Investigation on Neutron-Induced Fission Fragment Angular Distribution of ²³²Th and ²³⁸U

Abstract

Fission fragment angular distribution (FFAD) data would help obtain new insights into both the fission process and the mechanism of the projectile’s interaction with the nucleus. Recently, a structure has been reported in neutron-induced FFAD of even-even actinide nuclei near threshold. Statistical modelling is used in this article to analyse FFAD from neutron-induced fission of 238U and 232Th. The statistical variance K02 is obtained by fitting the measured fragment anisotropies with a theoretical model. Accurate analysis is performed to deduce the variance K02 of the K-distribution of the levels in the transition nucleus at neutron energies from threshold to 50 MeV. We show the method by which quantitative values of K02 can be obtained. The results not only present high-resolution data in these even-even nuclei but also show that for the 238U(n,f) reaction, the strength for the K-transition states comes mainly from the higher angular momenta, in agreement with the Nilsson model orbitals. We also study the periodic structure of anisotropy related to the set of (n,f) reactions in comparison with the related cross section performed with the TALYS code. The comparison of the variance with the cross section clearly illustrates the strong correlation between the value of the variance and the opening of a fission chance. We show that whenever the probability of reaction in a new channel and cross section increases, K02 decreases versus the incident neutron energy, so the minimum of K02 can show the maximum probability of the (n,f)xn reaction.