An exciton model analysis of (p, α) reactions induced in odd light nuclei

Abstract

Three years ago it has been suggested that the exciton model could satisfactorily reproduce the energy dependence of the excitation functions of (p, a) reactions induced in odd light nuclei (L~). However the ignorance of the decay rates of the composite nucleus for cxeiton-exeiton interaction, forbade the evaluation of the absolute values of the cross-sections. Since that time several attempts have been reported to evaluate the decay rates, starting from free nucleon-nucleon cross-sections and the Fermi-gas model of the nucleus (3.7). In this letter we report the evaluation of the excitation functions of (p, ~) reactions on 11B, ~SN, I~F, for transitions to definite levels of the residual nucleus. The calculations make use of the decay rates for exeiton-exciton interaction evaluated with the semi-phenomenological approach of ref. (~,~), which proved to be successfull in reproducing the cross-sections of nucleon and a-particle induced reactions (~-s). The energy dependence of the excitation functions of these reactions suggests tha~ the most usual configuration of the composite system, just after the interaction of the projectile with the target nucleus, is of the 2p-Oh type Q). Without attempting in this letter to explain this finding, the initial configuration of the composite system does not appear to be of the 2p-lh type, as in the case of proton interactions with heavier nuclei, presumably as a consequence of the large amplitude, in the wave functions of the light nuclei here considered, of configurations in which several nucleons are closely correlated. The interaction of the incident proton with single nucleons of the Fermi sea could be hindered so that only interactions with a loosely bound nucleon with no excitation of hole states could be enhanced.