Cross Sections for (α,n) Reactions for Medium-Weight Nuclei

Thick target ($\alpha ,n$) neutron yields have been measured from threshold to about 11-MeV $\alpha$-particle energy for twenty targets ranging in $A$ from 27 to 120. The targets of nickel, copper, and zinc were isotopically enriched. The absolute neutron yields, which were measured at approximately 100-keV energy intervals, have an accuracy of ±4%. The neutron detector was the $4\pi$, flat-energy response graphite sphere detector developed by Macklin. Cross sections for the ($\alpha ,n$) reactions were obtained to an accuracy of ±15% by differentiation of the smooth thick-target yield curves. For those nuclei in which the ($\alpha ,n$) cross section is thought to account for a significant percentage of the total reaction cross section, the observed cross sections are considerably larger than those predicted by Shapiro for his larger radial parameter of $1.5A^{\frac{1}{3}}$ F. Both the shape and absolute values of the observed cross sections agree fairly well with the optical-model analysis of reaction cross sections for $\alpha$ particles by Igo.