Direct lifetime measurements of the excited states in Ni 72
The lifetimes of the first excited 2 + and 4 + states in 72 Ni 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 72 Ni were populated by the one-proton knockout reaction of an intermediate energy 73 Cu 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 68 Ni, but do not confirm the trend of large B(E2) values reported in the neighboring isotope 70 Ni obtained from Coulomb excitation measurement. The results are compared to shell model calculations. Here, the lifetime obtained for the excited 4 + 1 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 72 Ni.