Nuclear state densities of odd-mass heavy nuclei in the shell model Monte Carlo approach

TL;DR

This paper advances the shell model Monte Carlo method to accurately calculate nuclear state densities of odd-mass heavy nuclei by combining level counting and resonance data, achieving results consistent with experiments.

Contribution

It introduces a novel approach to determine ground-state energies in odd-mass nuclei, enabling precise state density calculations using the shell model Monte Carlo method.

Findings

Accurate state densities for odd-mass nuclei were obtained.

Results closely match experimental data.

Method extends applicability to odd-even isotopic chains.

Abstract

While the shell model Monte Carlo approach has been successful in the microscopic calculation of nuclear state densities, it has been difficult to calculate accurately state densities of odd-even heavy nuclei. This is because the projection on an odd number of particles in the shell model Monte Carlo method leads to a sign problem at low temperatures, making it impractical to extract the ground-state energy in direct Monte Carlo calculations. We show that the ground-state energy can be extracted to a good precision by using level counting data at low excitation energies and the neutron resonance data at the neutron threshold energy. This allows us to extend recent applications of the shell-model Monte Carlo method in even-even rare-earth nuclei to the odd-even isotopic chains of $^{149-155}$Sm and $^{143-149}$Nd. We calculate the state densities of the odd-even samarium and neodymium isotopes and find close agreement with the state densities extracted from experimental data.