Lifetimes of Low-Lying Levels inNi57

Lifetimes of low-lying levels in ${}_{}{}^{57}{}_{}{}^{}\mathrm{Ni}$ have been investigated using the reaction ${}_{}{}^{54}{}_{}{}^{}\mathrm{Fe}(\alpha ,n){}_{}{}^{57}{}_{}{}^{}\mathrm{Ni}$ and the Doppler-shift attenuation method. The lifetimes are not in agreement with single-particle predictions and, together with the presence of new low-lying levels in ${}_{}{}^{57}{}_{}{}^{}\mathrm{Ni}$, confirm the presence of core excitation of ${}_{}{}^{56}{}_{}{}^{}\mathrm{Ni}$.