21+→01+ transition strengths in Sn nuclei

The lifetime of the 21+ state at 1256.7 keV in Sn112 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 21+ energy threshold and below that of the second excited level. Through the Doppler-shift attenuation method, the lifetime of the 21+ state is determined as 750-90+125 fs, which gives a B(E2;21+→01+) value of 10.9-1.6+1.5 W.u. This E2 strength in Sn112 also allows a redetermination of the B(E2;21+→01+) in Sn108 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.