Cross-section ratios of isomeric nuclides produced in the reactions ${}_{}{}^{41}{}_{}{}^{}\mathrm{K}(\alpha , n){}_{}{}^{44}{}_{}{}^{}\mathrm{Sc}$, ${}_{}{}^{55}{}_{}{}^{}\mathrm{Mn}(\alpha , n){}_{}{}^{58}{}_{}{}^{}\mathrm{Co}$, ${}_{}{}^{93}{}_{}{}^{}\mathrm{Nb}(\alpha , n){}_{}{}^{96}{}_{}{}^{}\mathrm{Tc}$, ${}_{}{}^{93}{}_{}{}^{}\mathrm{Nb}(\alpha , 2n){}_{}{}^{95}{}_{}{}^{}\mathrm{Tc}$, ${}_{}{}^{93}{}_{}{}^{}\mathrm{Nb}(\alpha , 3n){}_{}{}^{94}{}_{}{}^{}\mathrm{Tc}$, and ${}_{}{}^{136}{}_{}{}^{}\mathrm{Ba}(\alpha , 3b){}_{}{}^{137}{}_{}{}^{}\mathrm{Ce}$ are reported for the $\alpha$-particle energy range from reaction threshold to 42 MeV. Recoil ranges of products were also determined. Range information was used to help establish the bombarding-energy region in which a particular reaction was likely to be compound-nuclear. The ratio of cross sections, high-spin product to low-spin product, was found to increase initially as the bombarding energy increased. In the three ($\alpha , n$) reactions, the ratio goes through a maximum and decreases. The decreasing region is consistent with the onset of a process in which the product nucleus does not have all of the momentum of the incident $\alpha$ particle. In the ${}_{}{}^{93}{}_{}{}^{}\mathrm{Nb}(\alpha , 2n)$ reaction the cross-section ratio also goes through a maximum, but the range measurements in this energy region suggest that in only a small fraction of events is the momentum transfer much less than complete. Both ($\alpha , 3n$) reactions show a monotonic increase in the cross-section ratio with increasing $\alpha$-particle energy. Excitation functions of isomeric pairs indicate a shift to higher bombarding energy for the nuclide of higher spin.

• Received 29 March 1965

DOI:https://doi.org/10.1103/PhysRev.139.B886