Single particle effects in precompound reactions

Abstract

Double differential cross sections have been measured with time of flight techniques at 16 angles between 3.5\ifmmode^\circ\else\textdegree\fi{} and 159\ifmmode^\circ\else\textdegree\fi{} (lab) for the inclusive production of neutrons from reactions of 25 MeV protons with $^{50,52,53}\mathrm{Cr}$, $^{54,56,58}\mathrm{Fe}$, $^{59}\mathrm{Co}$, $^{60}\mathrm{Ni}$, $^{63}\mathrm{Cu}$, $^{89}\mathrm{Y}$, $^{90,91,92,94}\mathrm{Zr}$, $^{92,94,95,96,97,98,100}\mathrm{Mo}$, $^{110}\mathrm{Pd}$, and of 18 MeV protons with $^{90,91,92,94}\mathrm{Zr}$. The spectra from $^{90,91,92,94}\mathrm{Zr}$ targets are compared with qualitative and quantitative predictions based on proton-particle---neutron-hole state densities generated from different sets of single particle states using the recursion method of Williams et al. An end point gap of nearly 4 MeV in the $^{91}\mathrm{Zr}$(p,n) spectrum may be understood quantitatively in terms of such an extreme single particle model argument. The experimental peak structure shows reasonably good agreement with the calculated results. It is shown that precompound spectra exhibit pronounced structure which is dependent upon nuclear structure effects for near closed shell nuclei, and that these effects should and do disappear rapidly with nuclear deformation, as predicted earlier by Williams et al.