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

Abstract

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 \ensuremath{\gamma} 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 $^{31}\ensuremath{\rightarrow}^{31}$Al decay, a new decay scheme involves ten \ensuremath{\beta} 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 \ensuremath{\beta} branch, the experimental one being highly quenched as compared to theoretical expectations. Finally, new spectroscopic results have been obtained in the $^{32}\mathrm{Na}$ \ensuremath{\beta} decay. A previously noninterpreted 1436 keV \ensuremath{\gamma} 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 1\ensuremath{\Elzxh}\ensuremath{\omega} states at 0.40 MeV (7/${2}^{\mathrm{\ensuremath{-}}}$) and 1.57 MeV (11/${2}^{\mathrm{\ensuremath{-}}}$). The first experimental evidence for a \ensuremath{\gamma} cascade in the descendant $^{32}\mathrm{Al}$ is also obtained.