New approach to nuclear photofission reactions above 0.15 GeV

Abstract

A simple approach to evaluate nuclear photofissilities at energies above the pion photoproduction threshold has been developed. It is based on the current, two-step model for intermediate-energy photonuclear reactions, i.e. a photon-induced intranuclear cascade followed by a fission-evaporation competition process for the excited, post-cascade residual nucleus. The calculation method (semiempirical by nature) shows that fissility (i.e., total fission probability) is governed by two basic quantities, namely, the first-chance fission probability for the average cascade residual, and a parameter which defines an evaporative sequence of residuals in which the average, equivalent chance-fission probabilities of nuclides belonging to the same generation are located. The nat Pb photofissility data measured recently in the range ∼ 0.2 − 3.8 GeV at the Thomas Jefferson Laboratory could be explained very satisfactorily by the present approach.The simultaneous photofission cross section measurements carried out very recently at the Thomas Jefferson Laboratory on a number of actinide target nuclei and natural lead using tagged photons in the range ∼0.2-3.8 GeV and PPAD-detectors for detection of fission fragments [1,2] made possible to extract important conclusions about the nuclear photoabsorption and fissility of these nuclei. Among others, i) the 237 Np photofissility is very close to unity in the whole energy interval, thus indicating that its photofission cross section is almost completely equal to the total photoabsorption cross section, and ii) the photofissility for all other actinide nuclei (uranium isotopes and thorium) is less than unity, therefore their photofission cross section does not represent the photoabsorption cross section for these nuclei. Very recently, results of a detailed and refined description of photofission reactions in heavy nuclei covering a large photon energy range ( ∼0.07−4.0 GeV) became also available [3]. Since photofissility data for actinide targets have been already analysed to some detail [2-4], we decided in the present work to focus attention on the newest photofissility data of nat Pb reported in [1,2]. Photofissility data will be here analysed in the framework of a phenomenological, semi-empirical way, aiming to obtain average calculated fissility values from an approach which has been developed for the first time in describing intermediate-energy photofission reactions in the entire energy range of ∼0.2−3.8 GeV covered by the measured photofission cross section data. Monte Carlo calculations are, at present, the main tool to describe quantitatively both the cascade and fission-evaporation competition processes, as well as to obtain the total fission probabilities (i.e., fissility values) for a number of photofission reaction cases. However, for cases where the target nucleus is expected to have very low fissility-values (pre-actinide, intermediate-mass, and less massive nuclei), the available codes may reveal themselves very time-consuming in obtaining a calculated fissility-cur...