Direct Lifetime Measurements of the Excited States in Ni-72

Abstract

The lifetimes of the first excited 2(+) and 4(+) states in Ni-72 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 Ni-72 were populated by the one-proton knockout reaction of an intermediate energy Cu-73 beam. gamma-ray-recoil coincidences were detected with the gamma-ray tracking array GRETINA and the S800 spectrograph. Our results provide evidence of enhanced transition probability B(E2; 2(+) -> 0(+)) as compared to Ni-68, but do not confirm the trend of large B(E2) values reported in the neighboring isotope Ni-70 obtained from Coulomb excitation measurement. The results are compared to shell model calculations. The lifetime obtained for the excited 4(1)(+) state is consistent with models showing decay of a seniority nu = 4, 4(+) state, which is consistent with the disappearance of the 8(+) isomer in Ni-72.