Assessing the role of the (n,γf) process in the low-energy fission of actinides

Patrick Talou,T Kawano,A Couture,J E Lynn,O Bouland,S Mosby

doi:10.1051/epjconf/201612201013

Abstract

We review the role of the (n , γ f ) process in the low-energy neutron-induced fission reaction of 239 Pu. Recent measurements of the average total γ -ray energy released in this reaction were performed with the Detector for Advanced Neutron Capture Experiments (DANCE) at Los Alamos. Significant fluctuations of this quantity in the resonance region below 100 eV can be interpreted by invoking the presence of the indirect (n, γ f ) process. Modern calculations of the probability for such an event to occur are presented.

Highlights

The possibility for a heavy compound nucleus formed through low-energy neutron-induced reactions to emit a γ ray before fissioning was postulated theoretically long ago by Stavinsky and Shaker [1]
The Detector for Advanced Neutron Capture Experiments (DANCE) at the Los Alamos Neutron Science Center (LANSCE) is a 4π-calorimeter that consists of 160 BaF2 crystals, each 734 cm3 in volume with faces located 19 cm from the beam center
Very little information on the pre-fission γ-ray spectrum is available even though it is important to infer the particular nature of the γ transitions preceding fission

Summary

Introduction

The possibility for a heavy compound nucleus formed through low-energy neutron-induced reactions to emit a γ ray before fissioning was postulated theoretically long ago by Stavinsky and Shaker [1]. MeV, γ rays can be emitted leaving the nucleus with enough residual excitation energy to overcome the fission barrier, leading to an additional (n, γ f ) channel. Because the average neutron multiplicity ν plays such an important role in nuclear technologies, as for instance in the very accurate simulation of critical assembly and/or nuclear reactor benchmarks, a small change in ν has important practical consequences, as noted in [3, 4] Such fluctuations of ν have been observed already, as shown in Fig. 1 for incident energies up to. Besides the (n, γ f ) process, fluctuations in the particular fragment yields in mass and charge could lead to fluctuations in the number of emitted neutrons [5] Such an interpretation would not explain fluctuations in the observed total γ-ray energy, as will be explained below

Theoretical Framework

DANCE Experiment and Simulations

Results and Discussion

Conclusion