Direct Lifetime Measurements of the Excited States in Ni-72

The lifetimes of the first excited 2+ and 4+ states in Ni72 were measured at the National Superconducting Cyclotron Laboratory with the recoil-distance Doppler-shift method, a model-independent probe to obtain the reduced transition probability. Excited states in Ni72 were populated by the one-proton knockout reaction of an intermediate energy Cu73 beam. γ-ray-recoil coincidences were detected with the γ-ray tracking array GRETINA and the S800 spectrograph. Our results provide evidence of enhanced transition probability B(E2;2+→0+) as compared to Ni68, but do not confirm the trend of large B(E2) values reported in the neighboring isotope Ni70 obtained from Coulomb excitation measurement. The results are compared to shell model calculations. The lifetime obtained for the excited 41+ state is consistent with models showing decay of a seniority ν=4, 4+ state, which is consistent with the disappearance of the 8+ isomer in Ni72.