Implementation of the Moments Method in TALYS: Sensitivity of $^{56,57}$Fe$(n,γ)$ observables to shell model nuclear level densities

Abstract

The Moments Method (MM), a statistical spectroscopy framework built upon configuration interaction shell model Hamiltonians that enables shell model descriptions of nuclear level densities without requiring full diagonalization of the many body Hamiltonian, is implemented in the TALYS reaction code to investigate the sensitivity of Hauser-Feshbach calculations for the $^{56}$Fe$(n,γ)^{57}$Fe and $^{57}$Fe$(n,γ)^{58}$Fe reactions to the nuclear level density input. Spin- and parity-dependent MM level densities calculated in the pf shell are interfaced with TALYS and compared with the phenomenological and microscopic level density models available in the code. To construct TALYS-ready inputs, the MM level densities are extended to higher excitation energies using a back shifted Fermi gas continuation together with an experimentally constrained prescription for opposite-parity states. A sensitivity band for the MM calculations is estimated by varying a single parameter governing the statistical reconstruction of the level density. Neutron capture cross sections, Maxwellian averaged cross sections, and reaction rates are compared with evaluated ENDF/B-VIII.0 data and recommended KADoNiS and JINA-CEE values to assess the relative importance of the nuclear level density and $γ$-ray strength function inputs. The calculations show that the observables are considerably more sensitive to the choice of $γ$-ray strength function than to the MM level density variation considered here, while the MM-based results remain in good agreement with evaluated cross sections and recommended astrophysical reaction rates.