The first spectroscopy of 78Ni together with indications of shape coexistence just below the N=50 shell closure for 79Zn suggests that deformed intruder configurations could play a crucial role in low-energy structure properties in this region and towards the limits of the nuclear chart. Such configurations are predicted to originate from multiparticle-multihole excitations above the N=50 and Z=28 shell gaps pushed down in energy by neutron-proton correlations which enhance quadrupole collectivity.
Because these intruder states involve many-particle excitations more difficult to describe theoretically, their predicted energies vary more drastically between models than for yrast states originating from “normal” configurations on which they tend to agree. Characterising fully the properties of those states hence provides a good asset to benchmark microscopic models, or constrain effective shell model interactions.
This topic is the main goal of an experiment performed at the RIBF facility (RIKEN, Japan) in order to identify and characterise for the first time 2p-1h intruder states in 83Ge. Neutron hole states in this N=51 nucleus were populated via neutron knockout reactions from the N=52 nucleus 84Ge that has about two neutrons in the s1/2d5/2 valence space above N=50. This direct reaction allows in some cases to remove one of the neutrons from the quasi-full g9/2 orbital below N=50 and selectively populate the 9/2+ intruder states based on a ν(g9/2)−1(s1/2d5/2)+2 configuration. In order to identify the populated states, gamma-rays from their in-flight decay were measured using the HiCARI Germanium array that comprises six MINIBALL triple clusters, four Clovers, and two GRETINA-type detectors.
This talk will present the identification of the main intruder candidate in 83Ge based on lifetime and spectroscopic factor determination. Based on the comparison with Large-Scale-Shell-Model calculations using the PFSDG-U interaction, we will comment on future developments.
