Beta decay of proton-rich nucleus 24 Si and its mirror asymmetry

β-decay spectroscopy of the proton-rich nucleus Si 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 Al for the first time. The branching ratios were determined to be 31(4)% and 23.9(15)% for the 1+1 state 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 ∆E-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 Si were deduced. The B(GT) values were smaller than those of the mirror nucleus Ne by 22% and 10% for the 1+1 and 1 + 2 states, respectively. The mirror asymmetries of B(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 1s1/2 orbital. PACS numbers: 23.40.-s, 21.10.Hw, 23.20.Lv, 23.50.+z ∗ichikawa@ribf.riken.jp; Present address: RIKEN Nishina Center, RIKEN, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan