Nuclear Physics Rolling Grant

All of the matter around us originates utimately from the Big Bang, yet the Big Bang produces only the lightest chemical elements, hydrogen, helium and lithium. This begs the question where and how the other chemical elements are produced. In fact, Nuclear Physics underpins the processes that are responsible for transforming the lightest elements into the distribution of elements up to element 92 (uranium) which we see around us. Much of this processing is carried out in stars but to produce the very heaviest elements, and reproduce the known abundances of the elements, it is clear that much more extreme conditions of temperature, density and pressure are required. The most dramatic of these is the supernova where a dying star blows itself apart, but also important are phenomena such as novae where a star which has exhausted all its nuclear fuel (a white dwarf) suddenly flares up violently after stealing material from a neighbouring star in a binary system. Our research looks into the details of the Nuclear Physics at work in these explosive objects to understand which elements are produced as well as the energy generated. It turns out that even though such processes are highly complex and involve a huge number of possible nuclear reactions, it is only a small subset of these which impact on the final results. A focused research programme is therefore possible but many of the isotopes we need to study do not exist on earth and have to be produced as a radioactive beam in the laboratory. The question of what elements and isotopes can be generated by astrophysical processes is intimately related to the limits of nuclear existence - how many protons and neutrons can be added to a nuclear system before it falls apart? Our research focuses on nuclei on the so-called proton dripline - the limit where adding a further proton is not possible and the nucleus breaks up. This dripline lies very close to the line of nuclei with equal numbers of protons and neutrons (N=Z); such nuclei have special properties by virtue of this symmetry. Moreover, the nuclear force does not discriminate between protons and neutrons - the basic building blocks, and there are important consequences stemming from this. For example, so-called mirror nuclei which are the same under interchange of the number of protons and neutrons have near identical properties; it is the subtle differences, however, that tell us profound details of the balance of nuclear forces. Our research investigates these effects in detail and feeds back this knowledge into our understanding of stellar explosions. As well as the action of individual protons and neutrons, the overall behaviour of the atomic nucleus often reflects the result of the the protons and neutrons in the nucleus acting collectively. This collective behaviour can manifest itself as vibrations or rotations the nucleus. Rotational behaviour is ordinarily associated with a deformed nuclear shape, the commonest being prolate-deformed (rugby-ball shaped) and oblate-deformed (Smartie shaped). One of the striking properties of some atomic nuclei is their ambiguity with respect to which shape or configuration they chose to adopt. Adding a very small amount of extra energy to the nuclear system relative to the energy stored in the nucleus (which can be released, for example, through fission - E=mc^2) can prompt the nucleus to change from a spherical to a prolate or oblate shape, or vice versa. This phenomenon known as shape coexistence is highly sensitive to the properties of the individual nucleus concerned and is notoriously difficult for theoretical models to anticipate. Studying shape coexistence in nuclei therefore provides a strong challenge to our theoretical understanding of the nucleus. Naturally, a good understanding of nuclear properties underpins all aspects of Nuclear Physics discussed above. Shape coexistence therefore forms the third strand of our research programme.