Shell Evolution in the Region of 78Ni

The standard nuclear shell model predicts 78Ni to be doubly magic. However, shell evolution may weaken the Z=28 and N=50 closures; whether they persist in this region remains uncertain. The nucleus 80Zn lies on the N=50 shell closure, two protons above 78Ni, probing the shell closure strength. A sensitive probe for nuclear structure is the RBE2=B(E2,4+→ 2+)/B(E2,2+→0+). This work presents energy and lifetime measurements of excited states in 78,80Zn from an experiment at the RIBF employing a HPGe tracking array. A B(E2,4+→2+) value of 153+23−16 e2fm4 is measured in 80Zn, leading to an RBE2 value of 1.05+0.20−0.17, inconsistent with pure collective expectations. State-of-the-art Discrete Non-Orthogonal Shell Model calculations fail to reproduce these values, predicting a value of 0.616, suggesting additional effects, such as collectivity, may be necessary to describe this nucleus, indicating that the N = 50 shell closure may be weaker than previously predicted.

Further experiments are required for a complete understanding, such as the spectroscopy of 79Ni, which will necessitate developments in detector technology. The HYPATIA array is a planned next-generation scintillator array made up of GAGG and CeBr3 scintillators coupled to SiPM detectors. This work develops GAGG SiPM detectors for the array. The best energy resolution of 5.1(2) % FWHM at 662 keV has been achieved. Tests confirmed the detectors’ sensitivity to 10 MeV gamma rays and fast neutrons. A sum-amplifier SiPM readout board was developed, achieving a time resolution of 520(20) ps at 1.3 MeV. Improved timing will allow for a reduction in the background and open up the possibility for direct lifetime measurements. Finally, the detectors were clustered into prototype modules that were successfully used in experiments. The HYPATIA array will allow for the spectroscopy of 79Ni, providing a complete understanding of the N=50 shell closure in the region of 78Ni.