The capture cross sections of ${}_{}{}^{138}{}_{}{}^{}\mathrm{Ba}$, ${}_{}{}^{140,142}{}_{}{}^{}\mathrm{Ce}$, ${}_{}{}^{176}{}_{}{}^{}\mathrm{Lu}$, ${}_{}{}^{181}{}_{}{}^{}\mathrm{Ta}$ and the capture cross section of ${}_{}{}^{175}{}_{}{}^{}\mathrm{Lu}$ to the 3.68 h isomeric state in ${}_{}{}^{176}{}_{}{}^{}\mathrm{Lu}$ have been determined at 30 keV neutron energy using the activation technique. Neutrons were generated via the ${}_{}{}^7{}_{}{}^{}\mathrm{Li}\mathrm{}(p,n)\mathrm{}$ reaction just above the reaction threshold at a 3 MV pulsed Van de Graaff accelerator. The capture cross sections are of importance to stellar nucleosynthesis. With the ${}_{}{}^{138}{}_{}{}^{}\mathrm{Ba}$ and ${}_{}{}^{140}{}_{}{}^{}\mathrm{Ce}$ cross sections the time integrated average neutron flux for the s process was determined to 0.22 ${\mathrm{mb}}^{-1\mathrm{}}$. This result, the capture cross section of ${}_{}{}^{176}{}_{}{}^{}\mathrm{Lu}$ and the ${}_{}{}^{175}{}_{}{}^{}\mathrm{Lu}$ cross section to the 3.68 h isomeric state in ${}_{}{}^{176}{}_{}{}^{}\mathrm{Lu}$ were used to analyze the ${}_{}{}^{176}{}_{}{}^{}\mathrm{Lu}$ cosmic clock. The mean s process age of solar matter before the solidification of the solar system was estimated to 6 × ${10}^9$ yr. In addition the Hf/Lu abundance ratio was determined.

NUCLEAR REACTIONS ${}_{}{}^{138}{}_{}{}^{}\mathrm{Ba}(n,\gamma )$, ${}_{}{}^{140,142}{}_{}{}^{}\mathrm{Ce}(n,\gamma )$, ${}_{}{}^{175}{}_{}{}^{}\mathrm{Lu}(n,\gamma ){}_{}{}^{176}{}_{}{}^{}\mathrm{Lu}_{}^m{}_{}{}^{}$, ${}_{}{}^{176}{}_{}{}^{}\mathrm{Lu}(n,\gamma )$, ${}_{}{}^{181}{}_{}{}^{}\mathrm{Ta}(n,\gamma )$; $E_n=30\mathrm{}$ keV; measured capture cross sections; nucleosynthesis; ${}_{}{}^{176}{}_{}{}^{}\mathrm{Lu}$ cosmochronometer, Hf and Zr abundances.

• Received 2 October 1979

DOI:https://doi.org/10.1103/PhysRevC.21.534