Monte Carlo simulation of γ and fission transfer reactions using extended ℛ-matrix theory

Olivier Bouland

doi:10.1051/epjconf/202023903013

Abstract

This paper comes back on the accuracy of the surrogate-reaction method (SRM) historically used for neutron-induced average partial cross sections inference from measured surrogate-reaction probabilities. The SRM level of performance is examined in relation to a reasonably accurate reference calculation performed with the 𝒜𝒱𝒳𝒮ℱ-ℒ𝒩𝒢 code [1] through a challenging test case : the 240Pu* compound system. This paper argues on some ingredients of the reference calculation [2] and returns some hints about the failure now well-known of the neutron-induced γ average cross section inference. It shows also that in some special cases, the SRM can be poorly accurate also in terms of neutron-induced fission average cross section inference.

Highlights

The idea to supplement neutron-induced cross-section data for actinides and higher transuranic nuclides with particle-transfer-induced reactions has been raised a long time ago [3]
Major limitations in neutron fission cross section inference from surrogate-reaction data were promptly noticed [3, 5] with the difficulty to estimate a) the compound nucleus formation cross section by neutron absorption, b) the possible influence of the differences between the angular momentum distributions populated by neutron capture and direct reactions and c) the validity of the Weisskopf-Ewing (WE) hypothesis on reaction decay probability spin-parity independence [6]
The historical surrogate-reaction method (SRM) is based on the following hypotheses: A) the absence of width fluctuation correction factor (WFCF) in the cross section formulation. We know that it plays in low-energy neutroninduced reactions (En < 2 MeV) a major role in averaging over partial width distributions when calculating average cross sections. This was well illustrated in the neutroninduced WFCF model comparisons by Hilaire et al [15], B) the WE hypothesis [6] of reaction decay probability spin-parity independence applied to the BcJ′π (Ec′ ) term, C) the idealized matching between neutron-induced and surrogate-reaction spin-parity entrance distributions ; reading

Summary

Introduction

The idea to supplement neutron-induced cross-section data for actinides and higher transuranic nuclides with particle-transfer-induced reactions has been raised a long time ago [3]. In a previous paper [1], we have enlightened the actual possibility to carry one-dimensional fission barrier extended R-matrix simulations accurate enough to make predictions of low-energy neutron-induced fission cross sections for the isotopes of the Pu family for which no neutron spectroscopy measurements exist This has been accomplished thanks to Monte Carlo (MC) samplings of both first and second well resonance parameters (reaction widths and energies) of the actinide double-humped fission barrier and to model input parameters in part obtained from macroscopic-microscopic nuclear structure calculations [11]. This will give us the opportunity to quantify the reliability of present MC R-matrix technique for confident inference of neutron-induced capture cross sections below the energy range of second chance fission

Recalls on the SRM hypotheses

In-house Monte Carlo reaction decay probabilities algorithm

In-house simulated surrogate-reaction probabilities vs SRM inference

Conclusion