Intermediate Structures in Fission and Consequences on Average Partial Cross Sections

Abstract

This paper is willing to illustrate and quantify the impact of a double fission barrier in terms of average fission cross section and in particular, to status on both the additional class-II width correction factor and the degree of freedom of the overall double fission barrier which is a key parameter in the calculation of the standard Wn, f factor of Hauser-Feshbach theory. 1 Problematic related to average fission cross section simulation The purpose of this communication is to both recall and emphasize the existence of the class-II state width fluctuation factor, WII , whose treatment is usually disregarded in standard average cross section evaluation codes. The impact of fission channel Intermediate Structures (IS) is well pictured by Fig. 1 which displayed the observed and current evaluated fission cross sections within the neutron-incident Unresolved Resonance energy Range (URR) for a fissile and a fertile isotope borrowed from the plutonium family. Below the fission threshold energy ( En < 500keV), we observe large manifestations of the IS (Fig. 1- right side), relatively to the fission cross section magnitude, although IS e ffects show up as well for fissile isotopes (Fig. 1- left side). We guess that more complexity in the classically (1) used URR fission cross section model will be required for a fertile isotope but are we allowed to make some approximations for a fissile isotope without collateral damage? This is the question raised by this paper. In the next sections, the impact of both the barrier tunneling and WII factor for fissile and fertile isotopes is clarified using a new way of calculating the inextricably admixed IS and WII effects based on accurate formal R-matrix Monte-Carlo-fashion calculations which have been recently applied to the whole plutonium family in a consistent and macro-microscopic approach (2).