Beta decay ofNa31,32andMg31: Study of theN=20 shell closure

The ${}_{}{}^{31,32}{}_{}{}^{}\mathrm{Na}$ and ${}_{}{}^{31}{}_{}{}^{}\mathrm{Mg}$ beta decays were studied at the CERN on-line mass separator ISOLDE by gamma, gamma-gamma, and neutron-gamma measurements. In the ${}_{}{}^{31}{}_{}{}^{}\mathrm{Na}$ decay, the assignment of previously reported γ transitions and the observation of a new level at 3760 keV lead to a revised decay scheme which is found in good agreement with a calculation including two-particle–two-hole configurations in the model space, as far as only low-lying levels of ${}_{}{}^{31}{}_{}{}^{}\mathrm{Mg}$ are concerned. In the ${}_{}{}^{31}{}_{}{}^{}\to _{}^{31}{}_{}{}^{}$Al decay, a new decay scheme involves ten β branches and three states are reported for the first time. While satisfactory agreement with theoretical calculations is observed for excitation energies in ${}_{}{}^{31}{}_{}{}^{}\mathrm{Al}$, a strong discrepancy is observed for the intensity of the ground-state β branch, the experimental one being highly quenched as compared to theoretical expectations. Finally, new spectroscopic results have been obtained in the ${}_{}{}^{32}{}_{}{}^{}\mathrm{Na}$ β decay. A previously noninterpreted 1436 keV γ ray is now assigned in the ${}_{}{}^{32}{}_{}{}^{}\mathrm{Mg}$ scheme. The 240 keV ray is shown to arise from ${}_{}{}^{31}{}_{}{}^{}\mathrm{Mg}$ produced in the one-neutron channel, and to be related to the decay of an intruder state at ${\mathit{E}}_{\mathit{x}}$=461 keV. The latter is partially fed from the 1390 keV level. Both nicely compare with theoretical predictions locating 1ħω states at 0.40 MeV (7/$2^{\mathrm{-}}$) and 1.57 MeV (11/$2^{\mathrm{-}}$). The first experimental evidence for a γ cascade in the descendant ${}_{}{}^{32}{}_{}{}^{}\mathrm{Al}$ is also obtained.