Photofissility at 1 GeV for nuclei throughout the periodic table

Abstract

A new approach to evaluate nuclear photofissilities at incident photon energies above the pion photoproduction threshold has been recently developed and proved to describe successfully the fissilities of nat Pb and 232 Th target nuclei at energies ∼ 0.2 − 4.0 GeV. The method is merely a simple, semiempirical description of the photofission reactions in which fissility, f , is governed by two basic quantities, namely, i) the first-chance fission probability,f 1 , for the average cascade residual, and ii) the slope,s, of the chance-fission probability associated with the average evaporative sequence of fissionable residuals. In the present work we have extended this approach to analyse photofissity data that have been accumulated over the past fourty years or so, measured at 1 GeV, for nearly fourty target nuclei extending from Ti up to Np. Results have shown that the variation of fissility with Z 2 /A could be described quite satisfactorily by the proposed model.Recently, a new approach to evaluate nuclear photofissilities at incident photon energies E γ 200 MeV has been developed to overcome the calculational difficulties imposed by the available Monte Carlo codes when the target nuclei are expected to exhibit low, or very low, fissility-values ( 1%), as is the case for pre-actinide, intermediate-mass, and less massive nuclei [1]. The method is a simple, semiempirical approach that has proved to work quite well in describing the fissilities of nat Pb and 232 Th target nuclei induced by ∼ 0.2 − 4.0 GeV photons, which have been measured at the Thomas Jefferson Laboratory by Cetina et al [2,3].Since photofissility values for actinide targets have been already analysed to some detail in the framework of Monte Carlo calculations [3,4] we decided, in the present work, to extend this approach [1] to analyze photofissility data that have been accumulated over the past fourty years or so, measured at 1 GeV, for nearly fourty target nuclei extending from Ti up to Np. Results have shown that the variation of fissility with Z 2 /A can be described quite satisfactorily by the proposed model. We remark that the trend of fissility is seen as an inverse reflection of the behaviour of the height of the fission barrier with Z 2 /A [5,6].Monte Carlo calculations are, at present, the main tool to describe quantitatively both the cascade and fissionevaporation competition processes, as well as to obtain the total fission probabilities (photofissility values) for a number of photofission reaction cases. However, for cases where the target nucleus is expected to have 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-curve of acceptable uncertainty over a large energy-interval such as ∼ 0.2 − 4.0 GeV. This fact led us to develop an alternative method to calculate nuclear photofissility-values semiempirically. This approach is based on the current, two-step model for intermediateenergy photonuclear reactions...

Photofission and total photoabsorption cross sections in the energy range of shadowing effects

“…Afterwards, the MCMC and MCEF codes were coupled to give the CRISP code, which since has been used to calculate photoabsorption and/or photofission cross sections for several nuclei with mass numbers ranging from 12 up to 238, and for photons of energies from 40 MeV up to 1 GeV, covering the entire range of baryonic resonances [20]. Moreover, an alternative, semiempirical approach has been developed to analyze photofission data in the 0.2-4 GeV region for a number of target nuclei [21,22], where new features of the intranuclear cascade process are discussed. These calculations have shown good agreement with the experimental results.…”