Neutron effective single-particle energies above Ni 78: A hint from lifetime measurements in the N=51 isotones Se 85 and Kr 87

Background: While the N=50 shell-gap evolution towards 78Ni is presently in the focus of nuclear structure research, experimental information on the neutron effective single-particle energy sequence above the 78Ni core remain scarce. Direct nucleon-exchange reactions are indeed difficult with presently available post-accelerated radioactive-ion beams (especially for high orbital-momentum orbitals) in this exotic region. Purpose: In this study we probe the evolution of the ν1g7/2 effective single-particle energy which is a key to understanding the possible evolution of the spin-orbit splitting due to the proton-neutron interaction in the 78Ni region. To achieve this goal, a method based on lifetime measurements is used for the first time. The obtained lifetimes of the 7/2+1 states in 87Kr and 85Se are used to investigate the ν1g7/2 evolution. Method: Yrast and near-yrast states in the light N=51 isotones 85Se and 87Kr were populated via multinucleon transfer reactions, using a 82Se beam and a 238U target at the LNL tandem-ALPI facility. The prompt γ rays were detected by the AGATA Demonstrator and particle identification was performed using the PRISMA spectrometer. Lifetime measurements were performed by using the Cologne plunger device for deep inelastic reactions and the Recoil Distance Doppler Shift technique. Results: We obtain τ(7/2+1)=0.4+1.6−0.4 ps for 87Kr. In the case of 85Se an upper limit of 3(2) ps is obtained for the τ7/2+1 value. Conclusion: For 87Kr, the measured (7/2+1) lifetime is consistent with a core-coupled 2+⊗ν2d5/2 configuration for this state. This result is consistent with that obtained by direct reaction, which validates our method. For 85Se, the measured 7/2+1 lifetime limit indicates a very small contribution of the ν1g7/2 configuration to the wave function of this state.