Pre-equilibrium alpha emission induced by different incident channels: Evidence for alpha preformation in nuclei

A study of $\alpha$ emission induced by different incident channels has been undertaken. Measurements of energy spectra and angular distributions of $\alpha$ particles emitted in proton-, deuteron-, ${}_{}{}^3{}_{}{}^{}\mathrm{He}$-, and $\alpha$-induced reactions were performed. An analysis in the frame of the pre-equilibrium model succeeds in giving an over-all description of all particle spectra. The $\alpha$ particle being in this approach considered as an exciton, a preformation factor around 0.2 for four nucleons to be correlated as an $\alpha$ particle has been obtained. The $\alpha$ preformation probability deduced from this analysis is independent of the proton, deuteron, ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ incident channel, incident energy, and mass of target nuclei.