A semi-empirical systematics for 232–239Np(n,f) cross sections at 4–20 MeV

Abstract

This work presents a compact semi empirical systematics for neutron induced fission cross sections of neptunium isotopes 232-239Np for incident neutron energies from 4 to 20 MeV. The formulation adopts a multi chance fission description in which the total (n,f) cross section is written as a sum of penetrability weighted first, second, and third chance contributions scaled by the non elastic reaction cross section. Three isotope dependent coefficients are obtained by fitting to reproduce the plateau like behaviour associated with the sequential opening of the (n,f), (n,nf), and (n,2nf) channels. The semi empirical systematics is benchmarked against evaluated nuclear data libraries, ENDF/B VIII.1 and TENDL 2023, and against experimental datasets where available. Quantitatively, the agreement is strongest for 234Np, 235Np, 237Np, and 238Np, for which χ2/N remains between 0.001 and 0.006 with respect to ENDF/B-VIII.1 and between 0.006 and 0.036 against experimental data where available, whereas 236Np and 239Np exhibit somewhat larger deviations, with χ2/N values of 0.010 and 0.048 versus ENDF/B-VIII.1 and 0.042 and 0.030 versus experiment, respectively. No EXFOR measurements are available for 232Np and 233Np in this energy range. In parallel, TALYS 2.0 calculations are performed using 15 combinations formed by three level density models, CTM, BSFGM, and GSM, and five fission barrier options, FIS.MODEL 1 to 5. The calculated (n,f) cross sections show a clear dependence on these modelling choices, indicating that additional high precision measurements would be valuable for strengthening model validation and reducing uncertainties in neptunium (n,f) cross section estimates.