Broken axial symmetry as essential feature to predict radiative capture in heavy nuclei

TL;DR

This paper develops a global model for predicting neutron capture cross sections in heavy nuclei by incorporating broken axial symmetry, leading to accurate predictions across many nuclei relevant for astrophysics and waste transmutation.

Contribution

It introduces a novel parameterization that accounts for broken axial symmetry in level density and photon strength, using few parameters and based on a global fit to IVGDR data.

Findings

Level densities match nuclear matter values for small spins.

Significant collective enhancement due to symmetry breaking.

Reliable cross section predictions outside the valley of stability.

Abstract

Cross sections for neutron capture in the range of unresolved resonances are predicted for more than 140 spin-0 target nuclei with A > 50. Allowing the breaking of spherical and axial symmetry in nearly all these nuclei a combined parameterization for both, level density and photon strength is obtained which employs a surprisingly small number of parameters only. The strength functions used are based on a global fit to IVGDR shapes by the sum of three Lorentzians. They are based on theoretical predictions for the A-dependence of pole energies and spreading widths and add up to the TRK sum rule. For the small spins reached by capture resonance spacings are well described by a level density parameter close to the nuclear matter value; a significant collective enhancement is apparent due to the deviation from axial symmetry. Reliable predictions for compound nuclear reactions also outside the valley of stability (as important for nuclear astrophysics and for the transmutation of nuclear waste) are expected to result from the global parameterization presented.