On the β-decaying (21⁺) spin gap isomer in ⁹⁴Ag

The β decay of ⁹⁴Ag, a nucleus with three proton and three neutron holes with respect on ¹⁰⁰Sn, was investigated at the GSI on-line mass spectrometer by using an array of germanium and silicon detectors. On the basis of an β γ γ coincidence measurement, the previously reported decay scheme has been considerably improved and evidence is presented for a spin-gap isomer with a spin-parity assignment of (21⁺), i. e., the highest spin ever observed in β decay. The half-life of the low spin-spin isomer (7⁺) was remeasured to be 0.59(2) s, whereas the half-life of the high-spin isomer was determined as 0.47(8) S. For the states in ⁹⁴Ag and ⁹⁴Pd, large-scale shell-model calculations that include up to 4p-4h excitations across the N = Z = 50 shell gap and employ a realistic interaction were carried out. These core excitations play a crucial role in generating the E4 isomerism required for the interpretation of th long half-life of the (21⁺) state in ⁹⁴Ag.