Direct microscopic calculations of nuclear level densities in the shell model Monte Carlo approach

TL;DR

This paper introduces a new method within the shell model Monte Carlo approach to directly calculate nuclear level densities, enabling better comparison with experimental data across various nuclei and energy ranges.

Contribution

The authors develop a projection technique to compute level densities directly, removing spin degeneracy effects and aligning theoretical calculations with experimental measurements.

Findings

Excellent agreement with experimental level densities across multiple nuclei.

Accurate reproduction of low-energy level counting and resonance data.

Consistent results with Oslo method and fluctuation analysis at higher energies.

Abstract

Nuclear level densities are crucial for estimating statistical nuclear reaction rates. The shell model Monte Carlo method is a powerful approach for microscopic calculation of state densities in very large model spaces. However, these state densities include the spin degeneracy of each energy level, whereas experiments often measure level densities in which each level is counted just once. To enable the direct comparison of theory with experiments, we introduce a method to calculate directly the level density in the shell model Monte Carlo approach. The method employs a projection on the minimal absolute value of the magnetic quantum number. We apply the method to nuclei in the iron region as well as the strongly deformed rare-earth nucleus $^{162}$Dy. We find very good agreement with experimental data including level counting at low energies, charged particle spectra and Oslo method at intermediate energies, neutron and proton resonance data, and Ericson's fluctuation analysis at higher excitation energies.