THERMAL-NEUTRON CAPTURE BY SILICON ISOTOPES

We have studied primary and secondary gamma-rays (46 in Si-29, 107 in Si-30, and 33 in Si-31) following thermal-neutron capture by the stable Si-28, Si-29, and Si-30 isotopes. Almost all of these gamma-rays have been incorporated into corresponding level schemes consisting of 12 excited levels in Si-29, 28 in Si-30, and 9 in Si-31. In each case, the observed gamma-rays account for nearly 100% of all captures. The measured neutron separation energies for Si-29, Si-30, and Si-31 are 8473.56+/-0.04, 10609.24+/-0.05, and 6587.40+/-0.05 keV, respectively. The measured thermal-neutron capture cross sections for Si-28, Si-29, and Si-30 are 169+/-4, 119+/-3, and 107+/-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.