Gamma-ray angular distribution measurements following the ${}^{\mathrm{nat}}$Ni($n,n^{\text{'}}\gamma$) 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 ${}^{58}\mathrm{Ni}$ is determined as $\tau =1.{00}_{-0.10}^{+0.15}$ ps, which yields a $B(E2;2_1^{+}\to 0_1^{+})$ 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 $B(E2;2_1^{+}\to 0_1^{+})=10.0(4)$ 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 ${}^{60}\mathrm{Ni}$, 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.

• Received 24 March 2008

DOI:https://doi.org/10.1103/PhysRevC.77.064301