Questioning the 239Pu(n,2n)238Pu cross section shape above emission threshold

TL;DR

This paper uses advanced nuclear reaction modeling to reassess the 239Pu(n,2n)238Pu cross section shape above threshold, suggesting previous evaluations underestimated it below 10 MeV and proposing a correction factor.

Contribution

It introduces an upgraded modeling approach combining AVXSF-LNG and TALYS-ECIS06 to better estimate the (n,2n) cross section and proposes a correction to existing measurements.

Findings

Current evaluations underestimate the cross section below 10 MeV by about 7%.

A correction factor of approximately 1.24 is proposed for recent measurements.

Uncertainty in the model is estimated at 11.6% based on sensitivity analysis.

Abstract

In the light of the JEF(F) European project longstanding story according to the determination of the most exact shape of the 239Pu(n, 2n)238Pu reaction cross section and a recent measurement by Meot et al., this paper aims to shed another light on this topic by bringing new theoretical feedback. To achieve this goal, the AVXSF-LNG computer program has been upgraded to model second-chance reactions using its decay-probability module and, then chained to the TALYS-ECIS06 nuclear reaction system of codes. Present diligent calculation of the (n,2n) cross section over the energy range from the threshold to the onset of third-chance fission at about 12 MeV, suggests that current evaluations under-estimate the 239Pu(n,2n) cross section below 10 MeV; under-estimation of the order of 7% relatively to the JEFF-3.1 evaluation. On this ground, we propose an upward correction to the normalization of the measurement by Meot et al. Correction factor of about 1.24 with a maximum uncertainty on present fitted model estimated to 11.6%. Latter value is extracted from a sensitivity analysis of the calculation route to the level density model that is selected for the non-equilibrated residual nucleus and to alternative choices we can make in terms of neutron fission cross section measurement references for the 238Pu and 239Pu target nuclei.