Thermal-neutron capture by magnesium isotopes

We have studied the primary and secondary {γ} rays (33 in ²⁵Mg, 212 in ²⁶Mg, and 35 in ²⁷Mg) following thermal-neutron capture by the stable ²⁴Mg, ²⁵Mg, and ²⁶Mg isotopes. Almost all of these {γ} rays have been incorporated into the corresponding level schemes consisting of 9 excited levels in ²⁵Mg, 55 in ²⁶Mg, and 10 in ²⁷Mg. In each case, the observed {γ} rays account for nearly 100% of all captures. The measured neutron separation energies for ²⁵Mg, ²⁶Mg, and ²⁷Mg are, respectively, 7330.65±{}0.05, 11 093.18±{}0.05, and 6443.40±{}0.05 keV. The measured thermal-neutron capture cross sections for ²⁴Mg, ²⁵Mg, and ²⁶Mg are, respectively, 54.1±{}1.3, 200±{}3, and 39.0±{}0.8 mb. 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. We have also speculated on the mechanism responsible for the magnetic-dipole (M1) transitions which are quite strong in ²⁶Mg.