Time-correlated gamma-ray and electron spectroscopy can be complementary probes of the nucleus. Gamma-ray transitions are a common tool to determine the pattern of nuclear decays and hence the excited nuclear levels. Meanwhile, electrons emitted through internal conversion processes are a sensitive probe of the transition multipolarities, i.e the angular momentum and parity carried by nuclear transitions. In the special case of E0 multipolarities, that only proceed through electron emission, the electron measurements can also probe the nuclear shape.
The Department of Nuclear Physics operates a superconducting, gas-filled, 8T magnetic solenoid to transport the long-lived products of nuclear reactions to a spectrometer composed of 6 Si(Li) electron detectors and up to 7 high-purity germanium gamma-ray detectors. This combination, called Solenogam, has exceptional transport efficiency, as well as a high sensitivity to electrons and short-lived nuclei that cannot be matched by competing international devices.
We use this system to study the decay of weakly populated, long-lived nuclei that are difficult to study at the target position due to the presence of intense competing channels. Both ground-state decays, as well as the decay of long-lived excited states, called isomers, can be probed. A particular focus is the study of shape coexistence, where a nucleus can simultaneously manifest different nuclear shapes. Rather than being a simple round ball, the nucleus can be a quantum superposition of two or more different shapes, typically a mixture of spherical, prolate (rugby-ball) and/or oblate (discus) shapes.
A typical project will involve an experiment using the 14UD particle accelerator to initiate nuclear reactions and produce exotic nuclei that don’t normally exist in nature. Solenogam will be used to build the nuclear decay scheme and understand the nuclear structure, with a particular focus on isomeric decays, as well as nuclei that exhibit the strange quantum phenomenon of nuclear shape coexistence.