14.5-MeV Neutron Activation Cross Section for Some of the Rare-Earth Nuclides and Their Relation to the Nuclear Shell Structure

The 14.5-MeV neutron cross sections for the production of ${}_{}{}^{139m}{}_{}{}^{}\mathrm{Ce}$, ${}_{}{}^{140}{}_{}{}^{}\mathrm{Pr}$, ${}_{}{}^{141m}{}_{}{}^{}\mathrm{Nd}$, ${}_{}{}^{143m}{}_{}{}^{}\mathrm{Sm}$, ${}_{}{}^{143g}{}_{}{}^{}\mathrm{Sm}$, and ${}_{}{}^{149}{}_{}{}^{}\mathrm{Nd}$ nuclides have been measured using thick-target samples. The ($n,2n$) cross sections for the target nuclides, some of which were calculated from previously reported isomeric yield ratios, have been plotted as functions of mass number and of the separation energies of the last neutron from the target nuclides. The results are discussed in the light of previously observed neutron shell effects in the systematics of ($n,2n$) cross sections. The importance of the cross-section measurements to the 14-MeV neutron activation analysis of these rare-earth elements is also pointed out.