The HISPEC-DESPEC collaboration aims at studying the evolution of the shell structure and
exotic nuclear shapes in uncharted nuclear territory, providing spectroscopic information for the
nucleosynthesis of medium to heavy nuclei, exploiting the uniqueness of the GSI-FAIR laboratory.
In this first years after the restart of GSI, starting from early commissioning in 2019 to real
experiments...
Beta-decay rates are key quantities to understand both nuclear structure properties as well as the dynamics of nucleosynthesis processes. However, microscopic calculations based on the evaluation of nuclear matrix elements using eigenstates of the mother and daughter nuclei separately are scarce.
From a beyond-mean-field point of view, one of the main problems is the evaluation of the...
$^{71}$Kr was produced through the fragmentation of a $^{78}$Kr primary beam at the RIKEN-RIBF facility in Japan, in order to have the first comprehensive study of its $\beta$-decay leading to its mirror counterpart ($^{71}$Br). The $\beta$-decay of $^{71}$Kr has a significance from the astrophysical point-of-view, as it is a waiting point of the rp-process [1]. The question of the ordering...
A better quantitative understanding of $\beta$-delayed neutron emission rates and spectra is relevant for nuclear structure, astrophysics, and reactor applications. The field has experienced an increased activity during the last decades [1] thanks to the advances in nuclear experimental techniques and the radioactive ion beam facilities. More accurate measurements of $\beta$-delayed neutron...
S. Alhomaidhi$^{\mathrm{1,2}}$, V. Werner$^{\mathrm{1,2}}$, P.-A. Söderström$^{\mathrm{1,12}}$, P. R. John$^{\mathrm{1}}$, U. Ahmed$^{\mathrm{1}}$, H. M. Albers$^{\mathrm{2}}$, C. Appleton$^{\mathrm{6}}$, T. Arıcı$^{\mathrm{2}}$, M. Armstrong$^{\mathrm{2,4}}$, A. Banerjee$^{\mathrm{2}}$, J. Benito$^{\mathrm{11}}$, A. Blazhev$^{\mathrm{4}}$, P. Boutachkov$^{\mathrm{2}}$, A. Bozo$^{\mathrm{5}}$,...
Interpreting high-energy, astrophysical phenomena, such as supernova explosions or neutron-star collisions, requires a robust understanding of matter at supranuclear densities. However, our knowledge about dense matter explored in the cores of neutron stars remains limited. Fortunately, dense matter is not only probed in astrophysical observations, but also in terrestrial heavy-ion collision...
The phenomenon of quarteting in even-even $N=Z$ nuclei has a
long history in nuclear structure [1]. In spite of that, several unexplored and yet interesting aspects of this phenomenon have come to light only in recent years. In Ref. [2], we have evidenced on analytic grounds the key role played by the isovector pairing in the phenomenon of nuclear quarteting. We have indeed shown that...
The interpretation of the emergent collective behavior of atomic nuclei in terms of deformed intrinsic shapes is at the heart of our understanding of the rich phenomenology of their structure, ranging from nuclear energy to astrophysical applications across a vast spectrum of energy scales. A new window into the deformation of nuclei has been recently opened with the realization that nuclear...
The ISOLDE Decay Station (IDS) [https://isolde-ids.web.cern.ch/] was designed as a flexible tool for decay spectroscopy studies, operating since 2014 at ISOLDE. At the core of IDS there are 4-6 HPGe clovers to detect $\gamma$ rays with high energy resolution together with a moving tape system and a complex array of ancillary detectors such as LaBr$_3$:Ce crystals to measure excited-state...
The r-process produces roughly half of all nuclei heavier than iron, thus understanding the mechanism in which these nuclei are produced is an important topic of research. Properties of nuclei with magic numbers of neutrons are key to understanding the r-process. N=82 nuclei below $^{132}$Sn are connected to the mass abundance peak at A~130. In addition, studies of nuclei in this difficult to...
Various studies have shown that the proton-neutron (pn) pairing correlations can be accurately described not by a condensate of Cooper pairs, as considered in the majority of mean-field calculations [1, 2], but by a condensate of α-like quartets [3, 4, 5, 6, 7]. After a short review of the quartet condensate model (QCM), I shall discuss the effect of the pn pairing on the ground states of...
High precision mass measurement of exotic nuclei play an important role in shaping our understanding of the nucleus. It has become evident that the structure of the nucleus can change away from the valley of beta stability; new phenomena, as e.g. shell quenching, weakening or disappearance of classical and appearance of new magic numbers have been observed via characteristic signatures in the...
In nuclei along the N = Z line, as protons and neutrons occupy the same valence orbitals, proton-neutron correlation properties and quadrupole-quadrupole interactions emerges. In heavy even N = Z nuclei the competition between prolate and oblate quadrupole coherence is hitherto not measured. Well-developed deformation in the upper $fpg$ shell starts from $^{68}$Se. In $^{68}$Se, the intrinsic...
The $^{18}$O$+^{48}$Ti reaction was studied at 275 MeV incident energy for the first time under the NUMEN [1] experimental campaign with the main goal of investigating the $^{18}$O$(^{48}$Ti,$^{48}$Ca$)^{18}$Ne double charge exchange (DCE) process. The measurements were performed at the INFN-LNS in Catania, using the MAGNEX large acceptance magnetic spectrometer [2]. To fully understand the...
Direct reactions are fundamental tools to investigate the structure of exotic nuclei. Studies of nuclei far away from stability are usually performed with secondary radioactive beams, that suffer from low intensities and need to be compensated with thick targets and high efficient detection systems to increase luminosity. Active targets are invaluable devices that, among other important...
The concept of nuclear shape, most often of quadrupole type, is ubiquitous
in nuclear physics. We speak of spherical, prolate, oblate or triaxial nuclei;
of the evolution of the shapes along isotopic or isotonic chains, and of the
coexistence of different shapes in the spectrum of a given nucleus. The
last two phenomena, are closely related and prominent at the very neutron
rich...
A systematic study of neutron-deficient nuclei has been carried out by decay spectroscopy experiments with implanted radioactive ion beams (RIBs) at GANIL and RIKEN. Beta decay has a direct access to the absolute values of the Fermi and Gamow-Teller transition strengths. The comparison with complementary charge exchange reactions, such as the ($^3$He,t) reaction performed on the mirror stable...
Nucleon removal reactions at intermediate energies have proven of great value to extractspectroscopic information from exotic atomic nuclei. For the case of nucleon removal at inter-mediate energies, a trend was noticed in the early 2000s in which cross sections were found tobe significantly overestimated for the removal of deeply-bound nucleons in asymmetric nuclei,while the removal of the...
The concept of isospin has been introduced to explain the apparent exchange symmetry between protons and neutrons. However, if the nuclear force were the same for neutrons and protons properties such as excitation energies and masses would depend only on the mass number A. Recent studies have shown that the Coulomb force cannot account for all deviations, suggesting that other...
Transition probabilities for the yrast $2^+$ states in the midshell Te isotopes, 2 protons above Sn, are theoretically interesting in order to test shell model predictions. However, some values are still missing and some values have an error too large to successfully test shell model parameters. We have measured the lifetime of the $2^+$ to $0^+_{gs}$ transition in neutron deficient...
In the last decade, unprecedented fission experiments have been carried out at the
GSI/FAIR facility using the inverse kinematics technique in combination with state-
of-the-art detectors. For the first time in the long-standing history of fission, it was
possible to simultaneously measure and identify both fission fragments in mass and
atomic numbers [1] and obtain many correlations among...
The shape of nuclei is determined by a fine balance between the stabilizing effect of closed shells and the pairing and quadrupole force that tends to make them deformed. As other well known cases, located in the A = 100 mass region, as Yb, Zr or Nb for example, Sr isotopes are good candidates to study the existence of this nuclear deformation. In particular in this case, particle-hole...
Nuclei around the tin isotopic line have been a recent center of experimental interest, with a diversity of interesting phenomena from alpha decay to neutron skin as well as neutrinoless double beta decay candidates. In the meantime, the recent progress of nuclear ab initio methods based on chiral interactions now allows for meaningful predictions in such heavy systems. We will present here...
The long Sn isotopic chain is a formidable testing ground for nuclear models aiming at describing the evolution of the shell structure. Low-lying excited states roughly exhibits the typical behavior predicted by the generalized seniority scheme. However, the corresponding B(E2; 0⁺→2⁺) values, approaching the N=Z=50 shell closure, have shown a presumed deviation from the expected parabolic...
Nuclear fission of heavy (actinide) nuclei results predominantly in asymmetric mass-splits. Without quantum shells, which can give extra binding energy to these mass-asymmetric shapes, the nuclei would fission symmetrically. The strongest shell effects are in spherical nuclei, so naturally, the spherical "doubly-magic" 132Sn nucleus, was expected to play a major role.
However, systematic...
Very heavy nuclei owe their stability against spontaneous fission to quantum shell effects, which depend on the local density of single-particle states. High densities give rise to positive shell correction, meaning less stability and low densities translate into enhanced stability. Alternating stabilising effects may coexist on the pathway to scission as a function of deformation. In...
During the last ten years, the use of inverse kinematics in the experimental study of fission is bringing a wealth of new observables obtained in single measurements, which allows their analysis and, also importantly, of their correlations [1, 2]. An ongoing application of this technique the basis of a series of experiments performed with the variablemode, large-acceptance VAMOS++ spectrometer...
A thorough understanding of nuclear fission is still an arduous task due to its sudden transition from asymmetric to symmetric division, especially in the actinide mass region (near A=254 to 258). Recently, an attempt has been made to see the effect of compact and elongated configurations of quadrupole (β2) deformed decay fragments on the spontaneous fission of 242-260Fm isotopes using...
Nuclear fission has been used as a tool for the study of nuclear properties since its discovery in 1939. A new approach was performed in the context of the R3B collaboration, at the FAIR facilities, in which knockout reactions were used to induce fission in $^{238}$U, that will allow to characterise the excitation energy of the process. The CALIFA calorimeter, a key part of the set-up, will be...
In the last decade, unprecedented fission experiments have been carried out at the GSI facility using the inverse kinematics technique in combination with state-of-the art detectors, especially designed to measure the fission products with high detection efficiency and acceptance [1,2]. For the first time in the long-standing history of fission, it was possible to simultaneously measure and...
Tremendous efforts are undertaken worldwide to produce secondary beams at radioactive ion beam facilities. Their incarcerating in a dedicated ring or a trap is a straightforward way to achieve the most efficient use of such rare species. In this context, employing heavy-ion storage rings for precision physics experiments with highly-charged ions (HCI) at the intersection of atomic, nuclear,...
The AGATA-MUGAST-VAMOS set-up, which was recently available at GANIL, combined the state-of-the-art gamma-ray tracking array AGATA with the highly-segmented silicon array MUGAST and the large-acceptance magnetic spectrometer VAMOS. The mechanical and electronics integration provided a maximum efficiency for each device. The superb sensitivity of the complete set-up offered a unique opportunity...
Decay studies are of high demand since they complement existing in-beam data and provide a valuable information about non-yrast states. These states are of particular interest since they carry an information on the nuclear deformation. Deformation parameters, both axial and triaxial can be deduced from the spectra of excited states of the odd-mass nuclei. The particle-core coupling models [1]...
The assignment of the first 2$^+$ state in $^{62}$Ga has been a subject of debate in the last decades due to its implications in triplet energy difference systematics in this mass region[1]. To clarify this, an experiment was performed at the IFIN-HH 9-MV Tandem accelerator using the ROSPHERE[2] array in a mixed configuration of LaBr$_3$(Ce), HPGe and liquid scintillator neutron detectors. ...
Shell evolution [1,2] is one of the most discussed topics in the last two-decades in nuclear structure physics. It reveals that the traditional shell structure known for stable nuclei changes when we go towards the neutron-drip line. Recently, the low-energy states of neutron-rich $^{55}$Sc has been populated in an experiment [3], in which the first excited state $(3/2)^-$ has been found near...
In this study, the nuclear process 64Zn(n, a)61Ni induced by fast neutrons on 64Zn nucleus was investigated. Based on the authors' computer programs and Talys codes, cross-sections, angular correlations, and forward-backward asymmetry effects were investigated for incident neutrons with energies from 0.5 MeV to 25 MeV. We investigated how nuclear reaction mechanisms (direct, compound,...
A new solution for the Bohr Hamiltonian is proposed for an infinite square well potential in the axial deformation variable with an adjustable step [1]. The centrifugal contribution transforms the stepped potential into an effective double-pocket potential, making it suitable for treating shape coexistence phenomena [2,3,4]. The general model characteristics are studied as a function of the...
We address the current understanding of the nuclear structure of nuclei to evaluate their stability. The microscopic shel model approach and phenomenological models are applied to predict the α halflives for several yet-unaccessible isotopes of superheavy elements Z=118-122. We outline what might be expected for the nuclear stability from studying:i) couplings between the single particle and...
David Rodríguez García on behalf of DESPEC Collaboration
Our understanding of the production of the heaviest elements in the Universe is still incomplete. In particular the contribution of the rapid neutron capture (r-) process to the observed abundances of elements with A>180 and the astrophysical site for this process is uncertain. Combining astronomical observations (including...
One source of spin of the fragments during the fission process is the induction of a rotation at and after scission. Due to the small initial angle between the deformed fragment and the fission axis, the nuclear and Coulomb interaction creates a torque leading to a rotation of the fragments. The time-dependent Hartree-Fock theory is used to microscopically study this angular momentum...
The term halo nucleus was coined to describe a group of nuclei with an unusually large spatial extension, diverging from the standard $r=roA^{1/3}$. The first empirical observation of this behavior came from scattering experiments of Lithium isotopes [1,2], intended to measure the interaction cross-section of neutron-rich nuclei. As we move closer to the neutron dripline and jump from $^9_3Li$...
In nature several decays occur during which two particles of the same nature are emitted simultaneously, such as double beta decay in which two electrons are emitted together with two antineutrinos or double gamma decay. In addition, close to the proton and neutron drip lines nuclear decays with simultaneous emission of two protons and two neutrons have been observed. Simultaneous double alpha...
The experimental data collected during the S515 experiment performed by R3B collaboration at GSI/FAIR represent a great opportunity to investigate nucleon knockout reactions of exotic nuclei around 132Sn in complete kinematic measurements. These cross sections can be used to investigate the quenching in the knockout of the minority spice (neutrons or protons) in nuclei far from stability [2]....
The marked change in trend of the evolution of the charge radii of some isotopic families of nuclei versus the mass number A is known as kink effect. This is a consequence of the shell structure of nuclei and, obviously, cannot be explained by the droplet model. The fact that the density-dependent Hartree-Fock model with standard Skyrme functionals [1] or Gogny forces [2] were not able to...
