βdecay of the proton-rich nucleusSi24and its mirror asymmetry

$\ensuremath{\beta}$-decay spectroscopy of the proton-rich nucleus $^{24}\mathrm{Si}$ was performed. The decay scheme was reconstructed from results of delayed $\ensuremath{\gamma}$-ray and proton measurements. We observed two $\ensuremath{\beta}$ branches to bound states in $^{24}\mathrm{Al}$ for the first time. The branching ratios were determined to be $31(4)%$ and $23.9(15)%$ for the ${1}_{1}^{+}$ state at $0.426$ MeV and the state at $1.090$ MeV, respectively. The observation of an allowed transition to the $1.090$-MeV state enabled us to firmly determine its spin-parity as ${1}^{+}$. In the proton measurements performed with the $\ensuremath{\Delta}E\text{\ensuremath{-}}E$ method, we observed a new unbound level at $6.735$ MeV. The branching ratios to three unbound states, including the new level, were also determined for the first time. Based on the decay scheme, the $B(\mathrm{GT})$ values of $^{24}\mathrm{Si}$ were deduced. The $B(\mathrm{GT})$ values were smaller than those of the mirror nucleus $^{24}\mathrm{Ne}$ by $22%$ and $10%$ for the ${1}_{1}^{+}$ and ${1}_{2}^{+}$ states, respectively. The mirror asymmetries of $B(\mathrm{GT})$, observed in both the ${1}_{1}^{+}$ and the ${1}_{2}^{+}$ states, indicate changes in configuration in the wave function associated with the Thomas-Ehrman shift. To clarify the mechanism of this asymmetry, a comparison with shell-model calculations is also discussed. The calculations attribute the changes in configuration to the lowering of the $1{s}_{1/2}$ orbital.