Thermal-neutron capture by magnesium isotopes.

Abstract

We have studied the primary and secondary {gamma} rays (33 in {sup 25}Mg, 212 in {sup 26}Mg, and 35 in {sup 27}Mg) following thermal-neutron capture by the stable {sup 24}Mg, {sup 25}Mg, and {sup 26}Mg isotopes. Almost all of these {gamma} rays have been incorporated into the corresponding level schemes consisting of 9 excited levels in {sup 25}Mg, 55 in {sup 26}Mg, and 10 in {sup 27}Mg. In each case, the observed {gamma} rays account for nearly 100% of all captures. The measured neutron separation energies for {sup 25}Mg, {sup 26}Mg, and {sup 27}Mg are, respectively, 7330.65{plus minus}0.05, 11 093.18{plus minus}0.05, and 6443.40{plus minus}0.05 keV. The measured thermal-neutron capture cross sections for {sup 24}Mg, {sup 25}Mg, and {sup 26}Mg are, respectively, 54.1{plus minus}1.3, 200{plus minus}3, and 39.0{plus minus}0.8 mb. In all three cases, primary electric-dipole ({ital E}1) transitions account for the bulk of the total capture cross section. We have calculated these {ital E}1 partial cross sections using direct-capture theory. We have also speculated on the mechanism responsible for the magnetic-dipole ({ital M}1) transitions which are quite strong in {sup 26}Mg.