How does nuclear shell structure evolve with neutron excess? Using next generation radioactive ion beams facilities to explore exotic nuclei.

Next-generation radioactive ion beams will provide us with a means of extending our understanding of the atomic nucleus way beyond that of our current knowledge. This goes beyond the laboratory of the nucleus: it will allow us to answer questions about the universe with quantitative data: questions such as, why are the observed abundances of elements as they are? and, how does matter behave at the extremes of nuclear existence e.g. the neutron star? Key to understanding the atomic nucleus is an understanding of the underlying structure of the nucleus; the ordering and properties of single-particle states, and how these evolve with the addition of neutrons or protons to the nucleus. I propose to explore these the trends in these properties, or more simply, explore the evolution of nuclear shell structure as nuclei become ever more asymmetric in their neutron to proton number. This will involve exploiting the capabilities of various facilities. First, I plan to use the radioactive beams produced at Argonne National Laboratory in the USA, in conjunction with a new, novel type of detector. This detects particles taking part in reactions where a single neutron or proton is added to, or removed from, the fast-moving radioactive beam nuclei after it has struck a stationary foil. This all takes place in the centre of a large magnet, similar to those used for MRI techniques in hospitals. Light ions from these reactions are detected using silicon detectors, which measure their energy and position. From this we can extract the ordering and properties of single-particle states. This technique is very powerful, but relies on intense radioactive beams, which are not always available. In the next few years, newly-developed, cutting-edge facilities in Europe will provide us with access to high-intensity radioactive beams, in some instances with unique capabilities. I envisage the development of an optimised, second-generation version of the detector described above. This will allow us to capitalise on these unique radioactive ion beams, and push our understanding of nuclear structure further. The second theme in my research looks at the much-hypothesised decay process called neutrinoless double beta decay. Here a nucleus undergoes two simultaneous beta-decay processes, emitting two beta particles: there are several candidate nuclei for this process. This process is one of the most exciting and talked-about possibilities in physics today. Normal beta decay is accompanied by neutrinos (or antineutrinos)---extremely low mass, neutral particles. In neutrinoless double beta decay, these are not observed as they instantly annihilate with each other, meaning they are there own antiparticles, or that new physics is at work. This would have profound consequences for the Standard Model of Particle Physics, as it appears to violate lepton-number conservation, one of the unbreakable symmetries in the model. An observation of such a process would also provide us with the opportunity to access the mass of this elusive particle via it measured half-life and a quantity called the nuclear matrix element. These nuclear matrix elements are not well understood: theoretical calculations differ considerably. I have been working on extracting experimental data to help constrain these calculations, in particular for the 76Ge nucleus, a candidate for this decay process. Throughout my Fellowship I plan to continue such efforts exploring another candidate for this decay process, 130Te.