Nuclear Physics Rolling Grant

Lead Research Organisation: University of York
Department Name: Physics

Abstract

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.

Publications

10 25 50
 
Description The first direct reaction measurements for the 17O(a,p0 reaction were completed. this work has proved to be important for understanding the production of heavy elements in low metallicity rotating massive stars. Also the 18F(p,a0 reaction work, performed within the Gamow window, has provided key information to constrain the reaction rate over the novae temperature range.

In nuclear structure, we have obtained key information on isospin mixing via studies of E1 transitions in mass 31 and 35 mirror pairs. We also demonstrated that 75Rb has structures which retain substantial collectivity at the band termination points. this is at odds with what was predicted and indeed expected from rotational like structures that exhaust the spin in a given configuration. In addition, from an experiment at GSI we were able to firmly identify evidence for the long predicted spin-gap isomers in 96Cd and 97Cd. This work validates the results of long standing shell model calculations in the region approaching the doubly magic nucleus 100Sn.

A considerable amount of effort has also been put into instrument development for research purposes during the grant period. This included work on an active target gas counter and silicon detector array, SHARC, for nuclear astrophysics studies. On the structure side we have worked on the AGATA Ge spectrometer plus a new plastic charged particle veto box along with enhancements to the recoil-beta-tagging method developed by our group in the last grant. In addition, work has been done to help develop the PARIS scintillator array. These above instruments will be used in future grant work.
Exploitation Route The published work from this grant can be used to help develop nuclear models and nuclear reaction network calculations of astrophysical importance.
Sectors Education

 
Description ENSAR
Amount € 24,000 (EUR)
Organisation Sixth Framework Programme (FP6) 
Sector Public
Country European Union (EU)
Start 01/2008 
End 12/2012