Nuclear level density in the statistical semiclassical micro-macroscopic approach

TL;DR

This paper develops a semiclassical micro-macroscopic approach to derive nuclear level densities, incorporating shell effects, interactions, and deformation, and compares theoretical results with experimental data across various nuclei.

Contribution

It introduces an extended semiclassical method for calculating nuclear level densities that accounts for shell structure, deformation, and interactions, extending to low energies.

Findings

The MMA expressions match known asymptotes at high and low energies.

Fitted inverse level density parameter K varies significantly across nuclei.

Shell, isotopic, and pairing effects are prominent at low excitation energies.

Abstract

Level density $\rho$ is derived for a finite system with strongly interacting nucleons at a given energy E, neutron N and proton Z particle numbers, projection of the angular momentum M, and other integrals of motion, within the semiclassical periodic-orbit theory (POT) beyond the standard Fermi-gas saddle-point method. For large particle numbers, one obtains an analytical expression for the level density which is extended to low excitation energies U in the statistical micro-macroscopic approach (MMA).The interparticle interaction averaged over particle numbers is taken into account in terms of the extended Thomas-Fermi component of the POT. The shell structure of spherical and deformed nuclei is taken into account in the level density. The MMA expressions for the level density $\rho$ reaches the well-known macroscopic Fermi-gas asymptote for large excitation energies U and the finite combinatoric power-expansion limit for low energies U. We compare our MMA results for the averaged level density with the experimental data obtained from the known excitation energy spectra by using the sample method under statistical and plateau conditions. Fitting the MMA $\rho$ to these experimental data on the averaged level density by using only one free physical parameter - inverse level density parameter K - for several nuclei and their long isotope chain at low excitation energies U, one obtains the results for K. These values of K might be much larger than those deduced from neutron resonances. The shell, isotopic asymmetry, and pairing effects are significant for low excitation energies.