Narrow Results

Photofission cross-section of ²³⁸U was measured using bremsstrahlung radiation energy 7.4–9.0 MeV with energy step of 0.4 MeV by employing Lexan polycarbonate film as solid state nuclear track detector (SSNTD). The photon intensity from the Microtron accelerator at a distance of 15 cm from the bremsstrahlung converter (tantalum target) facility was estimated to be 10¹⁰ photons/sec using the code EGS-4. In this paper, details of the fission fragment angular distribution measurements of ²³⁸U target using Lexan polycarbonate have been discussed. The photofission cross-section was calculated using the angular distribution of fission fragments and the results were compared with those obtained using the code EMPIRE-II and various barrier parameters of the RIPL-1, RIPL-2 libraries and with the new analytical fission barrier formula based on the Hugenholtz–Van Hove theorem. The present experimental measurements were in good agreement with the results obtained from the Empire-II code predictions for potential parameter taken from RIPL-1 and a newly developed analytical fission barrier formula.

We study further a recent model for fission-fragment yield distributions based on Brownian shape motion on 5D potential-energy surfaces. Previously it was shown that this model describes well the transition between symmetric and asymmetric fission in the light Th region; here we study this transition near ²⁵⁸Fm and compare to scission-type yield models. We also study the impact of the relative density of grid points in the different shape coordinates. Although extreme changes in grid spacing affect the calculated yields to some degree, we find that the full 5D model with our origina grid choice is remarkably robust and that it is therefore suitable for applications, for example fission yields relevant for fission recycling in the r-process and its termination by fission.