Speaker
Mathis Wiedeking
(iThemba LABS)
Description
As excitation energy increases towards the particle separation energy, the level density increases rapidly, creating the quasi-continuum. Nuclear properties in this excitation energy region are best characterized using statistical quantities, such as the photon strength function (PSF) which is the ability of atomic nuclei to emit and absorb photons.
For several Fe isotopes an unexpected increase in the PSF has been observed [1] for which the probability to decay with low energy gamma-rays was found to be more than an order of magnitude larger than predicted by theories. Similar observations have since been reported in other isotopes across the nuclear chart.
The enhancement of low energy gamma-ray emission is controversial since it changes our view of how the nucleus emits gamma-rays in the region of high level density. No conclusive theoretical results exist which can reproduce or explain these experimental observations although different models suggest that the low-energy enhancement may be due to: 1) transitions within the single particle continuum producing E1 radiation [2], and 2) a reorientation of the spins of high-j neutron and proton orbits producing M1 transitions [3].
Although, the underlying mechanism remains largely unexplored and unknown the enhancement has nonetheless a dramatic impact on elemental formation in the universe. It has been shown that this modification to the low energy part of the PSF can have order of magnitude effects on the neutron capture rates of very neutron-rich nuclei which lie in the r-process path [4].
Unfortunately, our knowledge of the PSF comes exclusively from measurements using stable beams and targets which is then extrapolated to the neutron-rich regime where r-process nucleosynthesis takes place. Low-energy PSF measurements have never been performed in nucleonic systems with very asymmetric proton to neutron ratios and the persistence of the enhancement is only based on assumptions. Measurements in the neutron-rich regime are not only of fundamental interest to nuclear physics research but have wide reaching consequences in the field of nuclear astrophysics. In light of many open questions and their importance, a proposal has been submitted and was subsequently accepted by the INTC to run an HIE-ISOLDE experiment on 66,68Ni and investigating statistical decay properties of these nuclei.
In this talk I will give an overview of the current status on our understanding of the low-energy part of the PSF, its impact on nucleosynthesis, and discuss our plans to measure the PSF at HIE-ISOLDE.
*Supported by the National Research Foundation of South Africa.
[1] A. Voinov et al., Phys. Rev. Lett 93, 142504 (2004)
[2] E. Litvinova et al., Phys. Rev. C 88, 031302(R) (2013)
[3] R. Schwengner et al., Phys. Rev. Lett. 111, 232504 (2013)
[4] A.C. Larsen and S. Goriely, Phys. Rev. C 82, 014318 (2010)
Author
Mathis Wiedeking
(iThemba LABS)
Co-authors
Dr
Alexander Voinov
(Ohio University)
Andreas Goergen
(U)
Ann-Cecilie Larsen
(University of Oslo (NO))
Dr
Darren Bleuel
(Lawrence Livermore National Laboratory)
Dr
Dinesh Negi
(iThemba LABS)
Dr
Eda Sahin
(University of Oslo)
Francesca Giacoppo
(University of Oslo (NO))
Frank Leonel Bello Garrote
(U)
Gry Merete Tveten
(University of Oslo)
Hilde-Therese Nyhus
(U)
Prof.
John Rekstad
(University of Oslo)
Dr
Jon Wilson
(IPN Orsay)
Dr
Lee Bernstein
(Lawrence Livermore National Laboratory)
Ms
M. Klintfjord
(University of Oslo)
Prof.
Magne Guttormsen
(Department of Physics, University of Oslo, Norway)
Prof.
P Hoff
(University of Oslo)
Prof.
Peter Butler
(University of Liverpool (GB))
Ms
Sunniva Rose
(University of Oslo)
Dr
Sunniva Siem
(University of Oslo)
Tomas Kvalheim Eriksen
(University of Oslo)
Ms
Trine Hagen
(University of Oslo)
Mr
Vincent Kheswa
(iThemba LABS)
