β Decay of the proton-rich nucleus Si24 and its mirror asymmetry

β-decay spectroscopy of the proton-rich nucleus Si24 was performed. The decay scheme was reconstructed from results of delayed γ-ray and proton measurements. We observed two β branches to bound states in Al24 for the first time. The branching ratios were determined to be 31(4)% and 23.9(15)% for the 11+ 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 ΔE-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(GT) values of Si24 were deduced. The B(GT) values were smaller than those of the mirror nucleus Ne24 by 22% and 10% for the 11+ and 12+ states, respectively. The mirror asymmetries of B(GT), observed in both the 11+ and the 12+ 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 1s1/2 orbital.