Thermal-neutron capture by silicon isotopes.

Abstract

We have studied primary and secondary \ensuremath{\gamma} rays (46 in $^{29}\mathrm{Si}$, 107 in $^{30}\mathrm{Si}$, and 33 in $^{31}\mathrm{Si}$) following thermal-neutron capture by the stable $^{28}\mathrm{Si}$, $^{29}\mathrm{Si}$, and $^{30}\mathrm{Si}$ isotopes. Almost all of these \ensuremath{\gamma} rays have been incorporated into corresponding level schemes consisting of 12 excited levels in $^{29}\mathrm{Si}$, 28 in $^{30}\mathrm{Si}$, and 9 in $^{31}\mathrm{Si}$. In each case, the observed \ensuremath{\gamma} rays account for nearly 100% of all captures. The measured neutron separation energies for $^{29}\mathrm{Si}$, $^{30}\mathrm{Si}$, and $^{31}\mathrm{Si}$ are 8473.56\ifmmode\pm\else\textpm\fi{}0.04, 10609.24\ifmmode\pm\else\textpm\fi{}0.05, and 6587.40\ifmmode\pm\else\textpm\fi{}0.05 keV, respectively. The measured thermal-neutron capture cross sections for $^{28}\mathrm{Si}$, $^{29}\mathrm{Si}$, and $^{30}\mathrm{Si}$ are 169\ifmmode\pm\else\textpm\fi{}4, 119\ifmmode\pm\else\textpm\fi{}3, and 107\ifmmode\pm\else\textpm\fi{}3 mb, respectively. In all three cases, primary electric-dipole (E1) transitions account for the bulk of the total capture cross section. We have calculated these E1 partial cross sections using direct-capture theory. The agreement between theory and experiment is satisfactory.