Proton Spectra from 54.8-MeV Alpha-Particle Reactions: Precompound Emission

An experimental survey of 54.8-MeV ( alpha ,p) proton spectra is undertaken for studying pre-equillbrium emission; such deexcitation yields on the average cross sections about 250 mb, the average value measured for all the investigated reactions. Different quantitative formulations are discussed in tenns of exciton state mean lifetimes: the exciton and hybrid mcdels are found to give a good interpretation of the proton emision cross sections. A geometrical distinction between the equilibrium and nonequilibrium processes may be attempted. Odd-even effects appear in ( alpha ,p) reactions by means of partial state density: Odd-Z targets require an initial exciton number n/sib I/ equal to five (3p-2n-Oh) and even-Z targets equal to four (2p-2n-Oh). This result may be attributed to a pairing effect according to the odd-even character of residual nuclei.