β decay ofMg31: Extending the “island of inversion”

Abstract

The \ensuremath{\beta} decay of $^{31}\mathrm{Mg}$ has been investigated at GANIL. The ions were implanted in a silicon detector array surrounded by germanium \ensuremath{\gamma} detectors. \ensuremath{\beta} particles and subsequent \ensuremath{\gamma} rays have been observed. The $^{31}\mathrm{Mg}$ half-life has been measured (${T}_{1/2}=237\ifmmode\pm\else\textpm\fi{}25$ ms) in good agreement with previously reported values. The decay scheme of $^{31}\mathrm{Mg}$ has been enriched with two new low-spin levels and seven new \ensuremath{\gamma} transitions in $^{31}\mathrm{Al}$. Most importantly, very weak feeding to the $^{31}\mathrm{Al}$ ground and lowest excited states has been established. These results are compared to new shell-model calculations in the $\mathit{sd}\text{\ensuremath{-}}\mathit{fp}$ valence space, using an interaction that was modified in order to reproduce the recently established low-energy level scheme of $^{31}\mathrm{Mg}$. A good agreement for all observables is found, supporting the idea that $^{31}\mathrm{Al}$ is outside and $^{31}\mathrm{Mg}$ is inside the so-called ``island of inversion.''