Shell and odd-even effects on alpha-particle energy spectra from the (p,α) reaction on nuclei around neutron number 50

The energy spectra of α particles emitted from (p,α) reactions on ${}_{}{}^{90}{}_{}{}^{}\mathrm{Zr}$, ${}_{}{}^{92}{}_{}{}^{}$,94,96,98,100Mo, ${}_{}{}^{93}{}_{}{}^{}\mathrm{Nb}$, ${}_{}{}^{106}{}_{}{}^{}\mathrm{Pd}$, and Ag with 18 MeV protons and on some of them with 15 MeV protons have been measured in order to clarify the shell and odd-even effects in the preequilibrium processes of (p,α) and (n,α) reactions. From the experimental results, it was found that there exists no appreciable odd-even effect on the target nuclei in energy spectra for the preequilibrium (p,α) reaction except for the spectra corresponding to the low lying states of the residual nuclei. Considerable change in the energy spectra in the energy region higher than 16 MeV has been observed for target nuclei around the magic number N=50. We have calculated the energy spectra, by using the knockout model introducing effective Q values, the pairing correlation and a modified uniform spacing model in which the uniform spacing levels have a wide spacing at the magic number. The shell effect of the gross structure of the energy spectra for the nuclei near the magic nuclei can be explained very well in terms of the present knockout model, although the fine structures of the spectra above 16 MeV which correspond to the low lying states or groups of the states of the residual nuclei cannot be explained so well.