sample but also depends on the morphology of the samples and on the tip properties. ImAFM is particularly of interest for measuring visco-elastic properties i.e probing nanoscale properties of biomaterials.
Hollow-core photonic crystal fiber (HC-PCF) was used to develop a gas cell, which was incorporated in an Intracavity Absorption Spectroscopy (ICAS) system for the detection of gas at sub-ppmv levels of concentration. One drawback of using long lengths of HC-PCF is the relatively long time required to allow the gas sample to fill the entire cell. By segmenting the fiber there will be multiple points of entry for the gas to enter. The effect of segmenting the HC-PCF on sensitivity and fill/evacuation times will be examined. The working principle of the ICAS system will also be presented, along with a comparison between both segmented and un-segmented HC-PCF cells. The motivation for developing this system is to manufacture a compact and remotely operated system at a relatively low cost, which could be placed in a fertilized agriculture field to monitor the emission of greenhouse gases (e.g., N2O). Besides the HC-PCF, all other optical components used in the system are from the telecommunication industry which reduces the cost of manufacturing the system. A fiber Bragg grating (FBG) was used to select the lasing wavelength of the cavity. The lasing wavelength chosen for the system corresponds to the absorption line of the gas species of interest. When monitoring N2O, the system operates at a wavelength of 1522 nm. The wavelength was chosen in order to avoid strong absorption lines from other gases of the atmosphere. The research was financially supported by Natural Sciences and Engineering Research Council of Canada (NSERC), Canada Foundations for Innovations (CFI), Ontario Center of Excellence (OCE) and Nutrien.
We study the spreading of correlations in space and time after a quantum quench in the Bose Hubbard model. We derive equations of motion for the single-particle Green's function within the contour-time formalism, allowing us to study dynamics in the strong coupling regime. We discuss the numerical solutions of these equations and calculate the single-particle density matrix for quenches in the Mott phase. We demonstrate light-cone like spreading of correlations in the Mott phase in one, two, and three dimensions and calculate propagation velocities in each dimension. Our results show excellent agreement with existing results in one dimension and demonstrate the anisotropic spreading of correlations in higher dimensions. We also discuss how our results can be extended to disordered systems.
We have recently studied a set of 40 spectra of carbon monoxide in pure state and mixed with air, recorded at temperatures between 79 to 296 K. Our aim was to investigate if the temperature dependencies of line parameters, such as half widths and pressure induced shifts, stay the same at very low temperatures, close to 79 K. The spectra were all recorded over two decades using the 1-m Fourier Transform spectrometer located at Kitt Peak, AZ, USA and two temperature controlled gas cells. The spectra were calibrated using the line positions for residual carbon dioxide and water vapour and referencing them to the HITRAN database.
The constrained analysis technique was used together with the software Labfit [2]. Three line shape models were employed: Voigt, speed-dependent Voigt and Rautian. In the absence of experimental narrowing parameters, we have calculated them using computed diffusion constants [3].
The theoretical half-width coefficients for CO-N2 have been determined at several temperatures employing a potential energy surface of Tipping-Herman type, vibrationally independent potentials and by taking into account the electrostatic interactions. We will discuss what laws were fund to best represent the temperature dependencies over a wide range of temperatures, approaching cryogenic temperatures.
We thank D. Chris Benner for the Labfit software. The work of V.M. Devi was funded by NASA grants and contracts, and the research by M. A. H. Smith was performed as part of her employment at NASA Langley Research Center. No official endorsements are intended or implied. N. Islam and A. Predoi-Cross have been funded by NSERC. S. Ivanov and O. Byzykin received financial support from the Ministry of Science and Higher Education within the State assignment FSRC «Crystallography and Photonics» RAS and Russian Science Foundation (Project No.18-55-16006).
References:
1. I.E. Gordon, L.S. Rothman, C. Hill, R.V. Kochanov, Y. Tan, P.F. Bernath, et al. J. Quant. Spectrosc. Radiat. Transfer 203, 3–69 (2017).
2. D.C. Benner, C.P. Rinsland, V.M. Devi, M.A.H. Smith, D. Atkins, J. Quant. Spectrosc. Radiat. Transfer 53(6), 705-721 (1995).
3. J.O. Hirschfelder, C.F. Curtiss, R.B. Bird, Molecular theory of gases and liquids. New York: Wiley and Sons, 1952.
Material quality and crystallinity play an important role in the activity of plasmonic devices. Plasmonic structures made from monocrystalline metals may be expected to display much higher efficiency and stability than polycrystalline devices which are subject to many losses due to the presence of grain boundaries and defects. With the help of a novel epitaxial electroless deposition (EED) chemistry, ultrasmooth gold films can be grown on monocrystalline silver surfaces. In this approach, the electrochemical incompatibility of gold and silver can be overcome in concentrated sodium hydroxide (NaOH) (1 M) where the presence of OH⁻ causes a decrease in the reduction potential of gold cations by forming Au(OH)₄⁻ complexes (E≈0.55 V), an increase in the oxidation potential of the silver electrode (E≈1.45 V), and acts as a reducing agent. As a result, ultrasmooth monocrystalline gold films are grown with the same crystalline orientation as the underlying silver film. This chemistry enables the growth of gold from a few monolayers up to few hundreds of nanometers uniformly over a large area. Furthermore, this approach enables the fabrication of large area metasurfaces made of gold and silver epitaxially grown nanostructures that can be used in a variety of different applications. The growth of gold films and nanostructures can also be manipulated by the introduction of anionic species during the deposition, and leads to the formation of surface nanostructures with specific shape, due to preferential interaction of the anions with certain facets of the growing crystalline structures.
The decrease of the glass transition in polystyrene thin films with decreasing film thickness has given rise to the suggestion that there is a region near the free surface where there are faster dynamics than in the bulk. This surface mobility has been directly measured, but there are few studies of how the enhanced dynamics varies with distance from the free surface. We have used $\beta$-detected nuclear magnetic resonance ($\beta$-NMR) of implanted $^8$Li$^+$ to directly probe the temperature and depth dependence of the $\gamma$-relaxation mode, which is due to phenyl rings undergoing restricted rotation, in thin films of atactic deuterated polystyrene (PS-d8). I will show that the activation energy for the $\gamma$-relaxation is lower near the free surface and returns to bulk values at depths of $\sim$10 nm and discuss how the depth dependence of $\gamma$-relaxation is affected by sample processing (such as annealing, floating on water and the inclusion of a surfactant), and by the presence of a buried interface. I will introduce a novel hole-burning (selective saturation) $\beta$-NMR technique that has been used to measure the depth dependence of the fluctuation rate of a much slower relaxation process.
Thin polymer films are commonly used as samples in numerous areas of materials physics. Techniques applied to the preparation and application of thin films, including etching and nanolithography, often involve the addition of a solvent to the polymer surface. It is currently understood that there exist two classifications of polymer solvents. Good solvents are used to fully dissolve a polymer film while poor solvents are thought to have no lasting effects on a surface. However, recent research has shown that some poor solvents significantly impact polymer film surfaces and produce surface morphology on the nanoscale. In this study, polystyrene thin films of number average molecular weights ranging from 8000 g/mol to 545000 g/mol were produced through spin casting onto a silicon substrate. The films were then exposed to various poor solvents for differing time durations. The change in surface morphology was examined with an atomic force microscope and analyzed to determine the extent to which solvents change the polymer surface. It is shown that poor solvents produce a nanoscopic surface morphology with a characteristic length scale that is independent of the solvent used. The characterization results will allow for discussion of polymer surface morphology.
Vibrational sum frequency generation (SFG) spectroscopy is a non-destructive interface-specific non-linear optical spectroscopy that delivers information about the structure and composition of interface environments. Coupling and energy flow through vibrational modes at surfaces are important and of interest in areas ranging from membrane biophysics and biochemistry, to materials science, electrochemistry, and heterogeneous catalysis. SFG spectroscopy leverages the non-centrosymmetric environment of the interface to mix two incident electric fields and provide a coherent scattered field at their sum frequency. For fields resonant with the vibrational modes of interface molecules, the sum frequency signal provides valuable information about local interface structure and energetics. 2-Dimensional SFG (2D-SFG) spectroscopy extends these studies to provide direct information about molecular interactions and dynamics. Here, we describe the construction of a novel collinear 2D-SFG spectrometer and its application to the vibrational spectroscopy of highly ordered organic monolayers at the CaF2/Air interface. A three pulse IR sequence generated from a dual output Ti:sapphire-based optical parametric amplifier (OPA) and birefringent wedge pair create a highly phase stable train of IR pulses with controllable time delays. This 100 fs IR pulse sequence is combined with a narrow bandwidth picosecond up-converting 800 nm pulse to yield a broadband SFG spectrum. Spectra captured as a function of IR pulse sequence delays are then processed to yield 2D-SFG spectra, providing information about the molecular interactions, vibrational couplings and vibrational dynamics of the organic monolayer. Here we report the vibrational couplings and dynamics of the alkyl chain vibrational modes of supported ordered stearate chains in the CH spectral region to reveal their unusual dynamics and strong mode couplings.
It is well know that even in equilibrium the dynamics of a polymer in solution are affected by hydrodynamics and follow Zimm, rather than Rouse, dynamic scaling. However, due to the difficulties in correctly simulating a polymer in a real solvent, most simulations of polymers in solution have used Langevin dynamics which do not include hydrodynamics. My group has created a package for the open-source molecular dynamics package LAMMPS for simulations of particles in a fluid that includes full hydrodynamics. I will describe some of our studies of polymers in confined environments experiencing different flow fields. In particular, we will focus on polymers in channels with constricting and diverging flows and polymer margination in shear flow near a surface. The inclusion of full hydrodynamics is found to have a profound impact on the non-equilibrium behavior observed in these systems.
The isolation of single layer graphene in 2004 by Geim and Novoselov introduced a technique that researchers could extend to the broad family of van der Waals materials. Just as the van der Waals bonds responsible for holding together adjacent layers of two-dimensional (2D) materials can be broken, they can also be put back together. Crystals can then be reassembled with layers from different compounds or in different stacking orientations to create new materials previously inexistent in nature. In this work, we describe a home-built transfer setup that allows the user to create these new crystals (van der Waals heterostructures) by stacking distinct layered 2D materials with lateral alignment and, importantly, angular alignment. In addition to presenting the transfer set-up, we also describe some techniques for preparing the crystals that are subsequently stacked. Finally, characterization of the heterostructures is presented.
2D transition-metal dichalcogenides (TMDs) exhibit unique optical and electronic properties that make them highly appealing to the scientific community. Like graphene, they have strong in-plane bonds and weak out-of-plane bonds, allowing for easy fabrication of complex single layer structures or molecular heterostructures. In particular, MoS2 is a TMD semiconductor that displays emerging photoluminescence (PL) through its transition from exhibiting an indirect bandgap in its bulk form to a direct bandgap at few- to monolayer thickness. Although it holds great potential for use in novel nanoelectronics and optical devices, there is still much variability in the reported PL and electron mobility across studies involving 2D MoS2. This necessitates a better understanding of its excitonic properties. In this work, we report on our exfoliation and membrane transfer technique: a modified mask aligner and dry PDMS transfer for assembly of heterostructures. We developed simple optical methods to quantify layer number, which we compare with Raman spectroscopy, Atomic force microscopy, and PL spectrum analysis. We then compared the PL spectrum between layer number, h-BN encapsulated samples, and exfoliated vs. grown MoS2 via chemical vapour deposition (CVD). We found red shifted PL peaks with increasing layer number and in CVD-grown MoS2, indicative of crystal purity or structural differences for exfoliated devices. Also, PL intensity increased with decreasing layer number as well as with h-BN encapsulation. We further discuss the lithographic process used to integrate monolayer samples into a bottom-gated field effect transistor, followed by preliminary data.
Experiments were performed on natural crystals of biotite in an attempt to quantify the effects of impurities on acoustic phonon behaviour and elastic properties in this mica series. Electron probe microanalyzer(EPMA) was used to quantify the chemical compositions of biotite with differing concentrations of Fe and Mg. Brillouin light scattering spectra of these samples contained peaks due to pure transverse, quasi-transverse and quasi-longitudinal acoustic phonon modes. Analysis of these spectra permitted determination directional dependence of the velocities of these modes in the ac and bc crystallographic planes as well as the elastic constants $C_{11}$, $C_{22}$, $C_{33}$, $C_{44}$, $C_{55}$, $C_{66}$, $ C_{13}$, $C_{15}$, $C_{23}$, $C_{25}$, $C_{35}$, and $C_{46}$. In general, the acoustic mode velocities decrease with increasing Fe concentration. The values of some elastic constants also appear to show a dependence on impurity and concentration.
GaN nanowires (NWs) have been applied in devices including light emitting diodes (LEDs) photodetectors and laser diodes, but control of the dopant distribution has been a difficult task [1]. Also it is well known that polarization effects have an influence on device properties. A previous report on GaN NWs found an effect of strain on the charge distribution via off-axis electron holography EH [2]. We report the measurement of the electric potential and depletion width in GaN NW p-n junctions using EH which is carried out using transmission electron microscopy (TEM) [3]. Nanowire p-n junctions grown by Molecular Beam Epitaxy, with Mg and Si as the p and n-type dopants, had doping concentration of 5 x 1017 and 1 x 1019 cm3, respectively, based on planar growth calibrations. A high density of basal plane (0001) stacking faults was found to be present within the p-type side whereas the n-side was free of detectable defects. Also the diameter of the p–type segment of the wires was always larger and highly anisotropic compared to the more uniform diameter n-type side. Phase images and potential profiles, extracted from the EH measurements, showed both an effect of thickness and the presence of a junction. An average built-in voltage of 0.5 ± 0.1 V and depletion width of 80 ± 5 nm were measured which both indicate either a smaller carrier activation or reverse biasing from charging or polarization effects. Results from a variety of growth conditions and structures will be presented.
1. Wallentin, J et al. Doping of semiconductor nanowires. J. Mater. Res. 26, 2142 (2011).
2. Chen, X et al. Controlling charges distribution at the surface of a single GaN nanowire by in-situ strain. Prog. Nat. Sci. Mater. Int. 27, 430–434 (2017).
3. Darbandi, A et al. Direct Measurement of the Electrical Abruptness of a Nanowire p–n Junction. Nano Lett. 16, 3982–3988 (2016).
Silicon nanowires (SiNWs) are attractive for a variety of applications such as photoelectrochemical (PEC) devices for hydrogen fuel production which require precise compositional and morphological control for abrupt electrical interfaces. However, SiNWs grown by the vapour-solid-liquid method typically encounters problems such as nonselective deposition on sidewalls, catalyst-induced compositional gradients, inadvertent kinking and inhomogeneous doping. The bandgap of Si is 1.1 V, therefore multiple Si tunnel junctions are required for the operating voltage to exceed values required to split water (1.23 V).Tunnel junctions require degenerate-doping conditions such that the field in the depletion region is strong enough to induce tunnelling, with a sharp potential profile to increase its probability. Trap-assisted tunnelling in Si via gold impurity levels usually outnumbers band-to-band tunnelling at low field strength, therefore a very small concentration of gold impurities is believed to aid the tunnelling process. To this end electron holography (EH) combined with scanning TEM serve a critical role being able to non-invasively measure depletion region widths and their associated built-in voltages [1]. Although doping control has been demonstrated in VLS growth for SiNWs [2], growth of degenerate p-type Si, Boron (B) doped, has been challenging. Lower p-type doping is reached compared to n-type doping (Phosphorous) under the same conditions. We overcome these challenges synthesizing these structures by in situ chlorination of the NW surface with hydrochloric acid at temperatures ranging from 500 to 700 °C. We demonstrate degenerately doped p/n and n/p sections with abrupt potential profiles of $1.1 \pm 1$ V in SiNWs and depletion region widths as narrow as $10 \pm 1$ nm via EH. The results presented herein enable the growth of complex, degenerately-doped p-n junction nanostructures and are an essential step in the fabrication of PEC devices.
[1] A. Darbandi, J. C. McNeil,A. Akhtari-Zavareh, S. P. Watkings, K. L. Kavanagh, Nano Letters 16 (2016).
[2] D. J. Hill, T. S. Teitsworth, E. T. Ritchie, J. M. Atkin, J. F. Cahoon, ACS Nano 12 (2018).
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ZnO is a II-VI semiconductor with a wide bandgap of 3.37 eV. This makes it a good candidate for transparent solar cells and for near-ultraviolet LEDs. In the following, a Ta/Ru/Au thin film is sputter deposited on a (110) Si substrate as a seed layer. The Ta layer appears to be amorphous while the orientation of the deposited metal films is found to be Ru (001), Au (111) by X-ray diffraction. This allows for epitaxial c-axis ZnO nanowire (NW) growth by metalorganic chemical vapour deposition at temperatures from 400˚ - 600˚ C. Such growth may be independent of substrate choice, as the orientation of the metallic seed layers will depend only on the characteristics of the amorphous Ta layer. Transmission electron microscopy (TEM) and XRD are used to demonstrate c-axis growth of ZnO on the deposited metal film. Scanning electron microscopy shows densely packed NWs. The height and separation increase with growth temperature while the misalignment increases with growth temperature. This is corroborated by rocking curve measurements which show increasing FWHM with growth temperature.
Additionally, possible growth by vapour-liquid-solid (VLS) method is explored. Atomic force microscopy (AFM) shows the successful formation of liquid Au droplets on the substrate surface following exposure to Triethyl Gallium (TEGa) at temperature above the Au-Ga eutectic point. By adjusting length or flow rate of exposure the size, number, and height of nanoparticles can be controlled. Atomic force microscopy (AFM) shows that longer exposure (or higher flow) leads to fewer, larger particles of greater height on the substrate surface. Under VLS growth, this would allow tuneability of NW dimension and dispersion on the substrate surface. However, TEM and energy dispersive spectroscopy (EDS) indicate that higher temperature growths may be necessary to achieve NW growth by VLS.
Measurements of parity violation effects in atomic systems test the Standard Model at low energies and probe nucleon-nucleon interaction. Francium is a good system for atomic parity violation (APV) studies, because APV effects are predicted to be large in francium and its atomic structure is favourable for theoretical calculations. We are developing experiments to study APV effects in neutral francium atoms at the ISAC radioactive beam facility at TRIUMF. We use techniques of laser cooling and trapping to prepare the atoms for our measurements. Our current effort is based on optical spectroscopy of the electric dipole forbidden 7s-8s atomic transition aiming at the test of the strength of the electron-quark weak neutral coupling. In this talk, I will present our recent observation of the Stark (electric field) induced 7s-8s transition and our roadmap towards observing APV.
Operational funding by NSERC, NRC/TRIUMF, infrastructure support by DOE and NRC/TRIUMF, student support by U. of Manitoba, and travel support by CONACYT and Fullbright.
Dark Matter, neutrino masses, and the excess of matter over antimatter in the universe, are examples of gaps in our understanding of physics. An electric dipole moment (EDM) of the electron, either large or unobservably small, will be a feature of successful theories of these phenomena, and of other extensions of the Standard Model.
Our Electron EDM Collaboration is developing an experiment to search for an electron EDM using francium atoms produced at TRIUMF. Francium is an alkali atom, with its atomic structure well understood, and with its high sensitivity to an electron EDM established accurately from field theory.
Our experiment will use a francium atomic fountain. A fountain allows the measurement to be done in free space and in free fall, with no collisions with gasses or walls, with no confining lasers or A.C. Stark shifts, and with no applied static or time-varying magnetic fields. There will only be a static electric field between optical state preparation and state analysis, and the optical state preparation and state analysis occur in a region free of both applied magnetic and applied electric fields.
In this experiment we can measure our sensitivity to an EDM and even adjust it to zero. We will be able to measure our sensitivity to systematic effects and can make the systematics large to show that we have them under control. We can also test that any observed effect is linear in the electric field and that it reverses with electric field, both of which are definitive signatures of a permanent EDM.
Reactor neutrino oscillation experiments observe two anomalies in anti-neutrino energy spectra: a total deficit of events, and an event excess in the 5-7 MeV range compared to theory. A total deficit in anti-neutrino flux may support a non-SM sterile neutrino, but both discrepancies may result from inadequate understanding of the reactor fuel cycle. In the 5-7 MeV range first-forbidden $0^-\rightarrow 0^+$ decays account for 30% of the total anti-neutrino ($\bar{\nu}_e$) flux, with $^{92}Rb$ ground-state to ground-state branch (GSB) alone accounting for 30-50% of $0^-\rightarrow 0^+$ $\bar{\nu}_e$ flux. Model predictions will be improved by measuring the $\bar{\nu}_e$ energy spectra from $^{92}Rb$ decay, and measuring the strong GSB ratio with projected 2% accuracy and independent systematics to traditional total absorption spectrometers. Using the TRINAT neutral atom trap, and measured momenta of beta and recoiling $^{92}Sr$ daughter, kinematic reconstruction of $\bar{\nu}_e$ energy spectra, decay Q-value, GSB ratio, and beta-neutrino angular correlation coefficient $a_{\beta\nu}$ are examined experimentally.
Francium is an ideal candidate for atomic parity nonconservation (APNC) measurements, since APNC effects scale roughly with a $Z^3$ dependence. Knowledge of francium's atomic structure is key to interpreting APNC measurements, and can be studied through properties such as the Stark shift of the $7s\rightarrow8s$ transition. In our experiment we use magneto-optical traps to suspend francium atoms in vacuum between indium tin oxide coated field plates to measure the Stark shift. In this talk I will discuss our latest measurement for the single photon Stark shift of the $7s\rightarrow8s$ transition in francium 211.
We gratefully acknowledge UMGF and GETS support by the University of Manitoba.
The TRIUMF UltraCold Advanced Neutron (TUCAN) collaboration is currently engaged in building a next generation ultracold neutron (UCN) source, with a neutron electric dipole moment (nEDM) measurement as its flagship experiment. A permanent EDM is a violation of CP-symmetry, and thus the nEDM is relevant in standard model extensions and the baryon asymmetry problem. The nEDM measurement is currently in the design phase, for which detailed studies of systematic effects in the experiment are crucial. To study these effects the TUCAN collaboration is using PENTrack, a particle and spin tracking software package developed for simulating UCNs. Many of the systematic effects in the nEDM experiment arise due to inhomogeneities in the electromagnetic fields, which affect the neutron spin precession. PENTrack’s functionality allows these effects to be studied with realistic geometries and fields. This talk will discuss the use of PENTrack in performing such studies and how these results relate to the nEDM experiment.
Answering many of the outstanding questions in astroparticle and subatomic physics today requires the ultra-quiet environment provided by deep underground research facilities. SNOLAB is Canada's deep underground facility, at a depth of 2 km in Vale’s Creighton mine near Sudbury, Ontario. In this environment, cosmic radiation induced backgrounds are minimized to levels allowing the operation of sophisticated experiments. SNOLAB is home to a number of experiments that make use of this unique location. The facility is growing, and the number of projects hosted is increasing. I’ll review the new construction projects and the status of the currently running experiments, as well as improvements to the infrastructure.
Neutrino physics continues to be a rich and vibrant field. SNO+, situated as it is in the Vale Creighton Mine at 6800 m water equivalent depth, promises to address a number of topics within the subject. To understand the measurements taken by the SNO+ detector, a similarly multifaceted approach is required for its calibration. The different techniques used in the SNO+ calibration will be described here with special focus on the transition from the SNO prototyped water phase calibration measurements to the calibration of the scintillator filled detector. With the heightened requirements for background suppression in SNO+, new materials have been chosen for the construction of the calibration systems. The opportunity has also been taken to design new calibration sources with better characteristics for the new target medium. Early results from newly commissioned calibration systems will be presented in contrast to the old systems with the future prospects for the scintillator calibration program.
Only one supernova neutrino burst has ever been detected, and the detection of additional neutrinos from galactic core-collapse supernovae are expected to provide insight on the supernova explosion mechanism. One candidate for detecting supernova neutrinos is SNO+, a multipurpose ultra-low background particle detector. Within SNO+, a galactic supernova neutrino burst is expected to generate an unprecedented rate. Thus, it is necessary to stress-test and optimize the SNO+ data acquisition and electronics so that a supernova signal can be reliably read out. For this purpose, a Supernova Calibration Source is under development to mimic the light expected from supernova neutrino interactions [1]. Using one-dimensional simulated supernova neutrino datasets [2, 3], light profiles representing neutrino interactions are calculated and realised using a laser diode light source delivered into the detector via fibre optics and a deployed light diffuser. This talk will focus on the software conversion of neutrino datasets to light profiles, which define the light intensity and timing in the calibration source.
[1] C. Darrach. The SNO+ Supernova Calibration Source: Development and Testing. MSc Thesis, Laurentian University, Sudbury, ON, Canada (2016)
[2] L. Hüdepohl, B. Müller, H.-T. Janka, A. Marek, and G. G. Raffelt. Phys. Rev. Lett. 104, 251101 (2010)
[3] A. Mirizzi et al. Rivista del Nuovo Cimento Vol. 39 N. 1-2 (2016)
The flux of thermal neutrons, fast neutrons, and gamma photons, arising due to radioactivity in the materials surrounding the detector, constitute the principle component of the radiation environment for the underground experiments at SNOLAB. The flux of fast neutrons comprising ($\alpha$, n) and spontaneous fission constitutes a significant background and the SNO collaboration estimated this flux to be $\sim 4000~ \mathrm{neutrons} ~\mathrm{m}^{-2} $ $\mathrm{day}^{-1} (2\pi \mathrm{sr})^{-1}$, with an unknown spectrum. PICO-2L, operating as a very sensitive low background fast neutron detector with shielding removed, measured this fast neutron flux for $\sim$ 20 live-days. We performed a simulation of neutron propagation for PICO-2L to understand the spectral shape of the fast neutron flux, the detector sensitivity and the effect of the drift environment on neutron propagation. Through comparing and correlating the simulations with PICO-2L and previous measurements for gamma fluxes, slow and fast neutrons, and understanding the radioactive contamination of the materials around the detector, a coherent model for the radiation environment in the SNOLAB drifts can be constructed.
DEAP-3600 is a liquid argon (LAr) based spin-independent direct dark matter search experiment. It is designed to detect nuclear recoils induced by the elastic scattering of weakly interacting massive particles (WIMPs) on argon nuclei. DEAP-3600 recently reported its second physics result that included the best reported upper limit on the WIMP-nucleon spin-independent cross section on a LAr target of 3.9×10^−45 cm^2 for a 100 GeV/c^2 WIMP mass at 90% C. L. An essential component of this result involved measuring the rates of alpha-decays within the detector and determining their impact on the expected background rate in the WIMP search region. Alpha-decays from the uranium and thorium series were measured in the liquid argon and on the surfaces of the inner-detector. The techniques used to measure and characterize these rates in-situ are presented.
There is strong astrophysical and cosmological evidence for a new kind of matter known as dark matter, which does not interact via the electromagnetic or strong forces. The SuperCDMS experiment searches for dark matter particle candidates using cryogenic germanium and silicon detectors. The next generation of SuperCDMS is scheduled to start operating at SNOLAB in 2020. Before this facility is ready, the Cryogenic Underground TEst Facility (CUTE) will come online to test SuperCDMS detectors underground at SNOLAB, providing a low-background, low cosmogenic activation environment. The facility will have the capacity to hold up to six of the standard SuperCDMS detectors. The CUTE cryostat has a faster turnaround time to test detectors in an efficient manner and allows the study of detector performance well before the main experiment is ready. Given the low background of the facility as well as the expected threshold of the new detectors, the CUTE facility will have an opportunity to search for low-mass dark matter with the new SuperCDMS detectors. When SuperCDMS comes online, CUTE will be used to perform detailed characterizations of the detectors and test new R&D detectors. This talk will present the CUTE facility and discuss the status underground at SNOLAB.
Functional Magnetic Resonance Imaging (fMRI) is a powerful tool to map brain activity non-invasively. Over the past 25 years, fMRI has become widely used in neuroscience research and is the foundation of tens of thousands of published studies. However, fMRI does not measure neurons directly. Instead, in the most common form of fMRI called Blood Oxygen Level Dependent (BOLD), changes in the concentration of paramagnetic deoxyhemoglobin in the blood are detected. Although changes in blood oxygenation are correlated with neural activity, a quantitative relation has not been established, limiting the interpretation of data and the power of fMRI to study the brain in health or disease.
Animal models provide invaluable insight into brain physiology for studying questions such as the cellular origin of fMRI. In mice, state-of-the-art optical technologies have been developed to probe neuronal activity as well as blood flow and oxygenation across spatial scales while manipulating cell-type-specific neuronal activity. However, translating such detailed results from laboratory animals into predictions relevant for human imaging requires both technical and theoretical efforts.
In the first part of this talk, I will demonstrate a framework for imaging the brain of awake behaving mice across scales, from two-photon microscopy in individual ~10 micron wide cerebral arterioles and cells, to macroscopic fMRI. I will then show how high-resolution 3D images of cerebral vasculature measured in mice can be graphed into a connected network to model blood circulation using simple laws of physics. In this model, deep learning as well as equations of fluid dynamics, gas diffusion and the physics of magnetic resonance are leveraged to predict human brain imaging signals and improve their interpretation. Potential applications in neuroscience and fundamental cancer research will be discussed.
Nanotechnology is rapidly developing as an enabling technology for introducing new highly performant theragnostic tools for biomedical applications. Recently, plasmonic nanostructures (PNs) have shown a great potential for providing an effective method in this field of research and applications. PNs present very peculiar optical properties related to the presence of a large optical absorption cross-section due to the collective oscillation of the conduction electrons by incident laser field known as localized surface plasmon resonance (LSPR). The PNs are also capable of enhancing the incident laser field in the form of scattered near field for even an off-resonance irradiation where the incident laser wavelength is far away from LSPR. If the intensity of the incident laser is large enough, this field enhancement capability can be employed to generate a highly localized free electron (plasma) in the vicinity of the particles. The energy deposition in generated plasma can be considered as an energy source for photothermal ablation therapy, photoacoustic tomography, and light-controlled drug release. In this presentation we will present a model that will serve as a design tool for simulating ultrashort pulsed laser interaction with PNs in water for such applications. This tool is built based on the following physical phenomena that couples: 1) optical properties of the PN obtained from simulation of electromagnetic wave interaction with PN and coupled through resistive heating due to laser pulse to; 2) two temperature model to analyze transient heat transfer in ultrafast regime and temperature increase calculation, where temperature is used as a coupling parameter to; 3) plasma dynamics and 4) structural mechanics analysis for linear thermal expansion, stress and strain calculation for the input as a boundary condition in acoustic-structure interaction for; 5) acoustic pressure wave propagation modeling.
Detection of inflammation and classification of pathological lesions, for example in the oral diseases and cancer applications, has typically be restricted to expensive imaging modalities such as X-rays which suffer from poor spatial resolution and lack functional information for early diagnostics. Other traditional imaging such as computed tomography (CT) cannot be used during treatment or surgical procedures and are difficult techniques to access sensitive areas of a patients’ oral cavity. Moreover, CT and X-ray imaging are not sufficiently sensitive and specific to sense early stage biomarkers of inflammatory response in aging and pre-diseased tissue. Here we present the development of a non-invasive optical coherence tomography (OCT) technique to examine the structural integrity and connectivity at the interfaces of soft and hard tissue surfaces as a pre-diagnostic biomarker of oral disease. We evaluate the efficacy of this method for lesions discrimination from healthy tissue based on measurement of optical property variances, namely scattering and refractive index, in periodontal tissue multilayer interfaces. Specifically, we use stable and well-controlled optical phantoms to examine the OCT-derived properties across 1-2mm tissue-bone models to determine the limit of detection for periodontal pocket depth. We aim to compare these model systems with in vivo characterizations of bone structures through small layers of epithelial cells of the gum. We further introduce a multi-modal approach for in vivo time-resolved near infrared-OCT (NIR-OCT) which enables a novel in situ probe for structural-functional optical assessment of inflamed tissues through examination at these soft-hard tissue interfaces, such as through observation of variations of oxy-/deoxy- hemoglobin absorption local gradients of pH. We will also demonstrate the design and integration of biocompatible optical clearing agents for enhanced functional contrast. To compliment and overlay the high resolution optical scattering map of the gum-hard tissue interface, we aim to co-register it with NIR functional contrast associated with the local inflamed microenvironment. Finally, we discuss the broader relevance of this time-resolved NIRS diffuse reflectance method as a means for optically-stable biochemical contrast in other inflammatory environments and applications. Our technique presents new opportunities for exploring combined structure-function optical classification of soft-hard interfaces in connective tissues as biomarkers for understanding aging and early detection of degenerative diseases.
Obtaining accurate predictions of nuclear structure starting from the interaction between constituent protons and neutrons is a complex, computationally demanding problem. Accurate predictions depend on both short-range correlations as well as long-range clustering and collective dynamics, particularly when describing resonances and continuum states. We present state of the art methods used to obtain predictions of properties of both bound states and resonances in $^{12}\mathrm{C}$ with nuclear interactions from chiral effective field theory as the only input.
The study of radioactive isotopes is key to understanding the fundamental building blocks of matter. These investigations require state-of-the-art experimental stations, which exist only in select facilities around the world. The Gamma Ray Infrastructure For Fundamental Investigations of Nuclei (GRIFFIN), at the ISAC facility of TRIUMF is a powerful decay spectrometer that can be used to study $\beta$ decaying species. The tin isotopes are an important part of the nuclide chart due to their magic proton number, $Z=50$, a stable configuration analogous to the noble gases. They span a total of forty isotopes, two neutron shell closures, at $N=50$ ($^{100}$Sn) and $N=82$ ($^{132}$Sn), and extend up to $N=89$ ($^{139}$Sn), making them an important testing ground for nuclear structure theory. Furthermore they are important in the rapid neutron capture process (r-process), responsible for the production of the heaviest elements in our universe. An isotope of tin with 79 neutrons, $^{129}$Sn, was studied via the $\beta$ decay of its indium parent, $^{129}$In, at the GRIFFIN station. So far the analysis of the decay spectroscopy data has uncovered twenty new transitions and seven new excited states, never before seen in this nucleus. The $^{133}$Sn nucleus was also studied at the GRIFFIN spectrometer, though the data was dominated by the $\beta$n decay of the $^{133}$In parent into $^{132}$Sn. Newly outfitted with BGO shields for Compton suppression, the GRIFFIN spectrometer has entered into a new phase; a reduction in the Compton continuum will allow for the observation of very weak transitions, offering a more detailed look into the tin isotopes. Results from the study of $^{129}$Sn and $^{133}$Sn, detection mechanisms and potential implications will be discussed.
Studies of tin isotopes are important for understanding the structure of singly-closed shell nuclei. The $^{116,118,120}$Sn isotopes are examples of even-even singly-closed shell nuclei close to the line of stability. Using $(n_\text{th},\gamma)$ reactions provides a comprehensive study of the level scheme of a nucleus. Despite many previous studies for stable tin isotopes, many nuclear levels are missing spin and parity assignments. Experiments involving $^{116,118,120}$Sn were conducted in 2018 at ILL, Grenoble, using FIPPS (FIssion Product Prompt $\gamma$-ray Spectrometer), which uses eight highly efficient, clovered, n-type, HPGe detectors to measure low intensity gamma rays. This aids in identify previously unknown transition multipolarities, and thus, provide the spin states of ambiguous energy levels. % work on this In the study of $^{116}$Sn, sixteen ancillary LaBr$_3$ detectors were used in the experiment to provide ps timing measurements to deduce state lifetimes. The experiments involving $^{118,120}$Sn featured eight additional HPGe clovers, from IFIN-HH (Horia Hulubei National Institute of Physics and Nuclear Engineering), to further improve the array's efficiency. The high number of statistics that FIPPS provides aids in making polarization measurements and angular correlation measurements to identify the spin states of energy levels without definite properties. Study of the electromagnetic character of weak transitions in tin will establish the parity of the de-excited states, help complete the understanding of singly-closed shell nuclei, and nuclear structure as a whole. The current work being done to study the tin species will be presented.
Nuclei away from the line of stability have been found to demonstrate behavior that is
inconsistent with the traditional magic numbers of the spherical shell model that was
developed for nuclei close to stability. This has led to the concept of the evolution of nuclear
shell structure in exotic nuclei and the neutron-rich Ca isotopes are a key testing ground of
these theories. However, there have been conflicting results from various experiments as to the
true nature of a sub-shell closure for neutron-rich nuclei around 52Ca. In June of 2018, an
experiment was performed at the ISAC facility of TRIUMF; 52K was delivered to the GRIFFIN
gamma-ray spectrometer which was paired with the beta-tagger SCEPTAR and the Zero Degree
Scintillator auxiliary detectors. Using this powerful combination of detectors, the level
scheme of 52Ca populated following the β-decay of 52K has been constructed. Preliminary
results from the analysis will be presented and discussed in the context of an N=32 shell
closure in neutron-rich nuclei.
A recent survey [1] of potential candidates for spherical vibrational motion [2] concluded that very few passed the criteria; of those that did, $^{98,100}$Ru were the most promising. However, in part this may have been due to the lack of detailed spectroscopic data for $^{98,100}$Ru. In order to remedy this, we have performed a high-statistics measurement of the β decay of $^{98,100}$Rh using the newly commissioned β decay Tape Station at iThemba labs located near Cape Town, South Africa. Activities of $^{98,100}$Rh were produced using fusion evaporation reactions of $^{14}$N and $^{16}$O beams on an $^{89}$Y target, which was then transported from the irradiation station to a counting station. The counting station consisted of 4 clover-type high-purity germanium detectors, augmented by a TIGRESS detector, a plastic scintillator for the β particles, and an in-vacuum Si(Li) detector for conversion electrons. The decay of $^{98}$Rh and $^{100}$Rh to $^{98}$Ru and $^{100}$Ru was the first measurement to be completed on this new facility. Very high-statistics data sets were collected for $^{98,100}$Ru, resulting in considerable expansions of their decay schemes. In this presentation, we concentrate on the results for $^{98}$Ru from the analysis of the γ-γ coincidence matrix.
A main focus of this work has been on possible states associated with the first excited $0^+$ state, the $0_{2}^{+}$ level. Several weak E2 transitions $–$ the 495 keV $(2^{ +}_{3}→0_{2}^{+})$, 402-keV $(2_3^+→2_2^+)$ and 419-keV $(2_3^+→4_1^+)$, were newly observed. The E2 transition to the $0_2^+$ state possesses the largest relative B(E2) value, strongly suggesting that the $2^{+}_{3}$ state is a band member of the excited $0^+_2$ band. We have also observed a candidate level for the 4+ rotational band member, suggesting that $^{98}Ru$ possesses a more-deformed excited $0^+$ band coexisting with a less-deformed ground state.
Details of the analysis of the $^{98}Rh$ decay to date will be given.
[1] P.E. Garrett, J.L. Wood, and S.W. Yates, Phys. Scripta 93, 063001 (2018).
[2] J. Kern et al., Nucl. Phys. A593, 21 (1995).
In her academic career, Li-Hong Xu was a source of inspiration and enlightenment to many students, especially young women, both in the university setting and in the larger world. Her research itself was based on light, as she was a molecular spectroscopist, employing a variety of laser systems in her own lab and in recent years utilizing facilities at the Canadian Light Source in Saskatoon, the “brightest light in Canada”. The details of her life and work present a fascinating picture of a dynamic and determined woman who embraced a wide variety of challenges and rose to prominence in her international and national research communities, her university teaching community, and her wider home community in Saint John and Atlantic Canada. Here we will touch on some of her notable achievements and on activities she undertook in teaching and community outreach. Some of these were the innovative “Party Physics” in her first–year course, the “Physics Circle” she introduced for girls at the Public Library to arouse their interest in physics and science, and her demonstration shows where she would recruit keen girls from her class to show off a variety of physics “Magic” to an admiring and frequently amazed public at Science Fairs and the annual UNBSJ school Math competitions and open houses.
Physics continues to be a male-dominated field. Sometimes it is difficult for women in physics with diverse cultural backgrounds, to find mentors that they can trust and that will guide them through difficult times in their career. This talk will discuss how younger women in physics have benefited from the one-on-one mentoring offered by Li-Hong Xu during her distinguished academic career.
As a role model, Li-Hong Xu was leading by example, sharing her own stories and experiences, discussing how to balance work and personal life, guiding and providing support for the professional development of other women physicists. The mentees were encouraged to strive to achieve their full potential. Career expectations, career stages and paths to achieve them were often discussed. Participating together with Li-Hong at social activities and events contributed to building communities in the department or at national level through CEWIP.
Li-Hong’s work in outreach was exemplary and we all struggle to carry on her good work. I will try to show some of these attempts: from talking to 5th – graders here, to a 1am Skype lecture to an audience in Pakistan, to making preparations for the World Year of Basic Sciences for Development, as declared by the United Nations. This latter work is being carried out by Commission 13 of IUPAP – Physics for Development.
University of British Columbia (UBC) Physics Olympics is a high school physics competition held on UBC Vancouver campus ( https://physoly.phas.ubc.ca/ ). This annual outreach event attracts more than 500 high school students, competing in teams, accompanied by more than 70 teachers and coaches from across the province. This competition, organized by the Department of Physics and Astronomy and the Department of Curriculum and Pedagogy at UBC, is one of the largest and oldest high school physics competitions of its kind in North America. More than 70 UBC volunteers – physics and education faculty members, graduate and undergraduate students, and staff collaborate before and during the event to make it a reality. The competition consists of six hands-on events (heats), of which two require devices which are designed and built by the students in the weeks before the competition. In recent years, it has also included professional development workshops and networking opportunities for physics teachers and coaches who accompany the teams to the competition. The event engages students in physics in a friendly and collaborative environment where all students are welcome. Many of the teams come from the far reaches of the province of British Columbia, and the event has become an annual field trip for the students - one of the perks of getting involved in secondary school physics. Many of the high schools have been attending the event for decades, thus helping support a physics teaching community in the province.
In this talk we describe how young minds can be influenced by outreach programs aimed at the feminine gender. We pay a tribute to Dr. Li-Hong Xu and her contributions to this effort.
Devi. She is a warrior, a beautiful wife, a mother, a scholar. These attributes can equally describe Li-Hong. Apart from her strength as a physicist, Li-Hong was a huge patron of the Asian Arts in Saint John. This dance presentation in Bharatanatyam, a classical dance style of South India, describes the goddess Devi. As a dedication to Li-Hong, I will draw parallels between her and the goddess.
As Tony Bates once said, “nothing has changed more in higher education over the last 50 years than the students themselves.” Join us for a dynamic discussion around inclusive, innovative, and evidence-based teaching approaches, and how you could transform your classroom lectures and be inspired by the way your students consume and retain information. This session will be full of thought-provoking ideas that could easily be implemented in your classrooms tomorrow.
As Tony Bates once said, “nothing has changed more in higher education over the last 50 years than the students themselves.” Join us for a dynamic discussion around inclusive, innovative, and evidence-based teaching approaches, and how you could transform your classroom lectures and be inspired by the way your students consume and retain information. This session will be full of thought-provoking ideas that could easily be implemented in your classrooms tomorrow.
When biological science students walk through the physics lecture room door on their first day at the University of Guelph, they generally fall into one of two categories: terrified or ambivalent. Almost no one is excited to be there, as they’ve been putting off taking physics for as long as possible. Knowing this, we have designed our service courses so that by the end of week 12 they have a much greater appreciation for our discipline. Many even go so far as to say that our class was their favourite course of the semester! Come and find out more about our approach to building up confidence in first-year science students at the University of Guelph.
A wealth of physics education research in recent decades has helped identify innovations that offer the most promise for truly effective physics learning. There is no doubt that active learning, mastery of concepts coupled with student self-checks, effective use of technology, and problems that challenge without overwhelming are all important components. Active learning will naturally happen when students are sufficiently engaged with the topics and the environment, but how do we really foster that? Working as part of a collaborative author team, self-reflection, and writing about physics has been an effective way for us to learn insights on both physics and learning. We will share ideas on how you might apply similar experiences to your own physics classrooms.
The flux of angular momentum in electromagnetism cannot be expressed entirely in terms of the field's radiative degrees of freedom. Its expression also involves Coulombic pieces of the field, in the form of a charge aspect q(theta,phi), a function of polar angles whose integral gives the total charge of the system. Guided by the strong analogy between radiative processes in electromagnetism and gravitation, we ask whether the flux of angular momentum in general relativity might also involve Coulombic pieces of the gravitational field. Further, we ask whether such terms might have been missed in the past by specializing the flux to sources of gravitational waves that are at rest with respect to the frame in which the flux is evaluated. To answer these questions we bring together the Landau-Lifshitz formulation of the Einstein field equations, which provides specific definitions for angular momentum and its associated flux, and the Bondi formalism, which provides a systematic expansion of the metric of an asymptotically flat spacetime in inverse powers of the distance away from the matter distribution. We obtain a new expression for the flux of angular momentum, which is not restricted to sources of gravitational waves at rest nor to periodic sources. We show that our new expression is equivalent to the standard formula used in the literature when these restrictions are put in place. We find that contrary to expectations based on the analogy between electromagnetism and gravitation, the flux of angular momentum in general relativity can be expressed entirely in terms of the field's radiative degrees of freedom. In contrast to electromagnetism, no Coulombic information is required to calculate the flux of angular momentum in general relativity.
Thanks to recent gravitational wave observations, we have evidence for the validity of Einstein’s theory in the strong field region, including near black hole event horizons. The existence of black holes gives rise to theoretical issues, such as the necessary existence of singularities and the related information loss conundrum, that will hopefully be resolved by quantum mechanics. In the absence of a complete testable theory of quantum gravity, it is useful to study simplified models that hopefully retain essential, relevant features of the full theory. After a brief introduction to the subject, I will describe recent work on a class of such models that designed to describe the quantum dynamics of singularity free spherical black hole formation and evaporation.
The oral presentation will consist primarily of a recently published article in International Journal of Modern Physics D entitled "A connection between linearized Gauss–Bonnet gravity and classical electrodynamics", authored by MR Baker and S Kuzmin. In this article, a connection between a well known gravitational model and classical electrodynamics is derived. Given the excitement among physicists regarding the relationship between the fundamental interactions of nature, this should be an interesting talk for all audiences. A more formal abstract from the article regarding technical contents within is given in the next paragraph, however the talk will be less technical than the contents of the article.
Content abstract: A connection between linearized Gauss–Bonnet gravity and classical electrodynamics is found by developing a procedure which can be used to derive completely gauge-invariant models. The procedure involves building the most general Lagrangian for a particular order of derivatives (N) and a rank of tensor potential (M), then solving such that the model is completely gauge-invariant (the Lagrangian density, equation of motion and energy–momentum tensor are all gauge-invariant). In the case of N=1 order of derivatives and M=1 rank of tensor potential, electrodynamics is uniquely derived from the procedure. In the case of N=2 order of derivatives and M=2 rank of symmetric tensor potential, linearized Gauss–Bonnet gravity is uniquely derived from the procedure. The natural outcome of the models for classical electrodynamics and linearized Gauss–Bonnet gravity from a common set of rules provides an interesting connection between two well-explored physical models.
We show how the zeroth, first, and second laws of black hole thermodynamics emerge from a generic semiclassical theory of gravity. The zeroth law is a kinematic property of bifurcate Killing horizons. It depends neither on details of the semiclassical coupling nor on the dynamics of gravity. The first law is established for stationary spacetimes for which a Hamiltonian can be defined. The second law is established in "piecewise stationary" spacetimes, roughly speaking a stationary spacetime that is disturbed and eventually returns to a different stationary sate. Black hole entropy is consistently defined in the Noether charge approach, and the entropy for the matter in its exterior is the von Neumann entropy of the quantum fields. These results elucidate the meaning of these laws and strongly support the interpretation of black hole entropy as an account of the information that is hidden behind the horizon.
Spin-pumping is the phenomenon by which a pure spin current can be injected into a non-magnetic metal from an adjacent ferromagnet. Spin-pumping into Pt is of great interest since Pt exhibits large inverse spin Hall effect, important for many spintroincs applications. However, it is not clear that the mechanism of spin-pumping into Pt is well described by standard spin pumping theory considering that Pt is strongly polarized at an interface with a ferromagnet. Evidence of this is observed in literature results, which disagree with their finding; the magnetic damping on Pt thickness is found to be exponential, linear or even discontinuous. To reconcile this we present a clean, experimental system, to test the measure of spin pumping contribution to magnetic damping, α. By studying α in ferromagnet/Pt and ferromagnet/Pt/ferromagnet structures we provide a very rigorous test for spin-pumping into Pt. The induced magnetic moment in Pt also mediates interlayer exchange coupling between the two ferromagnets which results in an in-phase and out-of-phase resonances modes. From the standard spin-pumping framework, this would imply that the two ferromagnets actively communicate with each other via pumped spin-current and would result in very different behaviour of α. Surprisingly, we find remarkable agreement between experimental results and standard spin pumping theory which involves interlayer exchange coupling. Our results suggest that the enhancement in damping is entirely due to spin-pumping without any contributions from spin memory loss or interface damping due to proximity polarized Pt.
The rhenium (Re)-based compounds are difficult to synthesize owing to the element’s high melting temperature and low solubility into solid solution. In this talk, we will present an anomalous transport property observed in single crystal Re3Ge7. In zero field, the temperature dependence of electrical resistivity sharply increases below a phase transition temperature Tc = 58.5 K, showing a metal-to-insulator-like transition. Analysis of the Hall coefficient measurements indicates that the carrier density is 0.04 per formula unit at 300 K and drops by two orders of magnitude below Tc. When magnetic field is applied, the temperature dependence of resistivity develops a maximum around 30 K, deviating from an ordinary metallic behaviour. At low temperatures, the Shubnikov-de Hass quantum oscillations are detected on top of the linear field dependence of magnetoresistance.
Idealized two-dimensional ferromagnetic films are predicted to undergo a Kosterlitz-Thouless (KT) transition that involves topological excitations termed vortices. Whereas the idealized system is isotropic within the plane (2DXY symmetry), real ferromagnetic films grown on single crystal substrates display anisotropy and finite-size effects that can lead to more complicated behaviour. In this work, magnetic susceptibility measurements of 3-4ML Fe/W(001) films with four-fold in-plane anisotropy were compared to theoretical predictions for a finite-size KT transition, and found to be in excellent quantitative agreement. Susceptibility measurements on many films grown independently in ultrahigh vacuum were made in situ using the magneto-optic Kerr effect (MOKE). Since the susceptibility signal can become complicated by dissipation caused by domain walls at low temperatures, the analysis concentrated on the high temperature, paramagnetic tail of the signal where quantitative theoretical predictions apply. The paramagnetic tail was fit to the distinctive form predicted by KT theory for a gas of unbound vortex/antivortex pairs: χ(T)=χ0 exp(B/(T/TKT-1)a), using a four-parameter fit. An analysis of 16 susceptibility signals from 12 independently grown films gave fitted parameters of a=0.50±0.03 and B=3.48±0.16, in excellent agreement with the predicted values of a=1/2 and 3.2 < B < 3.8. In about one-third of the films, the dissipative signal Im(χ) was very small, so that the peak of Re(χ) occurred close to the finite size transition temperature, TC(L). In these cases, the fitted values of the KT transition temperature, TKT, were tens of K below TC(L), which is quantitative agreement with finite size KT theory. In contrast, fitting to a power law typical of a second order phase transition, χ(T)=χ0(T/Tγ -1)γ, gives an effective critical exponent of γ=3.7±0.7 and places the transition temperature Tγ ~10K below the peak temperature. Both of these results are unphysical.
The recent discovery of Anomalous Hall Effect in Mn3X (X=Sn,Ge) suggests the existence of Weyl nodes in the electronic band structure of these non-collinear antiferromagnets [1]. The resulting coupling of electronic transport and magnetism in Mn3X presents important technological opportunities. The magnetic properties of Mn3X are crucial to Weyl physics and to its implementation in devices but are still under debate due to the lack of experimental studies. In this talk, I will present polarized neutron diffraction and inelastic neutron scattering measurements performed on Mn3Ge to gain insight into its magnetism. I will show Mn3Ge has an anti-chiral spin structure with a peculiar field dependence. I will also propose a spin Hamiltonian for this material that can explain its magnetic ground state selection and can also parametrize it’s spin wave excitations. [1] Nakatsuji S. et al., Nature 527.7577 (2015).
Magnetic frustration occurs when a material’s lattice geometry prevents it from finding the classical ground state which minimizes pairwise magnetic interaction energies. In such situations, novel ground states with exotic properties can emerge: examples include classical and quantum spin ice and classical and quantum spin liquids. Many different states and excitations have been predicted theoretically, but the study of their physical properties requires real physical realizations of these models.
We have been searching for new geometrically frustrated magnetic materials to broaden the range of materials that can be studied to identify new ground states and novel excitations. I will describe our work to synthesis a stacked triangular system ErMgGaO$_4$ and the related garnet Er$_3$Ga$_5$O$_{12}$, which we have characterized with magnetic susceptibility, specific heat, muon spin relaxation and neutron scattering. We find that the spins in remain dynamic down to our lowest temperatures (50mK), making this system a candidate spin liquid.
I will also describe our ongoing work on new materials whose lattice is derived from the highly frustrated pyrochlore and kagome lattices where we have successfully synthesized single crystal and polycrystalline specimens of several new compounds.
Physics, the ultimate interdisciplinary science, is a challenge to learn. Conceptually counterintuitive, technically formidable, and creatively demanding, it can intimidate even the most seasoned veteran. Our task as physics teachers is to guide our students along this path, from their original exposure to the subject to their completion of its most advanced degrees. We will best succeed by realizing that our greater mandate is to bring an understanding of physics to as broad an audience as possible. This requires a range of pedagogical strategies that will best engage the scientific curiosity of the audience at hand. I propose that it is by teaching physics at all levels — from the youngest novice to the talented expert — that we will not only achieve excellence in undergraduate teaching at all levels, but will awaken a wider public into seeing that physics is both a wonder to discover and a joy to teach.
La physique, science interdisciplinaire ultime, pose un défi d’apprentissage. Contraire à la logique théorique, formidable sur le plan technique et ambitieuse du point de vue création, elle peut intimider même le vétéran le plus aguerri. À titre de professeurs de physique, notre tâche est de guider nos étudiants dans cette voie, dès qu’ils abordent ce sujet jusqu’à l’obtention des diplômes les plus avancés. Notre plus grande réussite sera de constater que notre mandat ultime est de faire comprendre la physique au plus vaste auditoire possible. Cela exigera une gamme de stratégies pédagogiques qui saura mieux éveiller la curiosité scientifique de l’auditoire. À mon avis, c’est en enseignant la physique à tous les niveaux — du plus jeune novice à l’expert talentueux — que non seulement nous atteindrons l’excellence en enseignement à tous les niveaux du premier cycle, mais que nous amènerons un plus vaste public à voir dans la physique tant une merveille à découvrir qu’un plaisir à enseigner.
The discovery of new superconductors often precipitates a race to identify the superconducting pairing state and clues to the mechanism driving the superconductivity. The journey is often a complex one, involving sample quality challenges, and a wide array of competing spectroscopic and thermodynamic techniques. I will give snapshots of this trail, highlighting the importance of materials growth, plus spectroscopic techniques at optical and microwave frequencies, and in a scanning tunnelling microscope. A side trip to look at the challenges of teaching scientific reasoning to first-year students will show surprising connections to the experts’ journey.
La découverte de nouveaux supraconducteurs déclenche souvent une course pour identifier l’état d’appariement des supraconducteurs et des indices du mécanisme déclenchant la supraconductivité. La quête est souvent complexe, comportant des défis de qualité d’échantillons et un vaste éventail de techniques spectroscopiques et thermodynamiques concurrentielles. Je donnerai un aperçu de cette démarche, faisant ressortir l’importance de la croissance des matériaux, ainsi que des techniques thermodynamiques à des fréquences optiques et micro-ondes, et dans un microscope à balayage à effet tunnel. Un détour par les difficultés d’enseigner le raisonnement scientifique aux étudiants de première année montrera d’étonnants rapports avec la quête des experts.
During this plenary session, you will hear updates about NSERC activitiers from Dr. Elizabeth Boston, Director, Mathematical, Environmental, and Physical Sciences, Dr. Kari Dalnoki-Veress, Physics Evaluation Group Chair, and Dr. Ritu Kanungo, Chair of the CAP-NSERC Liason Committee.
Au cours de cette session plénière, vous serez mise-à-jour sur les activités de la CRSNG par la Dre Elizabeth Boston, Directrice, sciences mathématiques, environnementales et physiques, le Dr. Kari Dalnoki-Veress, le président du groupe d'évaluation de la physique, et la Dr. Ritue Kanungo, présidente de la comité de liaison ACP-CRSNG.
More information here / Plus d'information ici: http://www.dineoncampus.ca/SFU/menus/Locations/Locations
Science faces an accessibility challenge. Although information/knowledge is fast becoming available to everyone around the world, the experience of science is significantly limited. One approach to solving this challenge is to democratize access to scientific tools. Manu Prakash believes this can be achieved via “Frugal science”; a philosophy that inspires design, development, and deployment of ultra-affordable yet powerful scientific tools for the masses. Using examples from his own work (Foldscope: one-dollar origami microscope, Paperfuge: a twenty-cent high-speed centrifuge), Dr. Prakash will describe the process of identifying challenges, designing solutions, and deploying these tools globally to enable open ended scientific curiosity/inquiries in communities around the world. By connecting the dots between science education, global health and environmental monitoring, he will explore the role of “simple” tools in advancing access to better human and planetary health in a resource limited world.
La science est confrontée au défi de l’accessibilité. Bien que l’information et la connaissance deviennent rapidement disponibles à tout dans le monde, l’expérience de la science est considérablement limitée. Une approche visant à résoudre ce défi est la démocratisation de l’accès à des outils scientifiques. Manu Prakash croit que cela peut être atteint par le biais de la « science frugale »; une philosophie qui sous-tend à la conception, au développement et au déploiement d’outils scientifiques à la fois extrêmement abordables et puissants pour les masses. En utilisant des exemples provenant de son propre travail (le Foldscope : un microscope en origami à un dollar et le Paperfuge : une centrifugeuse à haute vitesse à vingt-cinq cents), le Dr Prakash décrira le processus d’identification des défis, la conception de solutions et le déploiement de ces outils à l’échelle mondiale afin de faire naître la curiosité et des recherches scientifiques ouvertes dans les communautés du monde entier. En faisant le lien entre l’éducation scientifique, la santé mondiale et la surveillance environnementale, il explorera le rôle des outils « simples » pour faciliter l’accès à une meilleure santé humaine et planétaire dans un monde aux ressources limitées.
More information here / Plus d'information ici: http://www.dineoncampus.ca/SFU/menus/Locations/Locations
Just over one hundred and fifty years ago, Maxwell posed a thought experiment now known as “Maxwell’s demon.” Designed to understand more deeply the nature of the newly formulated second law of thermodynamics, the demon was to play a long, controversial role in the development of statistical physics. Just two months later, Maxwell’s paper “On governors” gave the first analysis of a feedback system. These two foundational works reflect the fundamental and practical aspects of control. I will present an experiment that unites the two: using feedback to create “impossible” dynamics, we make a Maxwell demon that can reach the fundamental limits to control set by thermodynamics. We test—and then extend—Rolf Landauer’s 1961 prediction that information erasure requires at least as much work as can be extracted from a system by virtue of information. Using these ideas, we report the first experimental measurement of the functional form for the Gibbs-Shannon entropy function for a system out of thermal equilibrium.
Polydispersity can significantly affect the physics properties of polymers. In this study we show the differences in poly(ethylene oxide) oligomer crystallization between monodisperse and polydisperse samples as an example. Commercial polymers even with a low polydispersity index (PDI) of about 1.01, still contain a rather broad distribution of different molecular weights. In this study we produced highly monodisperse PEO through evaporative purification of a M$_{n}$ 600 PEO sample, achieving a PDI significantly lower than the as purchased sample, as measured by mass spectroscopy. Melting temperatures were obtained using differential scanning calorimetry (DSC). Based on the Gibbs Thomson relation, we show that during crystallization, some monodisperse samples form crystal lamellae with both extended chains and once-folded chains. In contrast, polydisperse samples have been shown to only contain extended chains in the crystal form. The fact that we are able to tune the melting temperature through annealing treatment on the crystal validates the existence of both folded and extended chains in the monodisperse samples.
Understanding the origin of plasticity in amorphous systems remains a challenging problem. The absence of long-range order prevents a clear definition of structural defects and therefore of their evolution during deformation. Furthermore, the interplay with temperature or the rate of deformation makes their dynamics difficult to describe.
However, for systems slowly deformed in athermal conditions, plastic flow occurs through local rearrangements of particles. These shear transformation zones (STZ) are not independent and their interactions lead to intermittent collective rearrangements in the form of avalanches [1].
Recently, a scaling description of the yielding transition based on the depinning transition has been proposed [2]. The predictions of this theory have been validated by mesoscopic elasto-plastic models and particle scale simulations in the steady-state flow. However, conflicting points of views have emerged regarding the phenomenology in the transient regime [2-4].
In this work, we investigate the emergence of plasticity by considering molecular simulations of a Lennard-Jones glass former. In the athermal quasistatic shear limit, we study in a complementary way deformations at the macroscopic scale and at the local scale by using the frozen matrix approach. In particular, the evolution of the scaling properties of stress and strain distributions with deformation will be investigated.
[1] M.L.Falk and J.S. Langer, PRE, 57(6):7192–7205, 1998
[2] J.Lin et al. PNAS, 40, 111, 2014
[3] K. M. Salerno and M. O. Robbins , PRE, 88(6):062206, 2013
[4] H.G.E Hentschel et al. PRE, 92(6) 062302, 2015
Over the last years we have developed a method to produce monodisperse oil droplets in an aqueous environment. By carefully tuning the adhesion forces between the droplets they provide model systems for studying various physical phenomena that are not accessible by investigating the molecular counterpart. Here I will present recent work on the transition from 2D crystalline aggregates, prepared from monodisperse droplets, to disordered glassy aggregates prepared from a bidisperse blend of large and small droplets. The aggregates are compressed between two parallel boundaries; crucially, one of the boundaries acts as a force sensor. The compression forces provide a signature of the aggregate composition and give insight into the energy landscape as the system transitions from the crystal to glass. In addition to the idealized 2D aggregates the same system of droplets can be used to investigate the formation of 3D clusters if the adhesion between droplets is sufficiently strong. The buoyant droplets accumulate underneath a glass slide which acts as the top of a liquid cell, forming 3D aggregates. Droplets initially arrange to form crystals growing along the vertical direction. As a critical height is reached, the aggregate collapses and spreads horizontally on the glass slide, in an event analogous to avalanches in sand piles. We find that the geometry of such clusters is controlled by the balance between the adhesion strength and buoyancy.
ISAC is the ISOL facility at TRIUMF where rare isotope beams for use in nuclear reaction investigations are created by bombarding solid targets with high energy particle beams. TRIUMF is enhancing its rare isotope production capabilities through a new scientific infrastructure known as the Advanced Rare IsotopE Laboratory (ARIEL). The low-energy transport section of the ARIEL expansion is known as CANREB (CANadian Rare-isotope facility with Electron-Beam ion source) which aims to increase the mass range of high purity beams available for further acceleration.
To accomplish this the cocktail of ions from the solid target will be first sent through a high resolution mass separator where most of the isobaric contaminations should be removed. The continuous beam of singly charged ions of interest will then be fed into an RFQ cooler buncher where pulses of typically $10^6$ ions per bunch will be created before being transported to a pulse drift tube where the energy will be lowered to match that of the trap of the electron beam ion source (EBIS). Inside the trap of the EBIS the ions will be confined radially by a 6 T Helmholtz magnetic field and longitudinally electrostatically. They will then be charge bred through electron impact ionisation via interaction with an electron beam with a density of up to 20 000 A cm$^{-2}$. It is required that a mass to charge ratio of less than 7 amu/e be reached within a repetition time of 10 ms to allow for operation with short lived isotopes. The highly charged ions extracted from the EBIS will then be traversed through a NIER spectrometer for mass selection before reaching the goal of post-acceleration.
One of the most prominent theories in describing nuclei is the nuclear shell model, which has accurately predicted many experimental trends in the atomic mass region of A=20-60 and beyond. One way to examine this model is by studying mirror nuclei-which have exchanged numbers of protons and neutrons. Differences between mirror nuclei's analogue nuclear levels and their decay patterns help in quantifying isospin symmetry in the nuclear force. Of particular interest is studying the neutron deficient $^{34}$Ar and compare its structure to its mirror nucleus $^{34}$S. Whilst $^{34}$S has been extensively studied, $^{34}$Ar has only been studied at low energies so far. In addition, shell model calculations of mirror energies for A=34 Ar-Cl-S seem to disagree with available experimental data.
Excited states were populated in the $^{34}$Ar nucleus by a fusion evaporation reaction at the Laboratori Nazionali Legnaro (LNL-INFN) in 2015 using a $^{12}$C beam to bombard a stationary $^{24}$Mg target, and by evaporation of two neutrons from the $^{36}$Ar compound nucleus. Other stronger channels were opened in this reaction which produce a high background with respect to the $^{34}$Ar spectrum. Gamma rays were detected by the GALILEO array composed of 25 HPGe detectors. The EUCLIDES charged particle detector associates charged particles being evaporated, whilst the neutron wall array tagged neutrons. Coincident events between the charged particles, neutrons, and gamma rays are used to create spectra of specific nuclei. Analysis of this experiment will serve to provide more experimental data for the shell model to better extend our understanding of nuclei in this mass region.
Shape coexistence in atomic nuclei, the existence of structures with different degrees of deformation in a very narrow energy range, is an exciting phenomenon that is present across the nuclidic chart. A recent theoretical study$^{[1]}$ using large scale shell model calculations predicted a well deformed prolate band at a low excitation energy in the doubly magic $^{78}$Ni nucleus which indicated the presence of shape coexistence very far from stability. At the same time, another experiment$^{[2]}$ observed an intruder 0$_{2}$$^{+}$ state in $^{80}$Ge and interpreted this as evidence of shape coexistence. In our $\beta$-decay experiment, we aim to perform comprehensive spectroscopy of the $^{80}$Ge and $^{82}$Ge nuclei and investigate the evolution of their respective intruder excited 0$_2$$^{+}$ states.
An experiment to search for shape coexistence in $^{80}$Ge and $^{82}$Ge has been performed at the ISAC-TRIUMF facility. The $^{80}$Ge and $^{82}$Ge isotopes were formed from the $\beta$-decay of their parent isotopes, $^{80}$Ga and $^{82}$Ga respectively. The $^{80}$Ga and $^{82}$Ga beams were produced by the ISOL technique using a 500 MeV proton beam with a 10 $\mu$A current colliding with a UC$_x$ target. A specialized ion source was used to suppress Rb contamination. The $\beta$-decay was measured using the GRIFFIN spectrometer which was equipped with 15 HPGe detectors for gamma ray detection, plastic scintillators for $\beta$-$\gamma$ tagging, the PACES array which has 5 Si(Li) detectors for conversion electron spectroscopy and 8 LaBr$_3$ scintillators for fast timing measurements of nuclear levels. Using this versatile array, correlated $\gamma$-$\gamma$, $\gamma$-electron and electron-electron data have been acquired simultaneously, providing a highly detailed level scheme for $^{80}$Ge. The preliminary results of this data analysis will be presented. \newline
[1] F. Nowacki, A. Poves, E. Caurier and B. Bounthong, PRL 117 272501 (2016).
[2] A. Gottardo et al., PRL 116, 182501 (2016).
Experimental and theoretical studies of the germanium isotopes point increasingly toward exotic combinations of nuclear-structure effects, with indications of triaxiality, configuration mixing, and shape coexistence. A simple two-state mixing model has been used to describe 0$^+$ states in the even-even $^{72,74,76}$Ge isotopes, and reasonable agreement with experimental data from Coulomb excitation measurements was found. More recent studies on $^{72}$Ge led to the conclusion that its low-spin structure could be satisfactorily described via an admixture of two triaxial rotors.
$^{76}$Ge is a promising candidate for the observation of neutrinoless double-beta decay, but theoretical models have not reached agreement on the value of the nuclear matrix element for the process. A proper experimental description of mixing is thus crucial to provide a benchmark for theoretical work and obtain an accurate knowledge of the wavefunctions of the initial and final states involved.
Detailed spectroscopic studies of the $^{72,74,76,78}$Ge isotopes using the GRIFFIN spectrometer at TRIUMF-ISAC are in progress with the goal of clarifying the structure of these nuclei and providing constraints for neutrinoless double-beta decay nuclear matrix element calculations. The nature of low-lying configurations in the germanium isotopes are probed through measurements of the inter-band E0 transition strength, a highly sensitive experimental observable. Indeed, E0 transitions are still poorly studied along the germanium isotopic chain, and only one E0 strength is known among the isotopes under investigation. The present studies combine electron and $\gamma$-ray spectroscopy. Initial results will be presented from data collected for the decay of $^{72}$Ga to $^{72}$Ge.
Although the nuclear shell model does well to predict the structure of spherical nuclei, its ability to describe deformed nuclei far from shell closures is lacking. Models attempting to describe the structure of deformed nuclei, both from the microscopic and macroscopic perspective, require information on the excited energy levels of these nuclei in order to improve their predicting power. Incomplete and disagreeing information on the beta decay scheme of $^{160}$Eu to $^{160}$Gd has been previously published [1,2], so a comprehensive picture of this decay is not yet available. Recently published data [3] improves the decay scheme, though there are still many unplaced levels and gamma-ray transitions, and information on beta-feeding intensities is lacking. At TRIUMF-ISAC, $^{160}$Eu beta decay data was collected using the GRIFFIN (Gamma-Ray Infrastructure For Fundamental Investigations of Nuclei) array of high-purity germanium detectors coupled with LaBr$_3$ fast-timing detectors and a conversion electron spectrometer. Over 10 million decays were recorded, allowing a comprehensive analysis of the beta decay of $^{160}$Eu. New results from the ongoing analysis of this decay will be presented.
[1] N. A. Morcos et al., J. Inorg. Nucl. Chem. 35, 3659 (1973).
[2] J. M. D’Auria et al., Can. J. Phys. 51, 686 (1973).
[3] D. Hartley et al., Phys Rev. Lett. 120, 182502 (2018).
The isotopes of tin are of great interest to the study of nuclear shell evolution, as they span from doubly magic $^{100}$Sn to $^{132}$Sn and beyond. Due to the highly stable closed shell of fifty protons, the even-even tin isotopes mid-shell between $N = 50$ and $N = 82$ are known to be spherical in their ground state. However, low-lying deformed states due to 2p-2h excitations across the closed proton shell are also observed and it is important to determine the degree of mixing between the deformed states and the "normal" states for theoretical models. The 2p-2h rotational band built on an excited 0$^{+}$ state has been observed in many studies on $^{118}$Sn. New measurements to further characterize this rotational band in $^{118}$Sn have been made using the Gamma Ray Infrastructure For Fundamental Investigations of Nuclei (GRIFFIN) at the TRIUMF-ISAC facility. GRIFFIN's powerful array of sixteen HPGe clover detectors provides excellent energy resolution and efficiency for identifying and separating low intensity gamma rays, and can be used in coincidence mode to place newly observed transitions. Discrepancies between two studies on the intensities of a $\approx$285~keV triplet have been identified using $\gamma$-$\gamma$ coincidence measurements. Most notably, the branching ratio of a 284.6~keV transition within the intruder band was measured to be half of the previously reported value, leading to a change in its B(E2; 2$^+_2$ $\rightarrow$ 0$^+_2$) value from 39(7) W.u. to 18(3) W.u.. Further to these findings, forty four newly observed transitions and one new energy level have been placed in the level scheme and will be discussed.
An overview of dark matter detection in the Milky Way galactic halo will be presented, including constraints and cutting-edge searches for dark matter's interactions with nuclei and electrons. The theory of non-relativistic dark matter interactions, connections with early universe cosmology, and the status of underground dark matter experiments will be addressed, along with some new detection methods, including high-recoil energy searches for inelastic dark matter and multiscatter searches for very heavy dark matter.
Dark matter search results and a detailed background model for DEAP-3600 will be presented. DEAP-3600 is searching for dark matter interactions with a target of liquid argon, shielded from cosmic rays by more than 2 km of rock at SNOLAB in Sudbury, Canada. The spherical detector consists of 3.3 tonnes of liquid argon in a large ultralow-background acrylic cryostat instrumented with 255 photomultiplier tubes. DEAP-3600 is sensitive to nuclear recoils from dark matter particles, which cause the emission of prompt scintillation light. Backgrounds come from alpha particles on the inner detector surfaces, from external neutrons, from argon-39 beta decays, and from trace radioactivity in detector components. This talk details the model for each of these backgrounds, and the analysis techniques that were used to reject them. The latest results from DEAP-3600 demonstrate excellent performance for pulse-shape discrimination, event reconstruction, background rejection and sensitivity to dark matter.
Cosmological and astrophysical observations indicate that the vast majority of the universe’s matter content is made out of dark matter.
Over the past decades, the physics community has largely focused on searching for dark matter within the 10 GeV-1 TeV mass range (WIMPs).
The absence of a discovery has motivated us to broaden our experimental search program and to look for lighter dark matter particles in the <10 GeV mass range.
The forthcoming SuperCDMS SNOLAB experiment will test a range of low mass dark matter models using cryogenic silicon and germanium detectors. In this talk, recent results and detector R&D efforts will be described, as well as the latest status and prospects for the new SuperCDMS SNOLAB effort.
The PICO collaboration uses bubble chambers to search for dark matter with results leading the world in sensitivity to the direct detection of WIMPs with spin-dependent coupling to protons. PICO recently operated a 32 litre bubble chamber (PICO-60) at the SNOLAB underground laboratory and the next generation experiment is currently taking commissioning data. This new device (PICO-40L) will prove the viability of the “right side up” design and demonstrate the possibility of scaling up to the next ton-scale experiment (PICO-500, currently being designed). A discussion of the technology, recent results, and future plans will be presented.
A joint session of the Quantum Materials and Optical Sciences Symposia to showcase research on strong correlations in cold atomic systems.
If a single spin in a spin chain is suddenly flipped, the information regarding the disturbance propagates outwards at a maximum speed given by the Lieb-Robinson bound. This results in a light-cone-like structure in the space-time dependence of correlation functions which has been observed in experiments with cold atoms and ions. In this talk I will show that these ``light cones’’ are examples of quantum caustics. These are discrete matter-wave versions of the caustics known in optics such as rainbows and the bright lines on the bottom of swimming pools. Caustics are classified by catastrophe theory which endows light cones with certain characteristic features: 1) structural stability; 2) discretized Airy and hyperbolic umbilic functional forms for, respectively, the wave- and correlation functions near the cone edges; 3) existence of a lattice of vortex-antivortex pairs inside the light cone. The vortices are sensitive to the quantum phase transition exhibited in spin chain models and their rate of production is determined by the dynamical critical exponent.
The exquisite control available in ultracold quantum gas experiments has enabled the emulation of many different quantum systems and allowed us to better understand the many-body states of quantum matter. Using the technique of artificial gauge fields, a spatially- and spin-dependent gauge potential $A_{\sigma}(\vec{r})$ can be engineered in a BEC, in which the kinetic energy is modified from the free-particle value to create spin-dependent artificial magnetic field, $B$. In the limit of strong-enough $B$, vortices should enter the system, but will have opposite circulation for each spin. This raises several questions: Can vortices of opposite rotation be introduced into the same condensate? Is the threshold for vortex nucleation the same as a single-component system? What are the stable spatial configurations of vortices? How do inter-spin interactions affect all of these? This talk will discuss the theoretical simulations of a spin-dependent artificial magnetic field, and discuss our progress towards realizing these gases using ultracold $^{87}$Rb atoms in our laboratory.
Dissipation of electrical current in typical metals is due to scattering off material defects and phonons. But what if the material were a perfect crystal, and sufficiently stiff or cold to eliminate phonons -- would conductivity become infinite? We realize an analogous scenario with atomic fermions in a cubic optical lattice, and measure conductivity. The equivalent of Ohm's law for neutral particles gives conductivity as the ratio of particle current to the strength of an applied force. Our measurements are at non-zero frequency (since a trapping potential prevents dc current flow), giving the low-frequency spectrum of real and imaginary conductivity. Since our atoms carry no charge, we measure particle currents with in-situ microscopy, with which both on- and off-diagonal response is visible. Sum rules are used to relate the observed conductivity to thermodynamic properties such as kinetic energy. We explore the effect of lattice depth, temperature, interaction strength, and atom number on conductivity. Using a relaxation-time approximation, we extract the transport time, i.e., the relaxation rate of current through collisions. Returning to the initial question, we demonstrate that fermion-fermion collisions damp current since the lattice breaks Galilean invariance.
There is a particular interest to improve the characterization of the bitterness of beer which is derived primarily from the addition of the annual flowers (cones) of the perennial climbing vine Humulus lupulus, better known as common hops. The hops cones contain important acids called humulones. During the brewing process, the humulones are thermally isomerized into isohumulones which are highly bitter and contribute to the beer’s flavour profile. Despite significant contributions to beer flavour and quality, the degree of humulone isomerization is typically not monitored in microbreweries, due in part to the lack of easy-to-use, cost-effective and on-site testing methods. Raman spectroscopy is being explored to 1) measure humulones in hops plants and processed hops pellets and 2) measure isohumulones and the resulting sensory bitterness in beer. Raman spectroscopy probes molecular vibrations and, as such, is an attractive analytical tool for the identification and quantification of specific molecules of interest in plants/food. A portable, hand-held Raman spectroscopy system (NanoRam, B&W Tek) with excitation at 785 nm is being tested. Preliminary data demonstrates the capability to measure humulones in the glands on the underside of hops leaves (Chinook variety) and in hops pellets with varying humulone concentrations. Raman spectra of wort samples, provided by a local microbrewery, exhibit a strong fluorescence background. However, by adding methanol to the wort, at increasing concentrations, Raman peaks associated with methanol’s C-O and O-H bonds shift to lower wavenumbers, indicating a change in the vibrational/rotational frequency modes of these bonds. It may therefore be possible to use these types of peak shifts to monitor the hops thermalization processes. The overall goal of this research is to develop a field-ready, cost-effective Raman spectroscopy technique for monitoring and optimizing beer flavour and quality.
Collagen, the most abundant protein in mammalian organisms, is responsible for the cohesion of tissues and organs. It is a major structural component of our extracellular matrix, contributing to the mechanical stability, organization, and shape of a wide variety of tissues. Many collagen types have been reported in humans, all of which are triple-helical proteins that assemble into distinct higher-order organizational structures. To date, there has been greater focus on characterizing the more abundant fibrillar collagen types, leaving the mechanical properties of network-forming collagen type IV comparatively understudied. A key feature that differentiates fibrillar collagens from collagen IV lies in the characteristic triple-helical defining (Gly-X-Y)n sequence of the collagenous domain, where collagen IV has intrinsic discontinuities in its sequence. The role and structure of these interrupted Gly-X-Y regions remains unknown; however, it has been suggested that they play a role in the flexibility of the collagen molecule. To address this question, we used atomic force microscopy (AFM) to sample the two-dimensional conformations adopted by collagen on mica and performed statistical analysis to calculate its persistence length – a mechanical property that is used to quantify the flexibility of a polymer. By assuming homogeneous flexibility across the length of the molecule, we found the persistence length of collagen IV to be less than half of that of the continuously triple-helical fibrillar collagens. To investigate local sequence variations, we developed an algorithm that determines position-dependent persistence length profiles. We found significant variations in persistence length along the contour of the collagen IV molecule, where regions of higher flexibility correlated strongly with interrupted Gly-X-Y regions in its amino acid sequence.
High atomic number nanomaterials have been explored as a tool for improving cancer therapeutics. Gold nanoparticles are a system that has been introduced as they can act as effective radiation dose enhancers and anticancer drug carries. Gold nanoparticles have unique physiochemical properties that allow them to be probed in cells using techniques such as scanning electron microscopy and hyper spectral imaging. Optimization of gold nanoparticle uptake into 3D in-vitro models is essential to optimizing future cancer therapeutic applications and bridging the gap between in-vitro and in-vivo tumour environments. The optimization of the uptake of functionalized gold nanoparticles into 2D monolayer and 3D spheroid cell models was tested. Our initial findings reveal that smaller gold nanoparticles penetrate better vs larger gold nanoparticles within the 3D tumor models. This correlates very well with our recent in-vivo data. Hence, development of these 3D tissue models can be utilized to mimic tumor microenvironments in the lab, which would accelerate the use of gold nanoparticles in future cancer treatment.
In Duchenne muscular dystrophy (MD) patients, muscle fiber Na+-loading is linked to cytotoxic swelling. Quiescent fibers move away from their resting membrane potential Vrest (and towards Donnan equilibrium) due to concurrently open Na+ and Cl- conductances that together underlie the swelling and depolarization. Since a chloride conductance (ClC-1) underlies healthy Vrest in fibers, whatever underlies the Na+-loading is the pathologic conductance. Indeed, increased Na/K pump activity counteracts swelling and promotes repolarization (see review by Morris (Curr Topics Memb 81, 2018)). The classic animal model for Duchenne MD is the dystrophin-less mouse, mdx. Wild-type muscle fibers are better able to resist mechanically-induced (pipette aspiration) sarcolemmal blebbing than mdx fibers and they more effectively use caveolar tension buffering to maintain sarcolemmal integrity. In 3-day survival experiments, isolated fiber survival is markedly worse in mdx than in wild-type. The Na+ channel blocker Tetrodotoxin provides full protection for mdx fibers, indicating that the mdx fibers have “leaky” Nav1.4 channels. We postulate that it is specifically the Nav1.4 channels resident in bleb-damaged areas of dystrophic sarcolemma that would exhibit left-shifted operation. The result – an increased gNa(V) window conductance at abnormally hyperpolarized voltages -- would continually stress ion homeostatic processes, making it likely that cytotoxic swelling would eventually become evident. We built an MD muscle cell model including ion concentrations, conductances, pumps, volume, membrane potential and tensions. Damage is represented by a coupled left shift (CLS) of the activation and inactivation kinetics of a fraction of the Nav channels. The model predicts cytotoxic swelling with anoxia (no pump activity). It shows the tolerance of the muscle cell to damage, with no swelling until a critical state. It explains also how increased pump activity can keep the swelling under control.
One of the outstanding questions in protein folding is why the folding into some native state topologies, e.g. all-$\alpha$ folds, are more sensitive to sequence variations than other, more nonlocal protein folds, such as all-$\beta$ folds? To explore this question, we design and study three 35-54 amino acid sequences within a coarse-grained sequence-based model and show that they fold spontaneously into stable 3$\alpha$, 4$\beta+\alpha$ and $\beta$-barrel folds, respectively. Their thermodynamic behaviors, calculated using Monte Carlo techniques, exhibit features in line with experimental data including rank order of folding cooperativity and temperature-driven Hammond shifts of transition states. Using a novel generalized ensemble algorithm (A. Aina and S. Wallin, Journal of Chemical Physics 147, 095102, 2017) we then systematically study the effect of single- and double-point mutations on each of the three proteins. In total, $>$2,000 mutants are studied. We find that the proteins respond to sequence variations in a topology-dependent manner. In particular, the folding landscape of the $\beta$-barrel protein is the least perturbed of the three proteins, explaining previously observed mutational robustness of non-local folds. Moreover, we observe a link between the size of conformational fluctuations of these proteins and the divergence exhibited by their respective mutants. One consequence of such a link is that proteins with diverse folding pathways might be more sensitive to sequence variations than proteins with restricted folding pathways.
Lattice Monte Carlo (LMC) methods are frequently used to model drug delivery systems. In a typical case, the drug, initially encapsulated in a porous material (e.g., a hydrogel), is released via two processes: (i) disintegration of the material and (ii) diffusive escape. One way to control diffusive release is to design layered materials with regions that have different porosities. However, modelling inhomogeneous systems where both the porosity and the diffusivity are space-dependent is ambiguous in LMC simulations. In this talk, we examine two fundamental issues: 1) how to replace connected regions with different porosities by free-solution regions of different effective viscosities; 2) how to treat the LMC jumps between regions with different effective viscosities. We present computational and theoretical studies of 2D systems consisting in two different sets of immobile obstacles that create two media with different effective viscosities, as well as their equivalent obstacle-free 1D systems with effective diffusion coefficients. Using this toy model, we examine how interfacial diffusion is treated by the various flavours of stochastic calculi, and we demonstrate that Isothermal calculus is the correct choice as opposed to the generally employed Ito calculus. We then explore the corrections that must be considered to simplify such inhomogeneous systems while conserving the proper static and dynamic properties of the original system.
Collagen is the most abundant protein in vertebrates and plays a crucial role for the integrity of stress-bearing tissues, such as tendon and cartilage. All collagen types consist of three α chains, which are coiled about each other into a right-handed triple helix. It is well known that in vivo collagens’ mechanical properties provide stability and structure to tissues under a wide range of forces. However, a clear understanding of collagen’s mechanics on the molecular level is still missing. Force-dependent collagen experiments at the molecular level have generally employed magnetic or optical tweezers or atomic force microscopy, costly techniques that are generally low-throughput. The centrifuge force microscope (CFM) that we use for force-dependent structural studies helps to surmount these issues [1]. The CFM enables high-throughput single-molecule stretching experiments for force-dependent structural studies on an ensemble of objects using centrifugal force. Our system consists of a miniature light microscope mounted within a rotating device like a centrifuge bucket, with temperature control, live video, long run times (> 2 hours) and a low build cost (< $1000) . Our approach is answering the central question of how an applied force affects the quaternary structure of collagen. Unraveling the mechanical properties of collagen will then help to determine structural and mechanical aberrations associated with collagenopathies and to elucidate how mutations may change collagens’ behaviour under force.
[1] Kirkness MWH, Forde NR. Single-Molecule Assay for Proteolytic Susceptibility: Force-Induced Collagen Destabilization. Biophys J. 2018;114(3):570-576.
Stationary black hole solutions of classical general relativity satisfy certain identities relating their mass, angular momenta, and charge which are in close analogy with familiar thermodynamic laws. In this talk I will describe new identities for the physical variables of five-dimensional, asymptotically flat, stationary vacuum black holes. Unlike the well known Smarr relation, these identities depend on the topology of the black hole spacetime. The proof employs the harmonic map (sigma model) formulation of the vacuum Einstein equations for solutions in the presence of symmetries.
We discuss gravitational waves interacting with scalar particles and neutrinos. We discuss the physical implications of the results for detection on earth and early universe cosmology.
I describe results that show how black holes affect the entanglement of qubits. Making use of an approach called entanglement harvesting, which is an operational way to transfer correlations from the quantum vacuum to (idealized) detectors, I show how quantum correlations are affected by the structure of spacetime. This becomes particularly interesting when the spacetime has curvature, and even more so if a black hole is present. A variety of new phenomena occur, including asymmetric time-shift effects, forbidden entanglement islands in parameter space, entanglement inhibition near event horizons, and sensitivity to topology. I shall provide an overview of an ongoing research program in this area along with a description of these and other novel results.
We demonstrate the existence of stable, non-singular, negative mass solutions in de Sitter spacetime. The solutions correspond to the exact negative mass Schwarzschild-de Sitter spacetime on the outside of a thin, spherical wall and a smooth mass distribution on the inside, that is non-singular at the origin. The solution satisfies the Israel junction conditions at the location of the wall. The mass distribution satisfies the dominant energy condition everywhere, hence can represent physical matter. The central point of our analysis is that our construction of the effective potential for the location of the wall admits a stable minimum for the position of the location of the wall. Such negative mass bubbles could be important in understanding the formation and existence of voids in the universe.
A lot can be learned about spacetime by rescaling it à la Weyl. The familiar analogy between the laws of black hole mechanics and thermodynamics becomes ambiguous under such a rescaling. The deep reason for such an issue goes back to the fundamental dichotomy between matter and geometry inherent in Einstein equations. This dichotomy is actually so fundamental that the issue extends beyond black hole thermodynamics, affecting the more general theme of spacetime thermodynamics.
Black holes in a rotating and expanding space-time
In a previous paper [1], we have studied in detail the static non-empty symmetric geometry described by a metric based on an erfc gravitational potential. Although this new metric provides a consistent set of predictions and interpretations regarding some open problems in the solar system [2], the residual static offset incorporated in the erfc potential can also be reinterpreted in a dynamic context. In this paper, we propose, a mathematical equivalence to interpret and provide a plausible dynamic meaning for that constant scalar field. This can be done in two steps. First, the metric can be rewritten in an algebraically equivalent erf symmetric form. Then, interpreting the constant time offset into a spacetime rotation and the constant radial offset into a spacetime expansion, the static metric can be converted into a dynamic one. In other words, the problem is solved by proposing a more general axisymmetric geometry which describes a rotating massive body dragging its curved spacetime into a rotation and an expansion. The resulting length element is not invariant under time and azimuth coordinate transformations, which leads to revisit some basic properties of a rotating black hole dragging its spacetime in four dimensions. Although this asymmetric metric does not have any intrinsic singularity, it can give rise to non-singular black holes with a double horizon.
[1] Plamondon, R., (2018), General Relativity: an erfc metric, Results in Physics, 9, 456-462.
[2] Plamondon, R., (2017) Solar System Anomalies: Revisiting Hubble’s law, Physics Essays, 30(4), 403-411.
Nuclear Pore Complex (NPC) is a biomolecular “nanomachine” that controls nucleocytoplasmic transport in eukaryotic cells. The key component of the functional architecture of the NPC is the assembly of the polymer-like intrinsically disordered proteins that line its passageway and play a central role in the NPC transport mechanism. Due to paucity of experimental methods capable to directly probe the morphology and the dynamics of this assembly in intact NPCs, much of our knowledge about its properties derives from in vitro experiments interpreted through theoretical and computational modeling. Remarkably, despite their molecular complexity, much of the behavior of these assemblies and their selective permeability with respect to cargo-carrying transport proteins can be understood based on minimal complexity models relying on the statistical physics of molecular assemblies on the nanoscale.
Due to the unstructured nature of the proteins in the NPC passageway, it does not possess a molecular “gate” that transitions from an open to a closed state during translocation of individual cargoes. Rather, its passageway simultaneously contains multiple transport proteins carrying different cargoes in both directions. Although this feature increases NPC throughput, it remains unclear how the NPC maintains selective and efficient bi-directional transport under such crowded conditions. I will present of a coarse-grained computational model of the NPC transport and will discuss various proposed solutions to the crowding problem in light of the model results and the available experimental data.
We describe a simple Monte Carlo simulation method to calculate the free energy cost of localizing a single monomer of a polymer confined to a cavity for a localization site on the inside surface of the confining cavity. The method is applied to a freely-jointed hard-sphere polymer chain confined to cavities of spherical and cubic geometries. In the latter case we consider localization at both the corners and the centers of the faces of the confining cube. We consider cases of end-monomer localization both with and without tethering of the other end-monomer to a point on the surface. We also examine localization of monomers at arbitrary position along the contour of the polymer. We characterize the dependence of the free energy on the cavity size and shape, the localization
position, and the polymer length. The quantitative trends can be understood using standard scaling
arguments and use of a simple theoretical model. The relevance of these results to the initial
stages of polymer translocation through nanopores and polymer dynamics in porous environments
is discussed.
The liquid-liquid phase separation in biological systems has recently attracted intense interest in molecular biology, biophysics, and polymer sciences. Intrinsically disordered proteins (IDPs), proteins that do not fold into a unique structure when isolation because of the depletion in hydrophobicity and the abundance of polar, charged, and aromatic residues, have been discovered to undergo liquid-liquid phase separation in the cell, constructing various intracellular membraneless organelles with distinct biomolecular compositions and biological functions. Based on the sequence-function principle in molecular and structural biology, the phase behavior of a charged IDP must be determined by its unique amino acid sequence, the charge distribution on which is not necessarily fixed but can also be modified by phosphorylation or change of pH value. To understand phase separation in biology, we develop a sequence-specific random phase approximation (RPA) theory for charged IDPs. In addition, we take into account the sequence-specific structure factor of charged polymers by utilizing a variation principle to approximate the IDP by a Gaussian chain with an effective Kuhn length. We apply the theory to both polyelectrolytic biopolymers with large net charges and nearly-neutral polyampholytic IDPs with almost equal number of opposite charges, and obtain theoretical predictions consistent with experimental observations. Phase behavior under the influence of salt and counter ions is also investigated. The augmented renormalized-Gaussian RPA theory, termed rG-RPA, is a general theory for polymers with arbitrary charge sequences, providing a concise theoretical framework for not only studying sequence specificity in individual biopolymer systems but also conducting high-throughput sequence analysis.
We report a combined experimental, theoretical, and simulation study of the phase behavior and microstructural dynamics of concentrated binary mixtures of spherical nanocolloids with a size ratio near two and with a tunable, intrinsic short-range attraction. In the absence of the attraction, the suspensions behave as well mixed, hard-sphere liquids. For sufficiently strong attraction, the suspensions undergo a gel transition. Rheometry measurements show that the fluid-gel boundary of the mixtures does not follow an ideal mixing law, but rather the gel state is stable at weaker interparticle attraction in the mixtures than in the corresponding monodisperse suspensions. X-ray photon correlation spectroscopy measurements reveal that, in contrast with depletion-driven gelation at larger size ratio, gel formation in the mixtures coincides with dynamic arrest of the smaller colloids while the larger colloids remain mobile. Molecular dynamics simulations of the suspensions similarly observe gel formation driven by a structural arrest of the smaller particles that occurs at weaker strength of attraction than the gel point of the monodisperse suspensions. Characterization of the microstructure n the simulations indicates the arrest results from microphase separation that results from a subtle interplay of entropic and enthalpic effects and that drives the smaller particles to form dense regions. These observations suggest a potential new avenue for tailoring the gel-forming properties of colloidal suspensions.
Living systems at the molecular scale are complex (composed of many constituents with strong and heterogeneous interactions), far from equilibrium, and subject to strong fluctuations. Energy conversion in such contexts would seem to be challenging, yet nature has evolved numerous molecular machines that efficiently, precisely, and rapidly transduce free energy between non-equilibrium reservoirs. Here we discuss design principles for effective free energy transduction in a simple model of FoF1 ATP synthase, a rotary motor that is crucial for virtually all living things. In particular, we discuss the interplay between non-equilibrium driving forces, natural equilibrium fluctuations, and interactions between the strongly coupled sub-systems that comprise the full machine.
The Gamma-Ray Infrastructure For Fundamental Investigations of Nuclei (GRIFFIN) at TRIUMF-ISAC provides unique opportunities for decay studies with rare-isotope beams. GRIFFIN is located in the ISAC-I facility at TRIUMF and comprises 16 high-efficiency Compton-suppressed HPGe clover detectors. GRIFFIN is complemented by a suite of ancillary detector systems that allow for comprehensive studies of the decays of rare isotopes in order to investigate nuclear structure, nuclear astrophysics and perform tests of fundamental symmetries. An overview of the GRIFFIN spectrometer, along with selected highlights from the GRIFFIN research program will be presented.
The controlled collimation of ion beams is of paramount importance in particle accelerators, high energy beamlines, and detector systems, as it determines the sensitivity and resolution of the instruments. This is especially the case when dealing with radioactive ion beams, where high transportation yields are crucial due to the short lifetimes of certain nuclei. For this reason, it is essential to model the beam dynamics in order to optimise properties such as transmission and energy spread before the commissioning of new instruments. A plasma ion source (PIS), comprised of a heated filament followed by an anode, Einzel lens and X-Y correction steerers, was modelled and Monte Carlo simulations were run using SIMION. The simulations also included the integration of this beam into the existing beamline of TRIUMF’s Ion Trap for Atomic and Nuclear science (TITAN). Optimising the voltage configurations in these simulations has proved successful in the commissioning of the PIS at the TITAN experiment, which will provide a versatile ion source, capable of delivering stable isotopes from gas and solid samples. The PIS will be able to deliver important calibration beam to TITAN’s experiments and will also enable off-line, high precision isotope composition measurements with the Multiple-Reflection Time-Of-Flight Mass Spectrometer (MR-TOF MS).
Gamma ray spectroscopy in the Nuclear Science Laboratory (NSL) at Simon Fraser University (SFU) is used for nuclear structure studies, neutron activation analysis, and environmental radioactivity monitoring. The current detection system is the Germanium detector for Elemental Analysis and Radioactivity Studies (GEARS), and consists of a single high purity germanium (HPGe) detector which is housed in a lead box for passive shielding. Sensitivity is limited especially at low energies due to background radiation and Compton scattering. The detection capabilities of the NSL can be improved through the use of Compton suppression and time coincidence measurements. The time coincidence method allows for the possibility of gamma-gamma, beta-gamma, and alpha-gamma measurements that will help distinguish between events of interest, and background radiation or events caused by contaminant induced reactions. However a multi-detector system is required to take advantage of this method.
The 8-pi spectrometer, recently acquired by SFU from the ISAC-I facility at TRIUMF, is a set of 20 HPGe detectors equipped with BGO Compton suppressors arranged in a spherical array. The 8-pi is being rebuilt to its original design of a high multiplicity spectrometer to be used in fission studies. All photomultiplier tubes and 18 out 25 HPGe are currently operational. HPGe detectors operate at liquid nitrogen (LN2) temperatures and the required cooling system is being expanded from GEARS to support the 8-Pi. Implementation of the data acquisition (DAQ) system poses a greater challenge as it requires 332 channels for the BGO photomultipliers plus 20 channels for the HPGe detectors in order to operate the full array. A DAQ based on the TIG-10 and VF-48 digitizers is under development.
In the interim, a subset of six 8-Pi Compton Suppressed Spectrometers (CSS) will be arranged in a cubic array for the purpose of time coincidence measurements. A frame has been designed and will be constructed by the SFU machine shop. The HPGe detectors will be cooled using the LN2 cooling system intended for the full 8-pi array. The High Voltage distribution for the CSS shield photomultipliers built for the 8-pi will be used. Additionally NIM/CAMAC-based DAQ can be adapted from the existing GEARS system to allow operation of six CSS for the cube. Progress towards completion of this array will be presented and discussed.
The study of neutron rich nuclei far from the valley of stability has become an increasingly important field of research. One of the decay mechanisms that opens when the decay $Q$ value becomes sufficiently large is that of $\beta$-delayed neutron emission and the probability of its occurance, $P_n$, is important when studying the astrophysical r-process [1]. $\beta$-delayed neutron spectroscopy can also provide structural information being sensitive to the angular momentum in the process and the final state wave functions. The utilization of large scale neutron detector arrays in future experiments is imperative for these kinds of studies. One of such array is DESCANT (DEuterated Scintillator Array for Neutron Tagging) [2], designed to be coupled with the large-scale gamma spectrometers GRIFFIN [3] and TIGRESS [4] at the TRIUMF-ISAC and ISACII facilities, allowing for the study of both beta decay and reaction experiments, respectively.
The initial design of DESCANT was for the intended use as a neutron-tagging detector for fusion evaporation reactions [2] and thus obtaining precision on the neutron energy was not considered a priority. This limitation could be overcome through the use of thin plastic scintillators position in front of the DESCANT detectors. The energy of the neutrons can then be determined via the time of flight technique, similar to that implemented in the VANDLE array [5]. To investigate the viability of this augmentation, GEANT4 will be used to simulate and optimize the experimental design. A series of tests will also be performed using a single DESCANT detector and a plastic scintillator to verify the improvement in performance. The progress of both the simulations and the tests using the DESCANT detector will be discussed.
[1] Mumpower, M. et al. Prog Part Nucl Phys 86, 86 (2016).
[2] Garrett, P.E., et al. Hyperfine Interact (2014) 225: 137.
[3] Svensson, C.E., Garnsworthy, A.B. Hyperfine Interact (2014) 225:127-132.
[4] Svensson, C.E. et al. J. Phys. G: Nucl. Part. Phys. 31 S1663.
[5] Peters, W.A., et al. Nucl Instrum Methods Phys Res A 836 (2016) 122–133.
One of the recently developed approaches capable of describing both bound and scattering states in light nuclei simultaneously is the No-Core Shell Model with Continuum (NCSMC). This technique represents a state-of-the-art ab initio approach and combines the No-Core Shell Model (NCSM) description of short-range correlations with the clustering and scattering properties of the Resonating Group Method. Recent NCSMC calculations of $^7$Be and $^7$Li will be presented. The properties of these nuclei were investigated by analyzing the continuum of all the binary mass partitions involved in the creation of these systems, using chiral interactions as the only input. Our calculations reproduce all the experimentally known states in the correct order and predict new possible resonances with negative and positive parity. A positive-parity S wave resonance is found analyzing the continuum of p + $^6$He at a very low energy above the threshold, which produces a very pronounced peak in the astrophysical S factor of the $^6$He(p,$\gamma$)$^7$Li radiative capture. Possible implications for astrophysics have still to be investigated.
Low levels of cardiorespiratory fitness (CRF) are associated with high risk of cardiovascular disease (CVD) and all-cause mortality. Maximal oxygen consumption (VO2max) has been considered as a gold standard for determining CRF, which can be precisely measured by using the cardiopulmonary exercise testing (CPX) device with a maximal exercise protocol. However, this traditional methodology can be unavailable due to its high cost and risk. VO2max itself is not complete solution for CRF evaluation as it shows the level of CRF only. What limits the level of CRF and how to make improvement efficiently are not answered by VO2max. Recently, an alternative approach has been raised by researchers to evaluate VO2max by submaximal test based on the heart rate (HR). Compared with the maximal exercise based method, this submaximal test approach is more applicable and much safer, but at an expense of the VO2max estimation accuracy. Motivated by this, our group has proposed a novel method, which adds a new personalized performance related metric-muscle oxygen saturation (SmO2) that can be measured by a portage Near infrared spectroscopy (NIRS) device, to further improve the submaximal test accuracy and show the physical limitations that caused low levels of CRF. Furthermore, we also propose an adjustment algorithm to evaluate CRF with machine learning approach. Our test results show that the proposed adjustment algorithm improves the VO2max estimation in submaximal test and indicates the physical limitation for each subject at their own level of CRF, which thus demonstrates the feasibility of using SmO2 for CRF evaluation. To our best knowledge, our group is the first to propose this idea, which will be of great significance to clinical applications.
In-source laser resonance ionization spectroscopy aims to drastically increase the resonant laser ionization efficiency for element species. This can be achieved by finding auto-ionizing (AI) states suitable for laser resonance ionization. Isotope separator on-line (ISOL) facilities such as the isotope separator and accelerator (ISAC) at TRIUMF produce a wide variety of exotic radio-isotopes for experiments on nuclear structure, astrophysical processes and material sciences. These facilities are ideally suited for fundamental research on nuclear structure but also are the place to perform atomic spectroscopy in search of energy levels of radioactive elements. The study of atomic transitions along long chains of isotopes of a particular element allows also to study the evolution of nuclear charge radii as a function of neutron number via the hyperfine interaction.
The TRIUMF resonance ionization laser ion source (TRILIS) is an efficient tool to provide pure ion beams using the element selective resonance ionization process and an instrument for ultra-sensitive atomic spectroscopy by means of laser resonance ionization. .
For laser ionization scheme development on stable isotopes and ion source development, an off-line test stand was built and is being operated in parallel to the on-line laser ion source. This allows to conduct the required instrumentation developments for forefront experiments with rare isotopes in the service of nuclear physics experiments.
The Canadian Light Source 2.9 GeV electron synchrotron storage ring circulates 220 mA of beam current distributed over 265 bunches. These bunches travel through several in-vacuum undulators, large magnet arrays housed in tapered vacuum chambers with a complex internal geometry. The electrons traveling through these vacuum chambers carry electromagnetic fields whose distribution is determined by the geometry and material properties of the boundary. Electromagnetic wake fields are induced when the electron travel through a boundary condition transition. The wake fields of a lead particle can perturb the position and momentum of following particles or they may excite long lived but undesirable resonant modes in cavity-like vacuum chambers. Either case may drive beam instabilities. We have modeled these trapped modes with finite difference time domain codes. The results of these simulations, including the frequency and amplitude of these modes, are predicted in this report.
The main objective of the ALPHA-g experiment is to determine the behavior of the anti-matter when it is subjected to the Earth’s gravitational field. At the center of this experimental setup, anti-hydrogen atoms are created and trapped in the magnetic field produced by an octupole magnet. Once the magnetic field is lowered, these anti-atoms are released and move in accordance with gravity. Annihilation occurs when anti-atoms meet the walls of the trap. This annihilation produces pions, which are tracked by the second layer of the experiment: a radial Time Projection Chamber (rTPC) filled with an Argon/CO2 mixture. This first detector determine the path of the pions, and thus the original position of the annihilations.
The rTPC is sensitive to other energetic particles coming from outside of the experiment, like muons produced by cosmic rays which cross the rTPC at a significant rate. In order to distinguish the tracks produced by these particles from the annihilation products, a barrel scintillator has been installed around the rTPC to act as a veto.
This second detector is constituted by 64 plastics scintillator bars. Each bar has a length of 2.6m and a trapezoidal cross-section. When a particle crosses this barrel, scintillation light is produced by the scintillator and propagates up to the ends of the bar where Silicon Photomultipliers (SiPM) transform it into an electrical signal. This signal is then processed by the electronic read-out to determine the arrival time of the light on the SiPM, and the amount of photons received. This information allows us to determine when and where a particle has interacted with a bar, and thus if it comes from inside (annihilation) or outside of the barrel.
The design and realization of this scintillator apparatus were punctuated by several challenges. The space constraints were high and the barrel had to be self-supported without external structure. To ensure a proper rejection of the external particles, the read-out electronics had to be optimized to obtain a time resolution of the order of 200 ps.
This barrel scintillator was designed and assembled at TRIUMF, before being moved at CERN for its commissioning in September 2018. After being integrated to the rest of the experiment, the whole set-up was tested with antiproton annihilations.
The ALPHA-g experiment at CERN aims to undertake the first ever precision measurement of the gravitational mass of antihydrogen atoms, by releasing them from a gravity-compensated magnetic minimum trap. The magnetic minimum trap is nominally created by an octupole and two mirror coils. However, in order to achieve the level of magnetic control required to resolve gravity to the 1% level and beyond, many other magnetic elements are involved to tailor and correct the trap field to the O(10^-5) level, as well as to manipulate and cool the anti-atoms before the measurement. In this talk, we present the construction of the sophisticated, multi-purpose superconducting magnet system designed for this purpose, which composes of 24 coils, 5 overlapping octupole elements and a high-uniformity large volume solenoid. We will also showcase the simulated behaviour of the anti-atoms inside the magnet system, as well as the predicted gravity precision we may achieve under different experimental conditions.
Planned for the High-Luminosity LHC, the ITK (Inner TracKer) is the upgrade of the current inner detector of the ATLAS apparatus at the Large Hadron Collider (LHC). The ITK is a cylindrically-shaped, all-silicon semi-conductor charged particle tracker, with the ability to run at high rate in a high luminosity and high radiation environment. The ITK is comprised of two different technologies, an inner Pixel detector and an outer Strip detector. To make sure all 20 000 Strip modules are built with a high level of quality, a series of tests are performed for quality control (QC) and quality assurance (QA) of the production. These tests are crucial because once the ITK is installed in the underground cavern it will not be accessible for approximately 10 years. One of the tests performed as part of QA and QC is the thermal cycling, where modules are monitored while being cooled down and heated up in cycles to temperatures relative to potential operating conditions. These tests will occur in an environmentally controlled enclosure able to hold 4-5 modules simultaneously, known as a cold box. The subject of this work is to study the effect of thermal cycling on future ITK Strip R0 detector modules in terms of induced mechanical stress, and thus inform the design of the cold box. This was performed using a Finite Element Analysis simulation of the thermal cycling process. The first result is that the stress created in the module is mostly due to the vacuum applied to keep everything in place. Moreover, the maximum stress felt during thermal cycling is highly dependent on the thickness of the vacuum seal used: A thinner seal causes a lower stress. Finally, the stress in the module in our thermal cycling setup is between $\sim 20$ MPa and $\sim 100$ MPa depending on the selected seal thickness, which is consistent with the stress expected in the final detector design. We can then conclude that our proposed design for the cold box is a good candidate to perform the thermal cycling test.
Plasma is being exploited by industry to increase the wettability and adhesion of coatings to plastic automotive components, and to enhance the durability of printing inks on polymers and metals. These commercial applications of plasma treatments encouraged research on the plasma modification wood. The research mainly focused on the use of plasma to improve the adhesion and performance of glues and coatings on wood; plasma coating of wood to increase its hydrophobicity; plasma modification of wood fibres to improve their compatibility with polymers. Our own research on plasma modification began over 20 years ago with a study of the use of plasma treatments to improve the glue bonding of high-density eucalyptus species. We were able to improve the glue bonding of difficult-to-glue eucalyptus species, but improvements appeared to be related to modification of the microstructure of wood rather due to any changes in wetting characteristics of wood. A more recent study also found that modification of the microstructure of wood accounted for improvements in performance of polyurethane coatings on spruce wood. These findings led us to examine the effects of plasma on the microstructure and chemical composition of wood. We observed that the microstructure and chemical composition of wood surfaces is profoundly altered by plasma treatments. Plasma causes differential etching of wood’s cellular components, at the surface and also in sub-surface layers. Differential etching of wood’s major chemical components, cellulose, hemicelluloses and lignin also occurs. Cellulose is more susceptible to etching than lignin, and, as a result, the lignin-rich layers of wood cell walls are revealed at surfaces subjected to prolonged plasma etching. Plasma etching of wood can be rapid, depending on wood species and treatment parameters, which has opened up the exciting possibility of using plasma to ‘machine’ wood surfaces and create tailored microstructures with various interesting commercial applications.
During the 1970's a hot controversy emerged between in-situ measurements of the electron temperature in the ionosphere and ground measurements by incoherent backscatter radars. We suggest a possible explanation to this controversy. We define the ``local'' temperature of ionized species by the variance of the square root of the energy, and here by:
$$ \frac{3}{2} n_\infty kT(\mathbf{r}) = \!\! \int_{(-2q\phi(\mathbf{r})/m)^{1/2}}^{+\infty} \!\! \left( q \phi(\mathbf{r})+\frac{1}{2} mv^2 \right) \left( \frac{m}{2\pi kT} \right)^{3/2} e^{-\left[ q\phi(\mathbf{r})+\frac{1}{2} mv^2\right] /kT(\mathbf{r})} \,\mathrm{d}^3\mathbf{v} . $$
Here $n_\infty$ is the ambient density of the ionized gas, $k$ the Boltzmann constant, $T$ the temperature, $q$ the charge, $\phi$ the electric potential at a point $\mathbf{r}$ and $m$ is the mass.
For repelled species, this equation has an analytical solution, and we obtain:
$$\frac{3}{2}kT(\mathbf{r}) = e^{-q\phi(\mathbf{r})/kT(\mathbf{r})} \left[ q\phi(\mathbf{r})+\frac{3}{2}kT(\mathbf{r}) \right] ,$$
with the conditions:
$$\lim_{\mathbf{r}\to+\infty} \phi(\mathbf{r}) = 0;\ \ \lim_{\mathbf{r}\to+\infty} T(\mathbf{r}) = T_\infty;\ \ \lim_{\mathbf{r}\to+\infty} n(\mathbf{r}) = n_\infty .$$
If the electric potential vanishes, the local temperatures of ionized species go towards their ambient temperatures. The theoretical implication is that the distribution function $f(E,T(\mathbf{r}))$ is not a constant along a trajectory in the phase space, and that the Vlasov equation is violated.
Like Laframboise and Parker (1973), we shall consider an ionized gas without collisions and the ideal case of a potential well with a spherical symmetry, but there is no physical body. We define the number density as:
$$ n = \int f\,\mathrm{d}^3\mathbf{v} = n_\infty \left( \frac{m}{2\pi kT(r)} \right)^{\!\!3/2} \!\! \exp\left( \frac{-q\phi(r)}{kT(r)} \right) \times \!\!\!\! \int_{(-2q\phi(r)/m)^{1/2}}^{+\infty} \!\!\!\! e^{-mv^2/2kT(r)}\ 4\pi v^2 \,\mathrm{d} v . $$
For repelled species; we generalize Boltzmann's formula:
$$ n(r) = n_\infty \exp\left[ \frac{-q\phi(r)}{kT(\phi(r),r)} \right] . $$
We now calculate particles fluxes for attracted and repelled species. This will be a generalisation of Laframboise and Parker's (1973) formulas:
$$ J = \int fv_r\,\mathrm{d}^3\mathbf{v} = J_0 \left( 1 - \frac{q\phi(a)}{kT(\phi(a),a)} \right) , $$
where $J_0$ is the thermal flux and $a$ the radius of a spherical probe. For the repelled species, we have:
$$ J = J_0 \exp \left[ \frac{-q\phi(a)}{kT(\phi(a),a)} \right] . $$
In other words, the current due to repelled species is no longer an exponential! Our results also imply a modification of the PIC simulation methods. Because the temperature is no longer considered as a parameter but as a variable.
Reference:
Laframboise, J.G. and Parker L.W. (1973), {\it The Physics of Fluids}, p. 629.
Negative-ions (NI) may play an important role on discharge kinetics in low-pressure low-temperature plasmas, especially when using electronegative gases. NI are usually formed by dissociative electron attachment on molecules, and trapped inside the plasma volume due to the positive potential difference between plasma and surfaces. In certain circumstances, negative-ions are also formed on surfaces upon impact of neutral particles and capture of an electron to the material, or impact of positive ions (PI) and capture of two electrons by the incident particle. These negative-ions are then accelerated back towards the plasma by the sheath and may deposit some energy in the plasma volume. Low work function materials in interaction with plasmas are efficient for NI surface production due to the low energy required to extract an electron from the solid. However, other electronic property of materials, such as the presence of a band-gap or a negative electron affinity, may play an important role in electron capture as demonstrated in beam experiments. In this contribution we quantify and study hydrogen negative-ion surface production on various materials with a strong emphasis on carbon surfaces.
Samples of different kinds are installed on a sample holder in the diffusion chamber of a low-pressure capacitively or inductively coupled plasma, facing whether a mass spectrometer (MS) or a magnetised retarding field energy analyser (MRFEA). The sample is negatively DC biased and NI formed upon PI impact are accelerated towards the plasma. Considering the low pressure used in this study, they cross the plasma without any collision and are collected by the MS which provides a mass and energy analysis, or by the MRFEA which gives the NI current. A pulsed DC biased method has been developed to study NI surface production on insulating materials.
Details on experimental methods and modelling will be given. Basic production mechanisms will be detailed. It will be shown that electronic properties of materials, and not only its work function, can play a role on surface ionization efficiency in plasma.
From shortly after my arrival at TRIUMF I was swept up in the enthusiastic approach that John pursued. Starting with basic studies in pion exchange to separated isotopes at TISOL to cyclotron produced tracers for DRAGON and back again to off-line separated isotopes for medicine.
I will describe the breadth of the explorations John pursued and their impact on science in general and the arch of my career.
With the expansion of approved 18F based agents for medical imaging using positron emission tomography (PET), low energy (11-24 MeV) cyclotrons are now used at many commercial and academic centers to produce isotopes for medical imaging. The energy of these machines is ideal for isotope production via (p,n), (p,α) and in some cases (p,2n) reactions. These sites also provide ideal training grounds for the next generation of nuclear and radiochemists. Using the UAB TR24 cyclotron, our group has focused on the development of reaction routes, target materials and the separation chemistry of isotopes to expand the toolbox of nuclear imaging agents. These have included transition metals such as 52Mn, 55Co, 89Zr, 43,47Sc and 45Ti. Additional research has developed chemistry to incorporate these isotopes into new imaging radiopharmaceuticals for preclinical or clinical research. In particular, our group has been exploring the use of 89Zr radiolabeled antibodies for imaging of cell surface receptor expression in preclinical models and in clinical trials of metastatic breast cancer patients. Recent work has also resulted in new radiochemistry techniques for the development of new 43,47Sc and 45Ti radiopharmaceuticals for oncologic applications.
Positron Emission Tomography imaging coupled with an increasingly target specific tracer development is providing key insights into brain function in health and disease. Multiple neurochemical correlates to various brain diseases, mental states and personality traits have been uncovered. For example, multi-tracer imaging is revealing the impact of neurodegeneration, such as observed in Parkinson’s disease (PD), on several neurotransmitter systems and functional connectivity. At the same time there is an increasing awareness of the network type behavior of the brain and of the importance of the interactions between localized neurochemical alterations and longer-range functional effects. Distinct brain network-type behaviors are also being observed using functional magnetic resonance imaging (fMRI). This emerging knowledge highlights the importance of novel analysis approaches to the PET and/or multi-modality data which focus on identifying specific spatio-temporal patterns characteristics of brain function under different conditions. This talk will provide a brief history of how advances in imaging including radiotracer and instrumentation development as well as advances in data processing algorithms have contributed to revolutionize not only the understanding of brain function, but also the very concept of mental illness.
The NEWS-G direct dark matter search experiment is using spherical proportional counters (SPCs) with light noble gases as Ne, He, H to explore very low mass WIMPs parameter space. First results obtained with a SPC prototype operated with Ne gas at the Laboratoire Souterrain de Modane (LSM) have already placed NEWS-G as a leader in the search for low-mass WIMPs. Recent and planned improvements for the next phase of the experiment will be presented including the reduction of the background levels, detector performances and stability, and detector characterization. The next generation detector will consist of a larger volume 140 cm diameter SPC to be operated at SNOLAB with H and He gas. The use of lighter targets, improved thresholds and detector performance and with a significant reduction of the background levels will allow for unprecedented sensitivity to sub-GeV WIMPs down to 0.1 GeV. The current and future stages of the NEWS-G experiment in the context of the global dark matter search will also be discussed.
Models of particle dark matter (DM) that lead to the observed relic abundance today generically predict ongoing annihilation or decay into energetic standard model particles. Effective couplings to the standard model also imply that particle colliders such as the LHC can produce invisible particles which could be detected via enhancements in collision cross sections, or as missing energy and momentum. I will briefly review the theory behind indirect and collider searches, sketching out current constraints. I will then turn to exploring the myriad, complementary searches for DM annihilation and decay products using messengers such as gamma rays, cosmic rays, neutrinos, and the cosmic microwave background, as well as data from stellar astrophysics and cosmology.
One of the main focuses of the ATLAS experiment is the search for dark matter (DM) production in proton-proton collisions at the Large Hadron Collider (LHC). These searches are commonly interpreted in terms of simplified models with a spin-0 or spin-1 mediator particle propagating interactions between the visible and dark sectors. Simplified models lead to familiar Mono-X signatures where the presence of non-interacting DM can be inferred as missing transverse momentum caused by DM particles recoiling against a visible particle, X, (i.e. a jet, a photon, or a W, Z or Higgs boson). Since the mediators couple to SM particles, they can also be directly searched for though their decays to jets, top-quark pairs, and potentially even leptons. Simplified models describe dark-matter production kinematics with a minimal number of free parameters, but they do not represent complete theories. Recent theoretical efforts have focused on producing complete, renormalizable models of DM. In particular models involving extended Higgs sectors with an additional vector or pseudo-scalar mediator. These models offer rich phenomenologies and new experimental signatures. ATLAS has recently released summary plots gathering the results of more than 20 experimental DM searches. As well as first results utilizing the full 139 fb$^{-1}$ of 13 TeV proton-proton collision collected during Run 2 (2015-2018). ATLAS limits are compared to those set by direct detection experiments, and relic densities for the different DM models are calculated.
Dark matter is the dominant matter component of the universe. Despite its dominance over baryonic matter, the true nature of dark matter in unknown. One dark matter candidate is the weakly interactive massive particle (WIMP), invoked in several extensions to the Standard Model. The self-annihilation or decay of WIMP particles in regions of high dark matter density can produce secondary Standard Model particles such as very-high-energy (VHE, E> 100 GeV) gamma rays, allowing for indirect detection of dark matter. Indirect searches can be performed by making VHE observations of astrophysical regions of high dark matter density. As part of its long-term plan, VERITAS, a ground-based gamma-ray observatory sensitive to gamma rays in the ~85 GeV to greater than 30 TeV energy range, has observed a number of these regions. These observations have yielded constraints on the annihilation cross section of dark matter particles. In this talk I will discuss VERITAS observations of dark-matter-rich regions, such as dwarf spheroidal galaxies and the Galactic Centre, in the context of indirect dark matter searches.
D-Wave Systems produces superconducting quantum processing units (QPUs) that instantiate the quantum annealing algorithm for solving hard optimization problems. However, these QPUs have also proved to be an interesting platform for studying quantum magnetism within the context of the so-called transverse field Ising model. Recent publications have shown how such QPUs can be used to study paramagnetic-to-spin glass phase transitions and Kosterlitz-Thouless physics as a function of the transverse, or quantum mechanical, degree of freedom in this model. This lecture will provide a brief review of these latter studies and a discussion of ongoing work related to quantum magnetism.
Conventional magnets are characterized by symmetry breaking and the formation long-range magnetic order at low temperatures. New and unanticipated phases of matter can arise when such symmetry breaking is inhibited by highly frustrated magnetic interactions. A spectacular example of this is the pyrochlore lattice with isotropic antiferromagnetic interactions, where the collective behavior of fluctuating magnetic moments is described by an emergent divergence free field. The microscopic magnetic degrees of freedom continue to fluctuate even at zero
temperature, as if in a liquid state, but all configurations must obey the divergence condition; hence, this phase is termed a Coulomb spin liquid. In this talk, I will discuss a new pyrochlore lattice magnet, NaCaNi2F7, which realizes the isotropic Coulomb spin liquid with S=1 spins. I will present neutron scattering and calorimetric measurements that were used to uncover the magnetic correlations in this material and determine the magnetic Hamiltonian. Na+ - Ca2+ are randomly populated in the crystal structure of NaCaNi2F7 creating a rugged energy landscape that acts to freeze a small fraction of the magnetic degrees of freedom. However, the energy scale set by this disorder is small, and the Heisenberg interactions prevail. Only a small fraction of the available moment is frozen, and the magnetism in NaCaNi2F7 is dominated by a persistently fluctuating component that appears as a broad continuum of magnetic signal in inelastic neutron scattering measurements. These measurements demonstrate a beautiful realization of the Coulomb spin liquid and provide new insight into the interplay between disorder and magnetic exchange interactions in highly frustrated magnets.
Enormous interests generated by graphene physics have made honeycomb lattice one of the most studied two-dimensional lattice structures in recent years. In particular, the realization that bond-dependent anisotropic magnetic interactions can be found in honeycomb lattice materials with strong spin-orbit coupling has made a profound impact on quantum magnetism research. In this contribution, we will give an overview of recent experimental progress made in understanding honeycomb lattice quantum magnets with a special emphasis on the so-called Kitaev materials, such as $\alpha$-RuCl$_3$, which is a leading candidate for realizing a quantum spin liquid phase. Prospects of incorporating such honeycomb lattice quantum magnets in an artificial heterostructure with other two-dimensional materials will be also discussed.
The future can seem uncertain for those contemplating private/commercial sector jobs after graduating with a degree in physics. When graduates finally realize that their physics education has taught them some useful skills, and given them knowledge beyond the laws of physics, they become more confident. Hidden in their training are abilities that are useful to the outside world, and with some polishing and filling in the gaps, their job prospects excellent. But, there are many 'wake-up' moments along the way.
Generative models leveraging the recent advances in Deep Neural Networks (DNNs) have enabled incredible applications in diverse fields such as, machine vision, speech, and finance. After giving a brief historical perspective, this presentation introduces the concepts and principles behind deep generative models, focusing mainly on an important sub-class, namely, Generative Adversarial Networks (GANs). Using selected examples, we briefly explore applications of generative models to problems in physics and their implications. The presentation concludes with recent results using GANs for image synthesis and manipulation of satellite imagery to facilitate the training of object detection/segmentation networks.
To be provided later.
In honour of this year’s Nobel Prize recognition of Donna Strickland for her work on chirped-pulse amplification, we will celebrate the impact her work and highlight some current frontiers of optical science.
En l’honneur du Prix Nobel décerné à Donna Strickland pour son travail sur l’amplification d'impulsions par dérive de fréquence, nous célébrerons l’impact de son travail et soulignerons quelques initiatives aux frontières de la science optique.
Very intense hard X-ray beams (1.5µm X-ray source size, 5µJ-50µJ/shot in the 30keV-40keV band, 50mrad x 50mrad divergence, critical energy for the X-ray spectrum of 30keV) have been generated through ultra-relativistic self-guiding over long gas jet length (cm range). I will describe the experiments realized with our new laser facility (delivering up to 7J in 18fs at 2.5Hz on target) and I will discuss the empirical scaling laws we have obtained correlating the X-ray photon number to the laser and gas jet parameters. Our scaling indicates that a 40keV X-ray beam with energy of 1mJ range per shot can be produced with a driving laser with power in the 1 – 2PW range.
The X-ray source has been operated at the nominal 2.5Hz repetition rate giving an average power in the 12µW-125µW range in the 30keV-40keV spectral band. High throughput X-ray phase contrast imaging and 3D phase contrast tomography of various objects have been realized. We demonstrated that the phase contrast imaging was giving the possibility to see transparent very small objects (10µm to 300µm diameter range) embedded inside inhomogeneous and anisotropic thick (absorbing) environment. We will present the experimental demonstration and discuss the potential for non-destructive imaging.
There is a need for a stand-alone system dedicated for plants and seeds screening available on production sites. I will present our funded program in Canada in developing high throughput X-ray phase contrast plant imaging and screening using LWFA-based X-ray sources (10keV-100keV). This effort is realized through an initiative led by the Global Institute for Food Security (GIFS) at the U of Saskatchewan that aims to establish the correlation between the phenotypic expression of a plant and its adaptation to biotic and abiotic environmental stress.
High harmonic spectroscopy has been used for decades for sub-femtosecond resolution of molecular dynamics and attosecond measurements of electronic configurations. Recently, these ideas have been transferred to condensed matter to understand strong field processes in semiconductors and dielectrics. In developing these attosecond techniques, we have discovered that we can fully map the waveform of an ultrashort pulse, or create a broad spectrum sensor for THz radiation. In this talk, I will discuss the possibilities for spectroscopy using some of these attosecond technologies.
Since the advent of lasers, many different nonlinear optical techniques have led to shorter, higher-intensity pulses. At Waterloo, we are studying Multi-frequency Raman generation (MRG), which efficiently generates a large number of Raman orders spanning the spectral region from the infrared to the ultraviolet. The bandwidth of the Raman orders is sufficient to generate single-femtosecond duration pulses. While the pulse duration is longer than what is possible with high order harmonic generation, the conversion efficiency is much higher. While most research in this field is concerned with making as many Raman orders as possible, we noticed that the spectra of the individual Raman orders changed dramatically when changing either the dispersion in the nonlinear medium or the laser pump intensity. In this talk, I will discuss the possible physical process causing these changes in the spectra as well as how the changes effect the generation of ultrashort intense pulses.
Magnetic resonance imaging (MRI) is well known as a flexible and powerful non-invasive diagnostic imaging technique available to clinical medicine. MRI as a largely qualitative tool is well established, principally used to visualize the internal structure of biological systems through mapping the spatial position of hydrogen. The magnetic resonance signal lifetimes (relaxation times) and molecular diffusion usually manifest qualitatively in the MR image contrast. Quantitative analysis of these parameters will reveal molecular scale information, since magnetic resonance is sensitive to a range of physical/chemical characteristics including molecular structure, molecular motion, size and interactions, through the measurements of relaxation times and molecular diffusion. The potential for quantitative MR has been recognized. However, significant methodological developments are still required for this potential to be fully realized. I will present some recent progress and potential solutions to this challenging problem.
It is thought that the diameters of axons change due to disease. Until recently measurements of axon diameters (AD) could only be done invasively. AD can now be inferred non-invasively using a magnetic resonance imaging temporal diffusion spectroscopy (TDS) technique that requires many images to be collected with a range of diffusion-times or frequencies. Collecting many images to average together for a higher signal-to-noise ratio (SNR) becomes too time-consuming. Thus, low SNR (5-6) images of mouse brain were analyzed to infer axon sizes.
TDS methods require many measurements at different gradient strengths and frequencies making data collection time consuming and difficult to obtain high SNR in a reasonable amount of time. This work uses oscillating gradient TDS to study the corpus callosum (CC), fornix and optic tract (OT) of the mouse brain under the condition of low SNR to determine if reasonable results can be obtained with low SNR.
A 15.5 week-old female mouse (Kras/p53 on a C57BL/6 background) was perfused with formalin. Brain (in skull) was soaked in a formalin solution for 24 hours then transferred into a Phosphate-buffered saline (PBS) solution for 24 hours then transferred into another PBS solution for 24 hours prior to imaging to filter out any remaining formalin. All experiments were approved by both Universities’ Animal Care Committees.
The sample was imaged using a 7 T Bruker Avance III NMR system with Paravision 5.0 with a BGA6 gradient set with a maximum gradient strength gmax of 430357 Hz/cm, and a 3.5 cm diameter bird cage RF coil. Each 20 ms apodised cosine gradient pulse ranged from n=1-20, in steps of 1. Two different gradient strengths were used for each frequency and gradient pulses were separated by 24.52 ms.
TDS was able to infer ADs of 4.8±1.2µm in the CC and 2.4±0.6µm in the fornix using low SNR images. The models used to infer ADs assume the diffusion gradient direction to be perpendicular to the axons. The gradient direction was approximately parallel to the axons in the OT and we suspect this explains why we were unable to infer ADs in the OT. The gradient frequency range successfully targeted ADs ~2-4 µm in the fornix and CC. The low SNR did not seem to have an effect on the ability of the method to infer ADs. This study is the first step toward showing the feasibility of using OGSE TDS to infer ADs in situations of low SNR.
Acknowledgements
The authors thank NSERC for financial support.
Stereotaxy is a neurosurgical technique guided by preoperative imaging. Its procedure is typically performed through a small burr hole opening in the skull that prevents tissue visualization. The intervention is basically “blind” for the operator with limited means of intra-operative confirmation that may result in the reduced accuracy and safety. It may take several hours to reach a specific target. The magnetic resonance imaging (MRI)-guided robotic stereotactic neural intervention system offers the potential to reduce surgical duration while improving target accuracy and safety. Current studies show that the use of MRI for guiding robotic surgical devices has a great potential for performing precisely targeted and controlled interventions. However, the currently developed project of robotic system is limited to being compatible with clinical 1.5T-3.0T MRI scanners as well as expensive sales. The proposed project dedicates in developing and deploying MRI-compatible robotic systems, and enables the technologies for intra-operative MRI-guided based interventions. The preliminary work will be involved in the design of robotic mechanisms, the customization and development of motors as well as sensors. The developed robot will be used for MR safe and MR compatibility test, which is a new generation of 7.0T MRI developed by Sino Canada Health Engineering Research Institute. The follow-up work will be involved in the integration between the intervention robot and control system, network environment building, etc. Our aims are focus on developing stereotactic neurosurgical robotic assistant system with MR-compatible, high accuracy and low price, which could benefit both surgeons and patients.
Hyperpolarized 3He/129Xe gas pulmonary MRI provides physiologically relevant biomarkers of obstructive lung disease, including emphysema, bronchopulmonary dysplasia and alpha-1 antitrypsin deficiency (AATD). Recently, a stretched-exponential-model combined with under-sampling in the imaging and diffusion direction was used to generate 3He static-ventilation (SV), T2, multiple b-value diffusion-weighted (DW) MRI ADC and morphometry maps, demonstrating an acceleration factor (AF) of 7 to 10. The low gyromagnetic ratio of 129Xe coupled with clinically used gradient strengths, dictate that rapid acquisition strategies be developed to facilitate clinical uptake of 129Xe DW imaging. We hypothesize that the 3He method can be adapted to provide whole lung 129Xe MRI-based emphysema biomarkers, including SV, T2, ADC and morphometry maps. Therefore, in this proof-of-concept study, our objective was to extend the 3He method for accelerated 129Xe lung morphometry using single breath measurements for validation in a small group of patients.
Three healthy volunteers (<25yr>) and six AATD (<65yr>) patients provided written informed consent to participate in an ethics-board approved study protocol and underwent spirometry, plethysmography, and accelerated 129Xe MRI morphometry using a single xenon dose. Imaging was performed at 3.0T using whole-body gradients and a commercial human-sized xenon quadrature flex RF coil. For xenon measurements the diffusion-sensitization gradient pulse ramp up/down time was 500μs, constant time=2ms and diffusion time=5.2ms, providing five b-values of 0, 12.0, 20.0, 30.0, and 45.5s/cm2. For accelerated acquisition, a multi-slice (six interleaves) centric 2D FGRE DW sequence under-sampled in the imaging and diffusion direction for seven 30mm coronal slices. An extra interleave without DW (b=0) with significantly reduced TE (2ms) was utilized to generate a short-TE SV image and T2 map. A 7.4 degree constant-flip-angle (120 [20 per b-value] RF pulses-per-slice) was used for the AF=7 (all participants, 12sec single breath-hold) acquisitions.
To the best of our knowledge this is the first demonstration of 129Xe MRI morphometry measurements with AF=7. We have demonstrated that accelerated 129Xe MRI morphometry permitted to generate whole lung SV, T2, ADC and morphometry maps within a single 12sec breath-hold with typical spatial resolution.
Magnetic resonance imaging (MRI) is a non-invasive tool. Conventional MRI provides anatomical images with little information about the function of the anatomy. Positron emission tomography (PET) provides images of the function of the organs (e.g. glucose uptake) non-invasively with no anatomical information. Functional MRI (fMRI) gives information of blood oxygenation level within the central nervous system which is related to function. Combining both MRI and PET allows the visualization of functional and anatomical changes together providing much more information than either method can provide on its own.
When one image is acquired before another, it is difficult to align the images (image registration) because of motion of the subject. If the organ being imaged moves due to respiration, for instance, then image registration is even more challenging. If the subject's physiological response and/or anatomy varies with time, the images from the two different modalities when collected at different times do not necessarily capture the same changes, making correlation of results difficult. Simultaneous PET/MR imaging can be used to overcome many of these challenges.
Here we present data acquired from simultaneous PET/MR imaging. The MR images were acquired with a 7T 21 cm bore Bruker Avance III NMR system with Paravision 5.0. The PET images were acquired with a NuPET (Cubresa, Inc)with a 67.2 mm (axial) x 58.9 mm (transaxial) field of view which was inserted in the bore of the magnet. These data will demonstrate the feasibility and utility of simultaneous PET/MR imaging.
Acknowledgements
The authors thank NSERC for financial support.
In this talk, I will discuss two applications of holographic duality for CFTs with boundaries (BCFTs). First, I will discuss how to describe smooth spacetimes via
multipart entangled states of many non-interacting BCFT systems. As a second
application, I will describe the construction of certain black hole
microstates for which the behind-the-horizon geometry can be deduced
explicitly. The latter construction may provide a way to embed
cosmological physics within AdS/CFT.
How do we describe the local Hilbert space of a continuum field theory? And does this have lessons to each us about the emergence of spacetime in AdS/CFT? I will describe how to define the Hilbert space of an interval in a CFT without turning to abstruse mathematics. For CFTs with a gravitational dual, this leads to a theory of gravitational membranes and new insights for understanding spacetime entropy.
In this talk, I will discuss how to realize an asymptotically safe Standard Model by using the large number of flavours (N_f) techniques. This framework also opens the way to various safe extensions of the Standard Model. The first asymptotically safe Pati-Salam model and trinification model will also be discussed.
We point out that the Kalb-Ramond field of string theory can induce electroweak dipole dark matter, although the dark matter constituents of the dipoles do not carry weak charges. We also show that the model is compatible with direct search constraints from electron recoils. However, the recoil cross sections are above the neutrino floor and should be testable in future experiments.
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Attending.
The packing of spheres is an interesting problem in mathematics and physics with a long history dated back to the work of Kepler and Lord Kelvin. In recent years, intricate periodic and aperiodic spherical packing phases have emerged in a host of soft matter systems including supramolecular assemblies, surfactants and block copolymers, underscoring the universality of emergent order in condensed matter. In particular, the rich phase behavior of block copolymers provides an ideal model system to study the origin and stability of order phases in soft matter. Our recent study of block copolymer systems using the self-consistent field theory reveals that one key mechanism of forming complex spherical phases is the conformational asymmetry of the blocks. Furthermore, we have predicted that the segregation of different polymeric species in block copolymer blends provides another mechanism to stabilize spherical packing phases with very different sized-spherical domains. I will summarize recent progresses on this fascinating topic and discuss possible future research directions.
A brush-like structure emerges from the stretching of long polymer chains, densely grafted on to the surface of an impermeable substrate. This structure arises from a competition between the entropic elasticity of grafted polymer chains, and the intra and interchain excluded volume repulsion. Classical studies on polymer brushes focus on the structure of a brush, monomer density, end-distributions etc. but not on the stress. Recent advances in polymer-brush based soft active materials required us to understand the nature of stresses and forces in these material systems. In this talk two strong stretching theories (SST), based on Gaussian and Langevin elasticity of chains, are compared and contrasted with Molecular Dynamics calculations. Continuum mechanics plays a fundamental role in providing the analytical and semi-analytical theoretical methods to evaluate stress and its distribution within polymer brushes. For Gaussian chains, our theory predicts that the normal stress, parallel to the substrate, is a quartic function of the distance from the grafting surface with a maximum at the grafting surface. Idealizing the brush as a continuum elastic surface layer with a residual stress, closed form expressions for resultant surface stress and surface elasticity as a function of molecular weight and graft density are obtained. For higher graft density brushes, a (semi) analytical SST with Langevin chain elasticity will be discussed. Theoretical predictions are assessed by molecular dynamics simulation of a brush using bead-spring model. Experimental estimation of resultant stress due to a polymer brush will also be discussed. We conclude that classical scaling theories as well as the SST theories can be coupled with continuum mechanics to understand stress in a polymer brush.
While perfluorinated polymers dominate the commercial fuel cell industry, hostility to catalysts, difficult and expensive synthetic routes, and challenging disposal hamper wide adoption of fuel cell technology and impede further development. Hydrocarbon-based membranes utilize simple, well-developed synthetic routes that allow for rapid material development. We have investigated a promising series of sterically hindered methylated imidazole-based ionenes utilizing a combination of lab-scale X-ray scattering and OPLS-AA-based molecular dynamics simulations to elucidate the morphology of these materials
Myelin is the fatty insulating material that covers nerve axons in white matter brain tissue and spinal cord. A number of neurodegenerative diseases, the most prominent of which is multiple sclerosis, are associated with damage to myelin with consequent degradation of neuronal signal transmission and functional impairment. A number of magnetic resonance imaging techniques have been developed to characterize myelin in order to assess disease progression and the effectiveness of treatment and treatment candidates. Several of these imaging techniques will be discussed,with a focus on a new technique, misleadingly named inhomogeneous magnetization transfer (ihMT), which relies upon the quantum mechanics of the dipolar interactions between 1H nuclei in the hydrocarbon chains of myelin lipids. Working with in vitro bovine brain tissue and a variety of phantom samples, we have demonstrated that the original explanation for the technique, offered by its inventors, that the observed effect is associated with "hole burning" in inhomogeneously broadened NMR spectra, is not correct. We have shown that instead, ihMT is due to connections to the dipolar-coupled nuclear spin bath in the myelin lipid molecules. The equivalence between two coupled spin-1/2 particles and a single spin-1 allows a simple explanation of how the dipolar couplings in lipids give rise to the observed contrast in ihMT MRI images.
While many physicists agree that fusion energy has the potential to be an exceptional solution to the world’s energy challenges, few can agree on how and when it will be done. Historically, two main approaches to develop fusion energy have been pursued by academic research: magnetic confinement and inertial confinement. Over the years significant progress has been made, but fusion power plants based on these two methods are still a ways away. Magnetic confinement operates with a plasma density of 1020 ions/m3 for time in seconds. Inertial confinement plasmas have a density of 1032 ions/m3 for a duration of 10’s of ps. Encouraged by advancing plasma physics and supporting technologies, interest in new approaches between these two extremes has emerged. Called Magnetized Target Fusion (MTF) or Magneto-Inertial Fusion (MIF), these new approaches are in between the traditional ones, using compression (like inertial confinement) of a magnetized plasma (like magnetic confinement) to achieve fusion conditions. Some argue that MTF could lead to practical fusion energy faster and for less money than the more traditional approaches. In recent years, the potential timeline and lower cost of fusion energy has resulted in many new companies entering the field with various approaches to fusion. After an overview of the fusion energy field, General Fusion’s MTF approach will be discussed.
Magnetized Target Fusion (MTF) involves compressing an initial magnetically confined plasma on a timescale faster than the thermal confinement time of the plasma. If near adiabatic compression is achieved, volumetric compression of 350X or more of a 500 eV target plasma would achieve a final plasma temperature exceeding 10 keV. Power plant relevant fusion gains could be achieved provided the compressed plasma has sufficient density and dwell time.
General Fusion is developing a compression system using pneumatic pistons to collapse a cavity formed in liquid metal containing a magnetized plasma target. This approach offers a low-cost driver, straightforward heat extraction, good tritium breeding ratio and excellent neutron protection in power plant designs.
Through an active plasma and compression R&D program, General Fusion is conducting full scale and reduced scale plasma experiments and simulation of both. Although pneumatic driven compression of full scale plasmas is the end goal, present compression studies use reduced scale plasmas and chemically accelerated aluminum liners. We will review results from our plasma target development and dynamic compression program. In particular we will focus on the two most recent tests in which we have detected significant increases in fusion neutron rates.
The field-reversed configuration (FRC) is a class of magnetically confined plasmas characterized by self-generated poloidal fields and no toroidal fields. The FRC has a high beta value, the ratio of plasma pressure to magnetic field pressure, which means that the required strength of the external magnetic field is much lower when compared to other magnetic confinement schemes. Consequently, the achievable temperatures are much higher, which leads to the possibility of aneutronic fusion. The FRC has other technical benefits that make it a practical means to achieve nuclear fusion including simple linear geometry, and natural diverter systems for particle extraction and direct energy conversion. In this talk, I will discuss the technical benefits of the FRC as a practical fusion reactor core. I will also present the current research being performed in this area, the role of Canada in the worldwide fusion programme, and an outlook to the future of magnetic confinement fusion.
With four alpha particles in its decay chain, actinium-225 (225Ac; t1/2 = 9.9 d) is a promising candidate isotope for Targeted Alpha Therapy (TAT) when coupled with a disease targeting vector. The current limited global supply of 225Ac (67 GBq/year), and lack of appropriate chelating ligands able to complex this isotope has delayed the advancement of 225Ac-drugs towards the clinic [1]. Herein, we describe efforts to produce, purify, and evaluate the radiolabeling ability of 225Ac, by leveraging TRIUMF’s ISAC isotope separation on-line (ISOL) facility. 225Ac alongside, parent nuclide radium-225 ( 225Ra; t1/2 = 14.8 d), were produced via spallation of uranium carbide targets with 480 MeV protons on ISOL’s radioactive beam facility. Downstream from the target, a high-resolution mass separator was used to isolate 225Ra and 225Ac ions from other isotopes produced in the spallation process. The 28 keV beam was directed towards an aluminum holder in which the ions were implanted at a depth between 10 and 30 nm. Implantation yields of 1.6x108 and 5.7x107 ions/s resulted in isolation of 1.0 – 7.5 and 1.4 – 18.0 MBq of 225Ra and 225Ac, respectively. The implanted activity was etched off the sample stage with dilute acid, and 225Ac was separated in >99% yield from 225Ra using solid phase extraction (DGA resin) [2]. This method has resulted in the isolation of MBq quantities of both 225Ra and 225Ac, where the former can be stored and used as a generator for 225Ac. Subsequently, 225Ac coordination properties with a library of acyclic chelators based on picolinic acids (such as H4(CHX)octapa [3],[4] [N4O4], and H6phospa [5] [N4O6]) along with commercial standard DOTA (N4O4) were evaluated by testing radiolabeling efficiency, and complex stability. In conclusion, we have successfully established a production method for 225Ac which yields activities adequate for pre-clinical screening. Furthermore, several novel actinium-chelators showed promising 225Ac radiolabeling properties and kinetic inertness in vitro compared to DOTA, and will be tested in vivo in future studies.
The study of nuclei far from stability is one of the most active and challenging areas of nuclear structure physics. Studies of the most exotic neutron-rich isotopes require an unprecedented combination of beam intensities and detection sensitivity, which will soon be realized in the United States at the Facility for Rare Isotope Beams, with $\gamma$-ray spectrometers such as GRETA. The GRETINA array, being operated at NSCL and ATLAS at ANL is already pushing forward the limits of such measurements, with impacts in basic nuclear structure, nuclear astrophysics and applications. I will present an overview of the program of GRETINA/GRETA and highlight a few examples of the compelling physics being pursued.
An accreting compact star in a binary system can generate periodic thermonuclear runaways on its surface. In the case of a white dwarf star, the result is a classical nova, which enriches the interstellar medium with newly synthesized nuclides. In the case of a neutron star, a detectable burst of X-rays is emitted. Nucleosynthesis and energy generation in these events depends on thermonuclear reaction rates, which are especially challenging to measure directly in the laboratory when the reactants are radioactive. John D’Auria led a collaboration to surmount this challenge; in the process, he included and inspired a new generation of scientists. Some of John’s earlier work showed that beta decay is also an effective method to determine thermonuclear rates indirectly. A new program of beta decay experiments to constrain thermonuclear rates is being pursed at the National Superconducting Cyclotron Laboratory by a collaboration including one of John’s mentees. In particular, the Gas Amplifier Detector with Germanium Tagging (GADGET) system, developed to measure very weak low-energy beta delayed proton emission branches and gamma rays, is now operational.
Rather than being a single new species, the dark matter of the Universe could be a part of a more complicated collection of new particles with its own nontrivial dynamics that interacts weakly with the standard model--part of a ``dark sector.'' I will discuss motivations for considering this possibility, recent developments, and the new avenues for discovery that have opened up.
The leading dark matter (DM) paradigm over the past few decades has been that of a Weakly Interacting Massive Particle with a mass of tens of GeV to a few TeV. But in light of recent experimental constraints, attention is increasingly turning to models with lower-mass DM, especially in the context of a "dark sector" featuring dark mediators and multiple DM particle species. Probing such models requires exploiting complementarity between different types of DM searches, where electron-beam fixed-target experiments play an important role in the DM mass range of a few to hundreds of MeV. These experiments seek to generate dark sector particles, such as dark photons, via electron-nucleus scattering and emission processes analogous to standard bremsstrahlung. Identifying the visible decay products of the dark sector particles, such as electron-positron or muon-antimuon pairs, requires precise reconstruction of narrow mass resonances and/or displaced vertices; accounting for invisible decay products requires precise missing energy and/or momentum measurements. In this talk, I will give an overview of the landscape of current and planned fixed-target DM searches, with the Heavy Photon Search (HPS) and its planned successor LDMX (Light Dark Matter eXperiment) as specific examples.
The Belle II experiment has started operations at the SuperKEKB e+e- collider, located at the KEK laboratory in Tsukuba, Japan. Like its predecessors Belle and BaBar, Belle II will operate near the Y(4S) resonance. It has the goal of collecting a dataset 100x larger than that of BaBar. The experiment's trigger has been designed to allow searches for a wide variety of dark sector particles and other low-mass particles predicted by theories beyond the Standard Model, including possible dark matter candidates. A small amount of data was recorded in the Spring of 2018. I will review the status of searches using this initial data, and discuss future prospects, with particular emphasis on near-term discovery potential.
Despite the currently collected LHC datasets not seeming to show signs of easily observable physics beyond the Standard Model, there remains a number of areas still to be explored. One of these areas is the search for long-lived particles, typical signatures of Dark Sector models. These searches most often rely heavily on non-standard reconstruction within the ATLAS experiment. Standard reconstruction relies on the particles being promptly produced - or close to promptly produced for b-tagging - at the primary collision vertex. ATLAS is now carrying out a wide range of searches for long-lived particles whose decay lengths extend from slightly beyond typical B-hadron decays lengths (few mm) to several meters with decays within the muon spectrometers. A short overview of some recent such searches will be presented.
Recently, the long-standing notion that the overdoped cuprate superconductors conform to a Landau-BCS description has been challenged strongly by new data on superfluid density and optical conductivity of high quality LSCO films. We show instead that a wide variety of experimental data on LSCO and Tl2201 (superfluid density, optics, heat capacity, thermal conductivity) can be explained by dirty d-wave theory, subject to significant Fermi-liquid renormalizations, provided that the starting point is an accurate parameterization of the electronic dispersion, and the sometimes nonintuitive effects of disorder are properly treated. Our conclusions have important implications for future research on overdoped materials, and the overall understanding of cuprate physics.
The discovery of a superconducting phase in sulfur hydride under high pressure with a critical temperature above 200 K by Drozdov et al. [1] has provided a new impetus to the search for even higher Tc. The observation of a sharp drop in resistance to zero at Tc, its downward shift with magnetic field and a Meissner effect confirm superconductivity but the mechanism involved remains to be determined. Using the AILES beam line at Soleil, we provided a first optical spectroscopy study of this new superconductor[2]. Experimental results for the optical reflectivity of H3S, under high pressure, were compared with theoretical calculations based on Eliashberg theory using DFT results for the electron-phonon spectral density. One significant features stands out: a band with a depressed reflectance in the superconducting state in the region from 450 meV to 600 meV. The shape, magnitude, and energy dependence of this band at 150 K agrees with our calculations. This is strong evidence of a conventional electron-phonon mechanism. Also, this band provides a tool for a non-invasive probe of superconductivity[3].
Unexplained to this day, the mysterious pseudogap phase of cuprate high-temperature superconductors is one of their key defining universal properties. Many scenarios have been proposed for its origin, but none has provided a satisfactory description so far. Part of the problem stems from the absence of a clear and sharp signature of the pseudogap at low temperatures in the vicinity of its critical point at p. Here I will discuss our recent transport and thermodynamic measurements on a range of cuprate materials, which show that p is a quantum critical point. I will also show that hydrostatic pressure can be used as a tuning parameter for p* in Nd-LSCO, which puts constraints on theories of the pseudogap. Finally, I will mention very recent thermal Hall effect measurements that reveal a giant effect specific to the pseudogap phase and which seem to arise from neutral excitations.
Chilling a deep underground mine is costly and energy intensive. The temperature of the air increases as it descends due to the adiabatic lapse rate (called auto-compression in mining), the host rock, equipment and processes are additional sources of heat. There is an expectation that battery powered vehicles may allow for less air flow due to reduced airborne particulates, legislation changes pending, that increases the susceptibility to any additional heat. Battery powered vehicles and the charging process both add heat. A reduced air flow reduces the heat needed to raise the temperature to unacceptable levels, which may only require the operation of a few large machines. This paper discusses results from a prototype, a cryogenic co-generation system that stores energy and provides chilling, effectively the heat from the mine is converted to electricity (5 MWe electrical power absorbs 8 MWr of chilling). Additionally compressed air can be produced simultaneously producing chilling (5000 cfm produces 1.2 MW chilling) and motive force, engines for equipment would produce cool clean air as exhaust with about 1/3 motive power to 2/3 chilling. Results obtained from a bench scale prototype system, demonstrates the rapid response of the air flow to chilling. Using a cryogenic fluid to chill a deeper mine only requires a longer pipe when the mine depth is increased and the liquid is piped so the system can provide chilling where it is needed, which is cost effective.
In honour of this year’s Nobel Prize recognition of Donna Strickland for her work on chirped-pulse amplification, we will celebrate the impact her work and highlight some current frontiers of optical science.
En l’honneur du Prix Nobel décerné à Donna Strickland pour son travail sur l’amplification d'impulsions par dérive de fréquence, nous célébrerons l’impact de son travail et soulignerons quelques initiatives aux frontières de la science optique.
Abstract: We use intense vector beams to generate high harmonics or to create solenoidal currents in solids or gases. We predict THz magnetic fields reaching the scale of those only available at user facilities.
In the visible and infrared we can transform Gaussian beams into beams with spatially dependent polarization and/or phase structures. Orbital angular momentum is given to a beam when the phase varies by 2npi around its profile. One might ask if orbital angular momentum is conserved during high-harmonic generation? We show the conservation of orbital angular momentum [1] during high harmonic generation and show how the conservation of angular momentum leads to a method for coupling a controlled orbital angular momentum on any harmonic [2]. Our results open a pathway for attosecond science with similarly structured light.
Besides shaping the wave fronts, a Gaussian beam can also be transformed into beams with complex polarization states – so called vector beams.
We use an 800 nm, 2 mJ pulse, 35 fs pulse and a Q-plate (illustrated in the inset) to produce a vector beam with each quadrant circularly polarized, with adjacent quadrants delayed in phase by pi/2 and with different handedness for adjacent quadrants (encoded in red and blue in the figure). As such a vector beam propagates, it transforms into a beam with linearly polarized segments as illustrated (bottom left) and measured (bottom, middle). We transform this beam via high-harmonic generation to photon energy of 40 eV creating a new vector beam with linearly polarized segments and also with adjacent quadrants phase delayed by npi/2 where n is the harmonic order. For symmetric nonlinear media, this beam likewise transforms as it propagates into a beam with circularly polarized segments as illustrated in the 3-dimensional figure.
Cylindrical symmetric vector beams can be efficiently compressed to few cycles in hollow-core fibers and we conclude by discussing opportunities that arise from sub-focal spot coherent control with vector beams [3] allowing THz solenoidal magnetic field generation.
[1] G. Gariepy, et al, “Creating High-Harmonic Beams with Controlled Orbital Angular Momentum”, Phys. Rev. Lett. 113, 153901 (2014)
[2] F. Kong, et al, “Controlling the orbital angular momentum of high harmonic vortices”, Nat. Commun. 8, 14970 (2017)
[3] S. Sederberg, F. Kong, and P. B. Corkum, ”Ultrashort Magnetic Impulses Driven by Coherent Control with Vector Beams”, Arxiv 1901.07444v1 (2019)
Photons emitted from atomic transitions may be twisted; angular momentum opposite to the atom's internal configuration change is distributed between the photon's spin (polarization) and orbital angular momentum depending on the angle of observation. Photons twisted perpendicular to their angle of observation propagate through an aperture at an angle, and measuring the photon's displacement at an image plane constitutes a weak angular momentum measurement. Remarkably, photons from an elliptical dipole are not only twisted, but kinky, with more than $\pm \hbar$ of angular momentum per photon at the aperture, larger than the angular momentum of the dipole eigenstates or the corresponding atomic transitions.
Such `supermomentum' is a consequence of weak measurement amplification in a spin-orbit coupled optical field. Chiral light-matter interactions of this kind promise exciting new capabilities for photonic devices, and both consequences and opportunities for super-resolution microscopy. We measure optical spin-orbit coupling and supermomentum from two dipole photon sources.
We image single photons emitted from a trapped atom at the focus of a high-aperture objective and demonstrate wavelength-scale, chirality-dependent shifts between photons from opposing dipole transitions. Our elliptical dipole light source is a sub-wavelength gold nanoparticle and we observe supermomentum in the small-aperture nanoparticle image. This supermomentum becomes arbitrarily large as the aperture vanishes.
This optical spin-orbit effect can lead to systematic wavelength-scale errors in the localization of an elliptically polarized emitter. Such errors are present even for ideal, focussed, aberration-free imaging systems and reveal that the paraxial approximation is fundamentally inadequate for optical super-resolution microscopy, even when the observation aperture is very small. This finding applies to the localization of objects using any wave carrying angular momentum orthogonal to the direction of observation, and is relevant for super-resolution microscopy techniques, which achieve resolutions two orders of magnitude smaller than the supermomentum shifts observed here.
We measure the current dissipation rate of fermionic ultracold atoms in an optical lattice. A quantum gas microscope enables high-resolution fluorescence imaging of atoms pinned to lattice sites. Using micron-scale periodic displacements of an underlying harmonic potential to provide an oscillating uniform force, we measure the global current response of the atoms for multiple frequencies within the lowest band. We observe that the current response scales linearly with the forcing, providing experimental verification that data is taken in the linear response regime. Broadening of the current response spectrum for increasing lattice depth, interaction strength, and density provides a measure of the rate of dissipation. This dissipation occurs purely due to fermion-fermion collisions, given the absence of phonons or impurities in our potentials. It is observed to require a finite lattice depth in order to break Galilean invariance, as well as to enable Umklapp scattering events, which play a significant role in the dynamics. Measured dissipation rates collapse onto the predictions of a kinetic theory under the wide range of conditions studied.
Plasmon-based solar energy conversion relies on absorption and charge separation at rectifying, metal/dielectric interfaces. Hot electrons produced through plasmon decay can undergo internal photoemission (IPE) and injection into an adjacent dielectric material, generating useful photo-current and -voltage determined by the metal/dielectric material pair. Here, we describe our work to optimize plasmonic photovoltaic devices on smooth and nanostructured Ag/ZnO interfaces and identify the requirements for high quantum efficiency structures. We have (i) modelled the capture of solar radiation by plasmonic metal/dielectric structures using finite difference time domain (FDTD) simulation methods, (ii) fabricated test devices, (iii) evaluated their optical, rectifying, and photovoltaic response, and (iv) characterized their materials properties using electron microscopy, spectroscopy and x-ray diffraction methods. We describe the challenges and opportunities of this and related technologies and introduce a new bottom-up approach to deposit single crystal epitaxial metal films and nanostructures from solution. While this chemistry allows for the subtractive manufacture of nanostructure through ion beam milling, it also enables additive crystalline nanostructure using lithographic methods such as electron beam lithography to enable novel, large area, metamaterial arrays and high aspect ratio crystalline nanostructure. We anticipate that this new approach will have significant impact on this and other new plasmon-based nanotechnologies.
The tune-out wavelength is usually viewed a zero in the frequency-dependent polarizability [1,2]. This view is appropriate for an atom in an optical lattice that is fixed in space. However, for an atom interacting with a traveling plane wave from a laser, it is more appropriate to view the tune-out wavelength as a zero in the Rayleigh scattering cross section for coherent scattering. In lowest order, the two approaches are equivalent, but not when higher-order retardation corrections are taken into account. This paper presents a development of the theory, starting from the relativistic scattering matrix of QED to obtain a formulation of the problem in the velocity gauge [3]. Gauge invariance is discussed, and an equivalent length form is obtained for the leading retardation correction for S-states. The $xp_z$ retardation correction to the tune-out wavelength of helium near 304 nm is calculated to be $0.000\,560\,0236$ nm.
[1] B. M. Henson et al., Phys. Rev. Lett. 115, 043004 (2015).
[2] Y.-H. Zhang et al., Phys. Rev. A 93, 052516 (2016).
[3] G. W. F. Drake, J. Manalo and P.-P. Zhang, Hyperfine Int. submitted (2019).
Monte Carlo simulations of radiation transport have played an important role in medical applications over the past three decades. More recently, Monte Carlo methods have been used for such modern radiotherapy applications as 4D patient doses calculations that take into account anatomy variations during treatment, in-vivo patient-specific verification of the treatment delivery, or image-guided radiation therapy and the associated dose delivered to patients. Moreover, treatment machines with dynamic beam delivery and tumour tracking capabilities have been characterized by Monte Carlo generated phase space data. Several ultra-fast, GPU-based, Monte Carlo codes have been introduced in the past few years, opening the exciting prospect of using them for real-time adaptive radiation therapy. We will give an overview of these recent developments.
Radiotherapy is used to treat more than 50% of the cancer patients. Improving the therapeutic ratio, either by reducing the impact of side effects or enhancing the treatment effectiveness locally, is a major goal of current cancer radiotherapy. One of the ways to improve the local radiation dose while minimizing damage to healthy surrounding tissue is to introduction of high Z materials such as gold nanoparticles (GNPs) as radiation dose enhancers. They enhance radiation damage by producing a shower of secondary electrons when exposed to X-ray beams. Radiation dose enhancement properties due to GNPs is dependent on their localization within cells, due to the short range of these electrons. The exact magnitude of the sensitization depends on a number of factors, including GNP size, beam energy, and total dose. However, there has been limited translation of GNP-mediated radiosensitization to a clinical setting. One of the key challenges in this area is the wide range of experimental systems that have been investigated, spanning a range of particle sizes, shapes and preparations. Hence, mechanisms of uptake and radiosensitization have remained difficult to clearly identify. This has resulted in a significant barrier to the identification of optimal GNP formulations which strike a balance among their radiosensitizing properties, their specificity to the tumors, their biocompatibility, and their imageability in vivo. In this talk, I will review the current state of knowledge in each of the areas concerning the use of GNPs as radiosensitizers, and outlines the steps which will be required to advance GNP-enhanced radiation therapy from their current preclinical setting to clinical trials in the near future.
In 2012 BC Cancer became only the second institution worldwide to offer a novel radiation treatment for early stage breast cancer called Permanent Breast Seed Implant (PBSI). Briefly, patient specific treatment plans are designed that use strands of radioactive Pd-103 sources to deliver a prescription dose of 90Gy to the target region. Treatment consists of an operation where needles, pre-loaded with the planned sources, are used to permanently implant the Pd-103 into the breast guided by a template grid and freehand ultrasound imaging. Completed in a single outpatient session, PBSI is highly attractive to breast cancer patients compared to the standard 3.5 – 5 weeks of whole breast radiation therapy. However, at present this implant technique is technically challenging and largely limited to implementation by highly experienced clinical practitioners. Our research team is focused on developing tools and techniques aimed at reducing technical barriers to PBSI, with the goal of helping to enable widespread assess to breast seed implant for breast cancer patients. This talk will introduce PBSI and our experience with this advanced radiation treatment technique, present dosimetry results for our first 50 patients, and describe ongoing research efforts in developing 3D ultrasound for implant guidance and standardized, relevant dose reporting.
CHIME will use Intensity Mapping of the 21cm line of neutral hydrogen to map
large-scale structure between redshifts of 0.8 and 2.5. By measuring Baryon
Acoustic Oscillations (BAO) we will place constraints on the dark energy
equation of state as it begins to dominate the expansion of the Universe,
particularly at redshifts poorly probed by current BAO surveys.
In this talk I will introduce CHIME, a transit radio interferometer designed
specifically for this purpose. I will discuss the promise and pitfalls of
Intensity Mapping and describe how we plan to confront the many challenges of
such observations, in particular removal of astrophysical foregrounds which are
six orders of magnitude larger than the 21cm signal. CHIME started operating at
the DRAO in Penticton, BC at the end of 2018 and I will report on current
progress and lessons already learned.
Data analysis of the cosmic microwave background is often performed by making a map of the sky and then converting that map into its spherical harmonics. This is done because the expectation is that the spherical harmonic coefficients are Gaussian random variables, and the data agrees with this prediction extremely well. In this talk I will discuss a less often used map space approach. The approach requires finding extremal points in a map and stacking images of the CMB temperature and polarization around these points. In general this approach has equivalent constraining power to harmonic approaches though it deals with systematic effects in different ways. I show the simplicity and utility of this technique using specific models. I further extend this approach to include (never before used) saddle points and demonstrate that in certain cases they can be more constraining than extremal points.
I will discuss the current status and future prospects of searching for cosmic microwave background signatures of cosmic birefringence, primordial magnetic fields and cosmic strings, and implications for fundamental physics.
We argue that the presence of an inflationary epoch is a natural, almost unavoidable, consequence of the existence of a sensible effective action involving an infinite tower of higher-curvature corrections to the Einstein-Hilbert action. No additional fields besides the metric are required. We show that a family of such corrections giving rise to a well-posed cosmological evolution exists and automatically replaces the radiation-dominated early-universe Big Bang by a singularity-free period of exponential growth of the scale factor, which is gracefully connected with standard late-time ΛCDM cosmology. The class of higher-curvature theories giving rise to sensible cosmological evolution share additional remarkable properties such as the existence of Schwarzschild-like non-hairy black holes, or the fact that, just like for Einstein gravity, the only degrees of freedom propagated on the vacuum are those of the standard graviton.
The diffusion of macromolecules in cells and in complex fluids is often found to deviate from simple Fickian diffusion, and to have a strong dependence on lengthscale. Yet protein diffusion measurements usually only probe a narrow range of lengthscales. To circumvent this issue, we use variable-lengthscale fluorescence correlation spectroscopy (VLS-FCS), where the size of the volume of observation is varied over several orders of magnitude. We combine it with a numerical inversion of the data allowing to retrieve the mean-squared displacement of the process over over up to five decades in time. We performed computer simulations to examine the signature of several biologically relevant diffusion processes (simple diffusion, continuous-time random walk, caged diffusion, obstructed diffusion, two-state diffusion and diffusing diffusivity) in inverted VLS-FCS. We compare the results of our simulations to the diffusion of probes in gels and crowded polymer solutions. Although in both cases the diffusion is anomalous, it has astonishingly different characteristics in these two systems. Our work shows that even for noisy diffusion processes such as the ones encountered in cells, an unbiased discrimination between different classes of diffusion models is possible.
One of the basic characteristics of linear dsDNA molecules is its persistence length, typically of order 50 nanometers. The DNA chain inflicts a large energy penalty if it is bent sharply at that length scale. Viruses of bacteria, known as bacteriophage, typically have a dimension of a few tens of nanometers, of similar order of the magnitude as the DNA persistence length. Yet, it is known that a bacteriophage actively packages viral DNA inside the capsid and ejects it afterwards. The packaging process works under some extreme physical conditions: reduction of the DNA overall conformational entropy, competition between the persistence length and the capsid's size, and the relatively crowded density inside a packaged capsid to accommodate a long DNA chain. Here, adopting a commonly used polymer model known as the wormlike chain, we answer an idealized question: placing a linear DNA molecule inside a spherical cavity, what ordered states can we drive from known tools in statistical physics? Solving the model in a rigorous field-theory framework, we report a universal phase diagram for four orientationally ordered and disordered states, in terms of two relevant physical parameters.
Stochastic thermodynamics of mesoscale systems has been extensively studied using a dragged colloidal particle in a trap as a model system. The colloid is assumed to be a massive Brownian particle that undergoes stochastic motion governed by a Langevin-type equation. The Langevin description of the colloidal motion however completely ignores the hydrodynamic coupling of the colloid to the heat bath. Here, we present results for the stochastic thermodynamics of a dragged nanocolloid hydrodynamically coupled to a fluid heat bath. The motion of the colloidal nanoparticle is modeled using a hybrid fluctuating lattice Boltzmann (FLB) and molecular dynamics (MD) method that accounts for full nonlinear hydrodynamic effects [1, 2, 3]. The mesoscopic FLB-MD method employed in our simulations for the dragged colloidal particle in a trap allows testing of the fluctuation theorems in the truly transient regime of system evolution [1, 3]. FLB-MD also explicitly allows a study of the irreversible work done on the colloid-solvent system in the form of heat dissipation or entropy production in the fluid that has so far been impossible to achieve even in experiments.
References
With the advent of robust nanoscale fabrication, the study of soft matter has become intertwined with nano- and microfluidic device design. Applications range from building nanoscale devices with which to isolate single biomolecules, to studying colloidal particles in nano- to microchannels. Modeling and simulation of soft matter have followed this shift and have become powerful tools both for characterizing the biological material and for aiding in the design of the devices. In such work, common tasks include solving electric fields in complicated geometries and extracting quantities of interest such as the mean first passage time of polymers or particles moving through the devices. Traditional approaches to solving these problem include mesh-based techniques such as the finite element method for electric fields, and particle simulations such as Langevin dynamics for the mean first passage time. In this talk, I will present work from my lab in which we use deep neural networks to replace both of these traditional methods. The “slit-well” device will be used as an exemplar system. I will discuss the benefits of this machine learning approach, such as being mesh-free and parameterizable, as well as some of the remaining challenges.
In this talk, I will review my experience working with students at Queen's University to develop an open access textbook aimed at introductory calculus-based physics. In particular, I will discuss how we worked towards designing a text that is better adapted for the flipped classroom approach than current offerings, and how we managed the process of developing the text. While experimental physics is arguably half of the discipline, introductory textbooks tend to ignore this aspect of skill development completely; I will also discuss how we have incorporated a curriculum in experimental physics within the textbook and report on our preliminary experience using this new textbook during the last academic year. Finally, I will briefly report on two additional open source projects that we developed with our students in order to: (1) provide a free in-class response system in support of active learning, and (2) support the development of computer literacy in the labs.
We introduce ComPAIR, an open source, peer feedback and teaching technology developed at UBC that provides students a safe, flexible environment to develop the skill of evaluating another person’s work, and in turn, receive evaluations from their peers. We highlight its usage in a 300 level physics class.
The effectiveness of peer feedback can be limited by the relative newness of students to both the course content and the skills involved in providing good feedback. ComPAIR makes use of students’ inherent ability and desire to compare: according to the psychological principle of comparative judgement, novices are much better at choosing the “better” of two answers than they are at giving those answers an absolute score. By scaffolding peer feedback through comparisons, ComPAIR provides an engaging, simple, and safe environment that supports two distinct outcomes: 1) students learn how to assess their own work and that of others in a way that 2) facilitates the learning of subtle aspects of course content through the act of comparing.
To explore ComPAIR check out our sandbox site: https://compairdemo.ctlt.ubc.ca/
Details on how to set up ComPAIR at your own institution can be found here: https://lthub.ubc.ca/guides/compair/
In a series of experiments designed to engage students in reflective thinking in exam-like situations, we test the effectiveness of two different interventions in improving student performance on specific short-answer questions. These interventions (1) ask students to explain their answer after answering a short-answer question, or (2) pair questions together for concepts that students frequently confuse for each other. For both interventions we demonstrate that we are able to increase student performance on those specific questions. These results have implications for the design of short-answer physics questions in learning and assessment situations.
Gauss’s Law is one of the most elegant and powerful relationships in physics, but should it be taught in first year? On the one hand, it provides the framework to truly understand electric fields, is an entry point to Maxwell’s equations, and enables a beautiful introduction to important ideas of flux and symmetry. On the other hand, in first year students will not have the vector calculus background for Gauss’s law. Is it possible we do more more harm than good by introducing Gauss’s law too soon in the undergraduate curriculum? The session will feature a debate between physicists Robert Hawkes and Bob Hawkes on the question. The audience will be encouraged to add their voices on either side of the debate. At the conclusion the audience will be asked to vote.
When teaching the law of conservation of linear momentum to first-year university or senior high school students, it is quite popular to use the term "isolated system" to describe the condition of the conservation law. That is, the law of conservation of momentum is stated as that the momentum of an isolated system is constant. Our investigation suggests that the term isolated system, though useful if properly defined, can cause confusion, and furthermore may create a conceptual barrier in understanding and applying the law of conservation of momentum.
Keywords: Physics Education, Conservation of momentum, isolated system
The effects of hyper-resistivity on nonlinear tearing modes [1], on the nonlinear peeling-ballooning modes [2], and on magnetic reconnection in astrophysical plasma [3] was investigated. In this presentation, the nonlinear tearing modes with resistivity and hyper-resistivity H has been analytically investigated. In contrast to the flux average method used by previous works, the quasilinear method [4] has been extended to obtain the time evolution equation for nonlinear tearing modes.
Starting from the reduced MHD equations, the mode equation for nonlinear tearing mode including resistivity and hyper-resistivity can be derived by using the perturbation theory and boundary layer theory. The nonlinear evolution equation can be obtained by using the Fourier transforms and the asymptotic matching condition. This equation could describe the time evolution of nonlinear tearing modes with resistivity and hyper-resistivity. However, it is very difficult to get its analytical solution directly. At moment, two solutions for limiting cases and one solution for general case are presented here.
For the case where resistivity dominates in the inner region, one obtains , while for the case which is dominated by hyper-resistivity, one has [5]. For the general case where hyper-resistivity and resistivity are in the same order, recently we obtain a time evolution of perturbed magnetic flux as .
It is found that the destabilized effect of hyper-resistivity is lesser than that of resistivity on the growth of nonlinear tearing mode.
[1] P. K. Kaw, E. J. Valeo and P. H. Rutherford, Phys. Rev. Lett. 43 (19), 1398-1401 (1979).
[2] X. Q. Xu, B. Dudson, P. B. Snyder, M. V. Umansky and H. Wilson, Phys. Rev. Lett. 105 (17), 175005 (2010).
[3] G. Vekstein, Phys. Plasmas 23 (2), 024502 (2016).
[4] D. Li, Physics of Plasmas 2 (9), 3275-3281 (1995).
[5] W. Yang, D. Li, and X. Q. Xu, Chin. Phys. Lett. 35(6), 065201(2018).
A reasonably complete understanding of laser-plasma instabilities in directly-driven inertial confinement fusion has been gained over the past few years by a combination of advances in theory, model development and experiment. It is seen that, by application of laser bandwidth at the 1% level, prospects of ignition on a MJ-scale facility are much improved. I will describe the physical models that have led to these conclusions and the possibility of testing the results in near future. These tests will involve focused laser-plasma interaction experiments using the ultra-broadband laser program underway at the Laboratory for Laser Energetics (University of Rochester, NY, United States). I will also describe recent work that is exploring the use of structured, orbital angular momentum carrying, laser beams to achieve similar mitigating effects on existing laser facilities that lack a broad bandwidth capability.
The goal of our recent work is to determine the influence upon the fundamental physical properties of aromatic systems of various factors, notably the role of molecular size, heteroatoms, chemical functionalisation (amino, carbonyl, etc), and charge upon the molecule’s optical properties. These questions are addressed by performing electronic spectroscopy and mass spectrometry on functionalised polycyclic aromatic hydrocarbon (PAH) molecules in varying charge states in a cryogenically-cooled ion trap. Experiments are supported by calculations of ground and excited state geometries and energetics.
We illustrate the principle of our combined experimental and theoretical studies using examples of simple functionalised PAHs, from which we derive general laws that can subsequently be applied to larger molecules. This approach allows us to predict the optical properties of molecules too large to study experimentally and/or theoretically.
Noble, J.A.; Dedonder, C.; Jouvet, C., 2015, Astronomy & Astrophysics, 577, 79. doi: 10.1051/0004-6361/201425493
Noble, J.A.; Dedonder-Lardeux, C.; Mascetti, J.; Jouvet, C., 2017, Chemistry – an Asian Journal, 12, 1523. doi: 10.1002/asia.201700327
Noble, J.A.; Broquier, M.; Grégoire, G.; Soorkia, S.; Pino, G.; Marceca, E.; Dedonder-Lardeux, C.; Jouvet, C., 2018, Physical Chemistry Chemical Physics, 20, 6134. doi: 10.1039/C8CP00218E
John D'Auria had a distinguished career in nuclear structure and astrophysics, including pioneering work on the production and exploitation of beams of exotic unstable nuclei to expand the horizons of this field into new realms. His work and influence played a major role in promulgating the excitement of research on unstable nuclei and in propelling today's proliferation of major facilities for this research in Canada, the US, Europe, and Asia. I have known John since we were graduate students together at Yale in the mid-1960's. This talk will contain some reminiscences over this half-century plus span of friendship and reflections on his achievements and legacy.
The continuous 480 MeV proton beam from the TRIUMF H$^{-}$ cyclotron offers a unique potential for the production of rare isotopes via spallation, fragmentation and fission reactions. It was first investigated with the TISOL (Test Isotope Separator On-Line) facility, a project of John d’Auria$^{1}$. This ground-breaking work cleared the path for the present ISAC (Isotope Separation and ACceleration) and future ARIEL (Advanced Rare IsotopE Laboratory) facilities at TRIUMF$^{2}$.
Currently, the ISAC facility provides a wide range of isotopes$^{3}$ for basic research in the fields of nuclear astrophysics, nuclear structure and material science by irradiating targets containing a variety of refractory materials. Reaction products are extracted from the target via diffusion and effusion, ionized and transported through a network of electrostatic beamlines to the experiment. The isotope of interest determines the choice of target material and ion source.
A more recent application is the generation of pure exotic isotope samples from proton-irradiated targets for pre-clinical medical research towards therapeutic and diagnostic applications$^{4}$.
While isotopes delivered to online experiments can have half-lives as short as a few milliseconds, radioactive samples collected for offline medical and radiochemistry studies at the ISAC Implantation Station typically have half-lives in the range of hours to days.
The focus has been so far on isotopes for targeted alpha therapy (TAT) from composite uranium carbide targets. Samples of $^{225}$Ac, $^{224}$Ra and $^{209/211}$At (generated from $^{213}$Fr and $^{211}$Fr beams) have been collected. In a new development, alternative isotopes for TAT and Auger Therapy are collected from high-power tantalum metal foil targets which provide high-intensity lanthanide beams$^{3}$. In a first proof-of-principle test, a $^{165}$Tm/Er sample was collected and characterized.
TRIUMF's neutral atom trap (TRINAT) was started as a good match for surface-ionized alkali elements produced at TISOL (TEST/TRIUMF Isotope Separator On-line).
TRINAT had two main goals-- beta-neutrino correlations, and the chemistry of francium championed by John D'Auria.
Relativity makes francium's properties harder to extrapolate from the lighter alkali elements; e.g., the ionization potential is higher than cesium's due to a more deeply bound S ground-state orbital,