Formation of isobaric nuclides withA=131in the interaction ofU238with 0.8-11.5 GeV protons

Cross sections and thick-target recoil properties of ${}_{}{}^{131}{}_{}{}^{}\mathrm{Ce}$, ${}_{}{}^{131}{}_{}{}^{}\mathrm{La}$, ${}_{}{}^{131}{}_{}{}^{}\mathrm{Ba}$, and several neighboring Ba nuclides formed in the interaction of ${}_{}{}^{238}{}_{}{}^{}\mathrm{U}$ with 0.8-11.5 GeV protons have been determined. The results reveal a dichotomy between ${}_{}{}^{131}{}_{}{}^{}\mathrm{Ba}$ and the more neutron-rich products on the one hand, and the more neutron-deficient ones on the other. The latter have rising excitation functions, forward-to-backward ratios (F/B) that peak at 3 GeV, and ranges that decrease abruptly just below this energy. The former have decreasing excitation functions as well as F/B and ranges that decrease slowly and featurelessly with increasing proton energy. The energy dependence of F/B as well as the changes in the angular distributions of the very neutron deficient products observed between 3 and 11.5 GeV are explained in terms of a change in the nature of near-central proton-nucleus interactions. At the lower energies the interaction consists of a series of nucleon-nucleon collisions while at high energies the proton interacts collectively with all the nucleons lying in its path. The dropoff in the mean ranges of these same nuclides and the accompanying broadening of the spectra indicate a transition between fission and deep spallation. The possible connection between these two effects is discussed. Several experiments to test the proposed model are suggested.