Core-proton coupled nature of the 11/2+state in 131Sb probed by lifetime measurements

The structure of the 11/21+ state in 131Sb was investigated at the LOHENGRIN spectrometer of Institut Laue-Langevin via neutron-induced fission of 235U and lifetime measurements, yielding T1/2=3(2), at the edge of the sensitivity of the experimental method. This first result for the 11/21+ state half-life in neutron-rich Sb isotopes provides a quadrupole reduced transition probability to the ground state of B(E2)=1.4-0.6+1.5 W.u., very close to the B(E2;21+?01+) in 130Sn. Realistic shell-model calculations reproduce well both the experimental level scheme of 131Sb and the B(E2;11/21+?7/21+) value, indicting a dominant 2+(130Sn) -pg7/2 core-proton coupled configuration for the 11/21+ state.