(p, α) Reactions Induced by Protons in the Energy Range of 9.5-23 Mev

An earlier study of ($p, \alpha$) reactions induced by 23-Mev protons has been extended by observing the outgoing alpha particles from nuclear reactions induced by protons of various energies between 9.5 and 23 Mev in numerous elements throughout the periodic table. Alpha-energy distributions and absolute differential ($p, \alpha$) cross sections were measured at 90 deg. From the integrals of the alpha-energy distributions, excitation functions for ($p, \alpha$) reactions were determined. Excitation functions for ($p, \alpha$) reactions in targets for which compound nuclei are integral numbers of alpha particles are qualitatively different from those of other targets. Alpha-energy distributions for targets with $Z\lesssim 50$ (except for F and Al) are peaked at about the same energy for incident proton energies of 9.5-23 Mev; this may be interpreted as evidence that the Coulomb barrier is lowered for alpha-particle emission from excited compound nuclei.