Alpha-particle energy spectra from the (p,/alpha/) reaction on nuclei around atomic number 50

The energy spectra of /alpha/ particles emitted from (p,/alpha/) reactions on /sup 112/Cd, /sup 118/Sn, /sup 120/Sn, Sb, /sup 128/Te, and /sup 130/Te with 18 MeV protons have been measured in order to clarify the shell and odd-even effects in the preequilibrium processes of (p,/alpha/) and (n,/alpha/) reactions. From the experimental results, it was found that there exist no appreciable shell and odd-even effects on target nuclei in the gross (p,/alpha/) energy spectra lower than 20 MeV in the preequilibrium process. They can be explained well by the /alpha/ knockout model using the effective Q values which are shell independent. The fine structures of the energy spectra for the (p,/alpha/) reactions on nuclei around atomic number 50 can be qualitatively explained using the three-nucleon pickup model under the assumption that the states excited in the (p,/alpha/) reaction are formed by the coupling of two-neutron-hole states excited in the reaction such as the (p,t) reaction and one-proton-hole states excited in the reaction such as the (d,/sup 3/He) reaction. The similar three-nucleon pickup model can explain qualitatively the fine structures of the energy spectra for the (n,/alpha/) reactions on nuclei around neutron numbers 50 and 82.