Testing the spin-cutoff parameterization with shell-model calculations

TL;DR

This paper evaluates the accuracy of the spin-cutoff parameterization for nuclear level densities using shell-model calculations, confirming its effectiveness when sufficient statistical states are available.

Contribution

It provides a detailed assessment of the spin-cutoff parameterization's validity and introduces methods to extract <J^2> from thermal calculations.

Findings

Spin-cutoff parameterization is accurate with enough states.

The spin-cutoff factor relates to <J^2> at fixed excitation energy.

Error bars for sigma are quantified.

Abstract

The nuclear level density, an important input to Hauser-Feshbach calculations, depends not only on excitation energy but also on angular momentum J. The J-dependence of the level density at fixed excitation energy E_x is usually parameterized via the spin-cutoff factor sigma. We carefully test the statistical accuracy of this parameterization for a large number of spectra computed using semi-realistic interactions in the interacting shell model, with a nonlinear least-squares fit of sigma and finding the error bar in sigma. The spin-cutoff parameterization works well as long as there are enough states to be statistical. In turn, the spin-cutoff factor can be related to the average value of J^2 at a fixed excitation energy, and we briefly investigate extracting <J^2 (E_x)> from a thermal calculation such as one might do via Monte Carlo.