Cross sections for the formation of69Znm,gand71Znm,gin neutron induced reactions near their thresholds: Effect of reaction channel on the isomeric cross-section ratio

Abstract

Excitation functions were measured for the reactions ${}^{72}\mathrm{Ge}(n,\ensuremath{\alpha})$${}^{69}{\mathrm{Zn}}^{m,g},$ ${}^{69}\mathrm{Ga}{(n,p)}^{69}{\mathrm{Zn}}^{m,g},$ ${}^{70}\mathrm{Zn}{(n,2n)}^{69}{\mathrm{Zn}}^{m,g},$ ${}^{74}\mathrm{Ge}(n,\ensuremath{\alpha}{)}^{71}{\mathrm{Zn}}^{m,g},$ and ${}^{71}\mathrm{Ga}{(n,p)}^{71}{\mathrm{Zn}}^{m,g}$ over the neutron energy range of 6.3--12.4 MeV. Quasimonoenergetic neutrons in this energy range were produced via the ${}^{2}\mathrm{H}{(d,n)}^{3}\mathrm{He}$ reaction using a deuterium gas target at the J\"ulich variable energy compact cyclotron. Use was made of the activation technique in combination with high-resolution HPGe-detector \ensuremath{\gamma}-ray spectroscopy. In a few cases low-level ${\ensuremath{\beta}}^{\ensuremath{-}}$counting was also applied. In order to decrease the interfering activities in those cases, either radiochemical separations were performed or isotopically enriched targets were used. For most of the reactions, the present measurements provide the first consistent sets of data near their thresholds. From the available experimental data, isomeric cross-section ratios were determined for the isomeric pair ${}^{69}{\mathrm{Zn}}^{m,g}$ in $(n,\ensuremath{\alpha}),$ $(n,p),$ and $(n,2n)$ reactions, and for the pair ${}^{71}{\mathrm{Zn}}^{m,g}$ in $(n,\ensuremath{\alpha})$ and $(n,p)$ reactions. Nuclear model calculations using the code STAPRE, which employs the Hauser-Feshbach (statistical model) and exciton model (precompound effects) formalisms, were undertaken to describe the formation of both isomeric and ground states of the products. The calculational results on the total $(n,\ensuremath{\alpha}),$ $(n,p),$ and $(n,2n)$ cross sections agree fairly well with the experimental data. The experimental isomeric cross-section ratios, however, are reproduced only approximately by the calculation. For both the isomeric pairs investigated, the isomeric cross-section ratio in the $(n,p)$ reaction is higher than in other reactions.