Speaker
Dr
Sunniva Siem
(University of Oslo)
Description
The Oslo group has developed a technique to measure with high precission the level density from
the ground state up to the neutron binding energy. The method provides simultaneously the level density
and gamma-ray strength function in one and the same experiment. After establishing the level density
as a function of excitation energy, the entropy is known and we can explore various thermodynamical
parameters of the nucleus. The caloric curve, derived within the framework of the micro-canonical ensemble,
shows structures, which we associate with the break up of nucleon pairs. And the nuclear heat capacity
deduced within the framework of the canonical ensemble exhibits an S-shape as function of temperature,
indicating a phase transition.
Nuclear level densities and gamma strength functions are input parameters in large network
calculations of stellar evolution, and in the simulation of accelerator-driven transmutation of nuclear
waste. I will discuss the evolution of the level density and radiative strength function as one moves from
the well deformed Dy, Er and Yb nuclei to the close to spherical Sm nuclei. A pygmy resonance at around
3 MeV has been observed in several deformed rare earth nuclei and vanishes for the spherical nuclei.
This is as expected for a scissors mode pygmy resonance. Results from Oslo combined
with a thermal neutron capture experiment analysing two-step cascades finally establish the M1 multipolarity
of this pygmy resonance. Preliminary result for Sm will also be shown and a discussion of what happens to
the level density and the radiative strength function of samarium isotopes as one approaches and crosses the
N=82 closed shell.
Primary author
Dr
Sunniva Siem
(University of Oslo)
Co-authors
Dr
Alexander Voinov
(Ohio University)
Dr
Andreas Schiller
(Michigan State University)
Ms
Ann-Cecilie Larsen
(University of Oslo)
Prof.
John Rekstad
(University of Oslo)
Prof.
Magne Guttormsen
(University of Oslo)
Mr
Naeem U.H. Syed
(University of Oslo)
Dr
Rositsa Chankova
(University of Oslo)
