Spallation of Uranium and Thorium Nuclei with Bev-Energy Protons

Abstract

Cross sections have been measured for the production of isotopes of uranium, protactinium, thorium, and actinium in the irradiation U/sup 238/U2>s/ sup 35/, and Th/sup 232/ with 0. 68- and 1.8Bev protons. In addition, some yields were determined at other bombarding energies ranging up to 6.2 Bev. Calculations of the cross sections at 1.8-Bev bombarding energy were made, based on recent Monte Carlo calculations of the knock-on phase of the interaction, combined with published systematics of nuclear evaporation, and several assumptions as to fission-evaporation competition. Even without fission competition the calculated yields are considerably lower than the experimental ones, indicating a failure of the model for the knock-on phase of the reaction to predict sufficient probability for simple processes with low deposition energy. In view of the better agreement with experiment that previous workers have obtained with this type of calculation for 0.34-Bev protons on uranium, it is suggested that the present discrepancy may be due to an overemphasis of meson processes in the proliferation of the knock-on cascade in the calculation. (auth)