Abstract
Theoretical and experimental studies of neutron-rich nuclei have shown that the general concept of shell structure is not as robust and universal as earlier thought, but can exhibit significant changes as a function of neutron excess. New magic numbers appear and some other conventional ones disappear mainly because of a different ordering of the single-particle orbitals. In the present contribution, recent experimental studies of neutron-rich Cu isotopes, performed at RIKEN using β decay and one-proton knockout reactions, will be discussed. Neutron-rich nuclei near 78Ni were populated through in-flight fission of 238U on thick 9Be targets in both experiments. In the β-decay study, 75,77Ni nuclei were implanted into the WAS3ABi silicon array, while γ rays from excited states in 75,77Cu emitted after β decay of the implanted ions were detected with the EURICA Ge detector array that was surrounding the active stopper. In a second experiment within the SEASTAR campaign at RIKEN, the same 75,77Cu nuclei were produced in (p,2p) knockout reactions from 76,78Zn beam particles at around 250 MeV/nucleon impinging onto the MINOS liquid hydrogen target. In the latter experiment the DALI2 NaI array was used to detect de-excitation γ rays measured in coincidence with Cu nuclei identified in the Zero Degree Spectrometer. Both studies are complimentary and greatly contribute to our understanding on the nuclear structure in the 78Ni region.
Highlights
Over the last two decades, investigations at radioactive ion beam facilities have revealed that atomic nuclei undergo significant changes in their shell structure when moving towards neutron-rich isotopes
This so-called shell evolution for nuclei with large neutron-to-proton asymmetry is due to specific properties of the nucleon-nucleon interaction, which can lead to a different ordering of singleparticle orbitals and, to the appearance or disappearance of shell gaps
The results indicate that the inversion of the π2p3/2 and π1 f5/2 orbitals does not occur at N = 46 (75Cu), as was Experiments to study neutron-rich Cu isotopes in knockout reactions were performed within the SEASTAR col
Summary
Over the last two decades, investigations at radioactive ion beam facilities have revealed that atomic nuclei undergo significant changes in their shell structure when moving towards neutron-rich isotopes. The aim of the experiments that are presented here was to contribute to a more quantitative understanding of the shell structure near 78Ni. the evolution of the proton single-particle energies will be discussed as a function of neutron number, together with the consequences for the occupation of intruder configurations and the onset of deformation and collectivity. The β-decay experiment discussed above gave insight into the single-particle properties in the 78Ni region based on comparisons with shell model calculations. It gave a first indication for a 7/2− intruder state based on a proton excitation across the Z = 28 shell gap (see Figure 4).
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