Eur. Phys. J. A (2005) 24: 69-84
https://doi.org/10.1140/epja/i2004-10130-9

Intranuclear-Cascade model calculation of photofission probabilities for actinide nuclei

¹ Center for Nuclear Studies, Department of Physics, The George Washington University, D.C. 20052, Washington, USA
² Institute for Nuclear Research, Russian Academy of Science, 117312, Moscow, Russia

^* e-mail: briscoe@gwu.edu

Received: 11 August 2004
Accepted: 27 January 2005
Published online: 21 February 2005

Abstract

We have calculated the fission probabilities for ²³⁷ Np, ^{233, 235, 238} U, ²³² Th, and ^nat Pb following the absorption of photons with energies from 68 MeV to 3.77 GeV using the RELDIS Monte Carlo code. This code implements the cascade-evaporation-fission model of intermediate-energy photonuclear reactions. It includes multiparticle production in photoreactions on intranuclear nucleons, pre-equilibrium emission, and the statistical decay of excited residual nuclei via competition of evaporation, fission, and multifragmentation processes. The calculations show that in the GeV energy region the fission process is not solely responsible for the entire total photoabsorption cross-section, even for the actinides. The fission probabilities are 80-95% for ²³³ U, ²³⁵ U, and ²³⁷ Np, 70-80% for ²³⁸ U, and only 55-70% for ²³² Th. This is because certain residual nuclei that are created by deep photospallation at GeV photon energies have relatively low fission probabilities. The results of those model calculations are in reasonable agreement (at the ∼ 10% level) with recent experimental data on relative photofission cross-sections for ²³⁷ Np and ^{233, 235, 238} U (but not for ²³² Th or ^nat Pb) from the Saskatchewan and Jefferson Laboratories over a very wide range in photon energy. Using our calculated fission probabilities plus the total photoabsorption cross-sections per nucleon, estimated from previous cross-section data for nuclei from C to Pb, we can infer absolute photofission cross-sections for the actinide nuclei and compare them with the SAL and JLab results. The resulting discrepancies, however, clearly demonstrate the need for direct measurement of the total photoabsorption cross-sections for the heavy actinides.