Réactions entre protons de 30 à 155 MeV et noyaux lourds complexes

Abstract

Excitation functions have been established between 20 and 156 MeV for twelve (p, xn) reactions and nine (p, p xn) reactions induced on Bi by protons . In order to determine the cross sections for light polonium isotopes, new identifications were made for the disintegration schemes on 197–202Po. New isomeric states were found for 203Po, 201Po, 199Po and 197Po. The experimental excitation functions have been compared to theoretical values which were deduced from the two-step mechanism (Serber model). It has been shown that the experimental data are in agreement with a direct-interaction model in which prompt cascades are calculated in a diffusive-edge nucleus. However a model in which reflections and refractions are included for the penetration and the exit of the prompt nucleons, yields much too high proportions of compound nucleus, and therefore a simple model in which the impulse approximation is strictly applied for all collisions is a better description of the direct-interaction process than more elaborate calculations. The de-excitation process is described with great success by the statistical theory when at each step of the neutron evaporation chain, a competition is introduced for charged-particle evaporation and for fission. The best fit is obtained with level density parameters a = 13 MeV −1 for the particle emission and a f = 13.5 MeV − for fission. The agreement with experimental data obtained for all excitation functions along such a large energy range (20–155 MeV) seems to be a great success of the Serber-Goldberger model for the description of medium-energy, nuclear reactions.