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

Thick target ( alpha ,n) neutron yields were 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 plus or minus 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 plus or minus 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 1.5 A/sup 1/2/ 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. (auth)