A study of the γ-rays from the self-conjugate nuclei ⁶⁴Ge and ⁴⁸Cr using n-γ coincidence techniques

A neutron multiplicity detector system has been developed to study weak pure neutron evaporation channels from heavy-ion reactions. Such a system allows us to study nuclei far away from the line of β-stability - in particular, self-conjugate nuclei where the shell-effects for protons and neutrons are in phase and their combined effects may drive the nucleus towards superdeformation. The system consists of a neutron wall — 4 large quadrant neutron detectors and a small central detector with 4 large volume Ge(Li) γ-ray detectors positioned as close as possible to the target. Neutron-gamma coincidences were recorded and neutron multiplicity events could be extracted from the data. Rejection of gamma-ray events in the neutron detectors was achieved by time-of-flight and pulse shape discrimination methods. A surface barrier detector at 0° allowed a further rejection of charged particle events especially events which feature neutrons as well.The system was tested with the reaction 27 Al(24Mg, 2npγ) 48Cr at 62 MeV and a 1347 keV γ-ray transition was discovered. This has been shown to be the γ-decay of a new 8408 keV level in 48Cr.Gamma-rays from 64Ge, a nucleus predicted to have a superdeformed ground state shape, were investigated using the reaction 12C(54Fe, 2nγ)64Ge at 150 MeV beam energy. The first excited state was found to be 901.6±0.5 keV and the systematics of the region suggest a spin and parity of 2+. With this energy for the first 2+ state, the prediction of superdeformation can be ruled out. Nevertheless, this is the heaviest N=Z nucleus to be studied by in-beam γ-ray spectroscopy and this observation of pure neutron evaporation from a very neutron deficient compound nucleus is potentially important for observing heavier exotic self-conjugate systems.