Nuclear charge distribution in the region of asymmetric fission of 238U by protons of energy 20–85 MeV

Abstract

Independent formation cross sections of 148mPm and 150Pm and cumulative formation cross sections of 146Ce, 146Pr, 147Nd, 149Nd, 149Pm, 151Nd, 151Pm, 153Sm, 155Sm, 156Sm and 157Eu, produced in the fission of 238U by protons of energy 20–85 MeV have been measured. Independent cross sections of 146Pr, 149Pm and 151Pm have been estimated from the cumulative yields. From these data, charge distribution curves, typical of the mass region A = 146 ⋍ 151 were constructed. The values of ZA−Zp deduced from these approached stability with increasing bombarding energy with a rate of displacement slower than that observed with heavy products of lower mass. The full-widths at half-maximum of the charge distribution curves broaden with increasing mass, while remaining narrower than in the case of neighbouring lower masses, thus confirming a mass dependence. The total number of neutrons emitted during very asymmetric mass division at these energies was found to be lower than in less asymmetric fission processes. The results have been discussed in terms of a growing contribution from direct interaction mechanisms.