Activation Cross Sections for 14.8-Mev Neutrons and Some New Radioactive Nuclides in the Rare Earth Region

Abstract

Activation cross sections on 27 stable nuclides of elements Ba, La, Ce, Pr, Nd, Sm, Eu, Gd, Dy, Er, Yb, and Lu were measured for 14.8\ifmmode\pm\else\textpm\fi{}0.8-Mev neutrons. Highly enriched isotopes were used as targets in most cases, and in a few instances radiochemical separations were performed whenever it was necessary and possible in view of the product half-lives. The measured cross sections for ($n, 2n$) reactions were found to agree within an order of magnitude with predictions from statistical evaporation theory. However, experimental values of ($n, p$) and ($n, \ensuremath{\alpha}$) cross sections generally appear to be larger than calculated from continuum theory of the compound nucleus. The cross sections show no significant effects due to the 82-neutron closed shell and, furthermore, the Levkovskii effect, which is quite striking in the low $Z$ region, appears to be negligible for ($n, p$) and ($n, \ensuremath{\alpha}$) reactions in the rare earth region. The ($n, 2n$) cross sections show little variation with mass number at constant $Z$, and they exhibit a decrease with increasing mass number at $N=82$.Several previously unreported activities were observed; their half-lives, assignment, and gamma radiations are as follows: 12\ifmmode\pm\else\textpm\fi{}3 min ${\mathrm{Pr}}^{148}$ from the ${\mathrm{Nd}}^{148}(n, p)$ reaction; 0.5\ifmmode\pm\else\textpm\fi{}0.1 min ${\mathrm{Sm}}^{157}$ from the ${\mathrm{Gd}}^{160}(n, \ensuremath{\alpha})$ reaction, 0.57\ifmmode\pm\else\textpm\fi{}0.01 Mev gamma; 7\ifmmode\pm\else\textpm\fi{}1 min ${\mathrm{Tb}}^{163}$ from the ${\mathrm{Dy}}^{163}(n, p)$ reaction, 0.18\ifmmode\pm\else\textpm\fi{}0.05-Mev gamma; 3.3\ifmmode\pm\else\textpm\fi{}0.5 min ${\mathrm{Ho}}^{168}$ from the ${\mathrm{Er}}^{168}(n, p)$ reaction, 0.85\ifmmode\pm\else\textpm\fi{}0.05-Mev gamma; 40\ifmmode\pm\else\textpm\fi{}10 sec ${\mathrm{Ho}}^{170}$ from the ${\mathrm{Er}}^{170}(n, p)$ reaction; 4.4\ifmmode\pm\else\textpm\fi{}0.4 min ${\mathrm{Dy}}^{167}$ from the ${\mathrm{Er}}^{170}(n, \ensuremath{\alpha})$ reaction; 2.0\ifmmode\pm\else\textpm\fi{}0.5 min activity with gammas at 0.18\ifmmode\pm\else\textpm\fi{}0.01, 0.25\ifmmode\pm\else\textpm\fi{}0.01, and 0.36\ifmmode\pm\else\textpm\fi{}0.01 Mev which may be ${\mathrm{Tm}}^{176}$, ${\mathrm{Er}}^{173}$, or possibly isomeric ${\mathrm{Yb}}^{177m}$ from enriched ${\mathrm{Yb}}^{176}$ bombardments. Tentative assignment of a 5.5\ifmmode\pm\else\textpm\fi{}0.5-min activity to ${\mathrm{Tm}}^{174}$ is suggested from bombardment of enriched ${\mathrm{Yb}}^{174}$.