Prompt and delayed gamma-spectroscopy of neutron-rich krypton isotopes with N ≤ 60

Low-lying excited states of the neutron-rich ^94,95,96Kr were measured at the RIBF at the RIKEN Nishina Center for Accelerator-Based Science as part of the SEASTAR campaign. The nuclei of interest were populated in nucleon knockout reactions using the MINOS device surrounded by the DALI2 array for prompt γ-spectroscopy. Additionally, the EURICA array was run in a parasitic mode to measure the delayed isomeric transitions in ⁹⁵Kr. In all three nuclei, previously unknown γ-transitions were observed. Even-even Sr and Zr nuclei in the A=100 region show a sudden onset of deformation at N=60 while the lighter isotopes up to N=58 are rather spherical. Contrarily, the even krypton isotopes exhibit a smooth onset of collectivity up to N= 0. For ⁹⁶Kr, the measured new transitions imply the existence of low-lying low-spin non-yrast states, which we interpret as the shape coexisting states becoming yrast above N=60 in agreement with very recent IBM calculations. For ⁹⁵Kr, the analysis of prompt γ-radiation with and without coincidence of delayed radiation identified the prompt γ-rays as either feeding or bypassing the known isomeric state in ⁹⁵Kr.