Improved microscopic nuclear level densities within the triaxial Hartree-Fock-Bogoliubov plus combinatorial method

TL;DR

This paper introduces an advanced microscopic model for nuclear level densities that incorporates triaxial deformation effects, improving accuracy in predicting neutron resonance spacings and level counts across thousands of nuclei.

Contribution

The study develops a triaxial Hartree-Fock-Bogoliubov plus combinatorial method that accounts for nuclear shape deformations, enhancing the microscopic calculation of nuclear level densities.

Findings

Accurately reproduces experimental neutron resonance spacings.

Provides reliable extrapolations at low energies.

Supplies extensive level density data for over 8500 nuclei.

Abstract

New developments have been brought to our energy-, spin- and parity-dependent nuclear level densities based on the microscopic combinatorial method. Our new calculation is based on the BSkG3 mean-field model which relies on a three-dimensional coordinate-space representation of the nucleus, allowing for the spontaneous breaking of ground state rotational, axial and reflection symmetry. In particular, we now account for the impact of possible triaxial deformation of nuclear ground states on the level density. This has two effects on our calculations: the additional freedom of the single-particle levels affects the intrinsic level density while the absence of a rotational symmetry axis results in a larger collective correction. The present model reproduces the experimental s- and p-wave neutron resonance spacings with a degree of accuracy comparable to that of the best global models available. It is also shown that the model gives a reliable extrapolation at low energies where experimental data on the cumulative number of levels can be extracted. The predictions are also in good agreement with the experimental data extracted from the Oslo method. Total level densities for more than 8500 nuclei are made available in a table format for practical applications. For the nuclei for which experimental s-wave spacings and enough low-lying states exist, renormalization factors are provided to reproduce simultaneously both observables. The same combinatorial method is used to estimate the nuclear level densities at the fission saddle points of actinides and at the shape isomer deformation. Finally, the new nuclear level densities are applied to the calculation of radiative neutron capture cross sections and compared with those obtained with our previous combinatorial model.