Nuclear Level Density: Shell Model vs Mean Field

TL;DR

This paper compares nuclear level density calculations from shell-model quantum chaos methods with traditional mean-field approaches, highlighting differences in smoothness and the impact of residual interactions on thermalization.

Contribution

It introduces a practical algorithm for calculating nuclear level densities without diagonalization, demonstrating its agreement with exact methods and contrasting it with mean-field models.

Findings

Shell-model level densities are smoother than mean-field estimates.

Residual interactions significantly influence thermalization and level density.

Shell-model results align well with exact diagonalization, validating the approach.

Abstract

The knowledge of the nuclear level density is necessary for understanding various reactions including those in the stellar environment. Usually the combinatorics of Fermi-gas plus pairing is used for finding the level density. Recently a practical algorithm avoiding diagonalization of huge matrices was developed for calculating the density of many-body nuclear energy levels with certain quantum numbers for a full shell-model Hamiltonian. The underlying physics is that of quantum chaos and intrinsic thermalization in a closed system of interacting particles. We briefly explain this algorithm and, when possible, demonstrate the agreement of the results with those derived from exact diagonalization. The resulting level density is much smoother than that coming from the conventional mean-field combinatorics. We study the role of various components of residual interactions in the process of thermalization, stressing the influence of incoherent collision-like processes. The shell-model results for the traditionally used parameters are also compared with standard phenomenological approaches.