Speaker
Description
The analysis of level densities and photon strengths has been bedeviled by misconceptions and physics errors that have persisted for over 50 years. In this talk I will discuss deficiencies in the Constant Temperature (CT) level density model as described by Gilbert and Cameron. This model describes the “total” level density using a nonphysical back shift parameter. I will show that every spin/parity sequence follows the constant temperature model, but not the total level sequence. With my CT-JPI model the level densities for each spin/parity sequence can be fit with a single constant temperature and back shift parameters for each spin and parity determined from the experimental level sequences constrained by the standard spin distribution function at the neutron separation energy.
A second misconception is that the Giant Dipole Resonance (GDR) is “due to gamma rays exciting a motion in the nucleus in which the bulk of the protons move in one direction while the neutrons move in the opposite direction”. No such collective motion exists in nature and it is readily shown that the energy of the GDR corresponds to the harmonic oscillator shell gap energy and the width of the GDR corresponds to the nuclear deformation. The GDR is due to an increase in level density as a new set of levels appear at the shell gap. I will show that all standard GDR fitting parameters can be accurately calculated as a function of mass (A) and deformation ($\beta_2$). There are also weaker “GDR” peaks that occur at each successive shell gap.
A third misconception is that photon strength can be modeled directly. Photon strength is defined as the product of gamma ray strength and level density. The simple, empirical Brink-Axel (BA) model accurately determines the photon strength. Dividing the BA photon strength by the CT-JPI model level densities determines that the gamma-ray strength up to the at least the neutron separation energy. The gamma-ray strength varies smoothly with energy and can be extrapolated to higher energies. The level density at higher energies can then be determined by dividing the BA photon strength by the extrapolated gamma-ray strength. It is found that the higher energy level densities agree well with the BSFG model.
For more complete information see my preprints
[2104.02693] Spin/Parity Dependent Level Density (arxiv.org)
[2009.03356] The Origin of the Giant Dipole Resonance (arxiv.org)
[2106.09088] Level Densities from 0-30 MeV (arxiv.org).
*Retired