Collectivity in A ∼ 70 nuclei studied via lifetime measurements in 70Br and 68,70Se

Transition strengths for decays from low-lying states in A ∼ 70 nuclei have been deduced from lifetime measurements using the recoil distance Doppler shift technique. The results confirm the collectivity previously reported for the 21+→0gs+ 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 an fp   model space including the Coulomb, spin-orbit and isospin non-conserving interactions. The 31+→21+ 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 f52 protons and neutrons. The results suggest that the g92 orbit does not play an important role at low spin in these nuclei. The B(E2) values for the decays of the (T = 1) 21+ states in 70Br and 70Se are almost identical, suggesting there is no major shape change between the two nuclei at low spin.