Determination of the 21+→01+ transition strengths in 58Ni and 60Ni

Gamma-ray angular distribution measurements following the Ni-nat(n, n'gamma) reaction were carried out at 1.6 and 1.8 MeV neutron energies. Through the Doppler-shift attenuation method, the lifetime of the 2(1)(+) state in Ni-58 is determined as tau = 1.00(-0.10)(+0.15) ps, which yields a B(E2;2(1)(+) --> 0(1)(+)) value of 9.4(-1.2)(+1.0) W.u. From previous measurements and this work, average values of tau = 0.94(3) ps and B(E2;2(1)(+) --> 0(1)(+))=10.0(4) W.u. are recommended as standards for normalization. In addition, a longer lifetime of tau = 1.30(-0.20)(+0.30) ps has been determined for the 2(1)(+) state in Ni-60, which yields an E2 strength of 10.7(-2.5)(+1.7) W.u. Our results support an enhancement of proton-core excitations and related quadrupole and pairing strengths in the light Ni isotopes, in agreement with mean-field and shell-model calculations.