Determination of the21+→01+transition strengths inNi58andNi60

Abstract

Gamma-ray angular distribution measurements following the ${}^{\mathrm{nat}}$Ni($n,{n}^{'}\ensuremath{\gamma}$) 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 $\ensuremath{\tau}=1.{00}_{\ensuremath{-}0.10}^{+0.15}$ ps, which yields a $B(E2;{2}_{1}^{+}\ensuremath{\rightarrow}{0}_{1}^{+})$ value of $9.{4}_{\ensuremath{-}1.2}^{+1.0}$ W.u. From previous measurements and this work, average values of $\ensuremath{\tau}=0.94(3)$ ps and $B(E2;{2}_{1}^{+}\ensuremath{\rightarrow}{0}_{1}^{+})=10.0(4)$ W.u. are recommended as standards for normalization. In addition, a longer lifetime of $\ensuremath{\tau}=1.{30}_{\ensuremath{-}0.20}^{+0.30}$ ps has been determined for the ${2}_{1}^{+}$ state in $^{60}\mathrm{Ni}$, which yields an $E2$ strength of $10.{7}_{\ensuremath{-}2.5}^{+1.7}$ 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.