2{sub 1}{sup +}{yields}0{sub 1}{sup +} transition strengths in Sn nuclei

The lifetime of the 2{sub 1}{sup +} state at 1256.7 keV in {sup 112}Sn has been determined using the (n,n{sup '}{gamma}) reaction. Angular distribution measurements were carried out at a neutron energy of 1.7 MeV, above the 2{sub 1}{sup +} energy threshold and below that of the second excited level. Through the Doppler-shift attenuation method, the lifetime of the 2{sub 1}{sup +} state is determined as 750{sub -90}{sup +125} fs, which gives a B(E2;2{sub 1}{sup +}{yields}0{sub 1}{sup +}) value of 10.9{sub -1.6}{sup +1.5} W.u. This E2 strength in {sup 112}Sn also allows a redetermination of the B(E2;2{sub 1}{sup +}{yields}0{sub 1}{sup +}) in {sup 108}Sn 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.