2(1)(+)-> 0(1)(+) transition strengths in Sn nuclei

The lifetime of the 2+1 state at 1256.7 keV in 112Sn has been determined using the (n,n′γ) reaction. Angular distribution measurements were carried out at a neutron energy of 1.7 MeV, above the 2+1 energy threshold and below that of the second excited level. Through the Doppler-shift attenuation method, the lifetime of the 2+1state is determined as 750+125−90fs, which gives a B(E2;2+1→0+1) value of 10.9+1.5−1.6W.u. This E2strength in 112Snalso allows a redetermination of the B(E2;2+1→0+1) in 108Sn as 10.8(3.0) W.u. These values result in a symmetric trend around the neutron midshell in the systematics of E2 strengths in the even-mass tin isotopes and do not support N=64 or N=66 subshell gaps. The symmetric trend is in agreement with recent shell model predictions, where proton-core excitations were allowed in the calculations.