Gamma-ray Spectroscopy and Lifetime Measurements of Nuclei in the A = 70, N = Z Region

An experiment was performed at the National Superconducting Cyclotron Laboratory at Michigan State University, USA. Transition strengths for gamma decays from low-lying excited states in A ~ 70 nuclei have been deduced from lifetime measurements using the recoil distance Doppler shift and gamma-ray lineshape methods in conjunction with nucleon knockout and inelastic scattering reactions. The results confirm the collectivity previously reported for the 2^+ -> 0^+ decay in 68Se and reveal a relative decrease in collectivity in 70Br. This trend is reproduced by shell model calculations using the GXPF1A interaction in the fp model space including the Coulomb, spin-orbit and isospin non-conserving interactions. The 3^+ -> 2^+ decay in 70Br is found to have a very small B(M1) value, which is consistent with the configuration of the state being dominated by the coupling of f5/2 and p1/2 nucleons. The results suggest the g9/2 orbit does not play an important role at low spin in these nuclei. The B(E2) values for the decays of the T = 1 2^+ states in 70Br and 70Se are almost identical, suggesting there is no major shape change between the two nuclei at low spin. New gamma rays have been identified in 68Se and 69Se. gamma-gamma coincidence measurements have allowed one of the two new gamma rays observed in 69Se to be placed in the level scheme. Shell model calculations with the GXPF1A interaction in the fp model space suggest that this new gamma ray comes from a previously unobserved excited 1/2^- state. Lifetimes of excited states in 72Kr, 66Ge, 69Se and 65Ga have also been extracted and, where possible, compared to previous work.