Theoretical Nuclear Physics

The description of nuclei far from stability in terms of the underlying forces is one of the most important scientific questions of the day. Studies of nuclei on the neutron-rich side of the valley of stability probe the equation of state of nuclear matter at lower than normal densities, in contrast with ultra-relativistic heavy-ion collisions which seek to create nuclear matter at high temperature and density.

Our knowledge of the detailed structure of neutron-halo nuclei comes largely from detailed reaction studies. In particular, reactions involving multi-particle continuum states will provide important information on the nature of the correlations in the halo, as long as their interpretation relies crucially on good reaction theories. Surrey has been in the forefront of the development of realistic few-body models of nuclear reactions, working to a full theory of break-up spectra in the presence of both Coulomb and nuclear forces.

We aim to strengthen our position as world leaders in the theory of nuclear reactions involving halo nuclei. Our theories of single nucleon transfer and knockout reactions are vital spectroscopic tools for light and medium mass nuclei, in support of our collaborations with GANIL and MSU. We are also developing and exploiting these strengths for photonuclear reactions leading to charge exchange and pion production.

At the interface between the theory of nuclear structure and reactions, we are involved in developing calculations of asymptotic normalising coefficients and one-body overlap integrals. These are necessary for extracting structure information from reaction studies, and new approaches have been developed to apply to halo nuclei and for systems of astrophysical interest.

Complementary to the reaction studies, we have a vigorous research programme in nuclear structure theory. Some of these are directly geared towards improving the structure input to the reaction calculations of light exotic nuclei, using techniques such as the hyperspherical functions formalism for many-body systems, which has also been used in the search for bound multineutron states. Other techniques used include a Sturmian approach to few-body structure calculations.

A long standing interest of the group is in the properties of K-isomers, or excited spin trap states, which are long lived excited states found in heavy systems. The properties of these are studied with regard to deformation and residual interaction effects. These studies are closely related to the experimental work of the group. Other possible classes of excited states in medium to heavy mass nuclei are being investigated, including tetrahedral nuclei (pyramid-shaped). It has been calculated that some nuclei should be able to exist in a metastable tetrahedral shape for a short time. These interesting states have analogues in molecular physics and await a definite observation in nuclei. Meanwhile, calculations of possible collective modes (rotation or vibration) built on these exotic states are underway which will help to pin down the experimental signature.

Although most nuclear ground states are spherical or axially deformed, the more general triaxial deformation manifests itself in nuclear excited states. Such deformations can give rise to new kinds of motion which have recently been seen experimentally. In particular, chiral bands and wobbling motion appear in the Osmium region of nuclei which reveal details of the underlying structure not seen in ground states. To calculate the properties of these nuclei, self-consistent rotating mean field calculations are used with a residual interaction as a starting point, beyond which states of good angular momentum are projected out. These, then are used in a Generator Coordinate Method, to build up a correct picture of these new kind of nuclear states. The methods have application also to other many-body systems, such as metal clusters and Bose-Einstein condensates.

Another line of our research uses a full effective interaction in many-body perturbation theory to develop a sophisticated picture of ground states, including correlations - an effect which becomes more important as one moves away from the realm of stable nuclei towards the unstable nuclei which are the focus of much of contemporary nuclear experimental research. The effective interaction developed in this approach has also been applied to the properties of nuclear matter and neutron star structure.

Please contact Jeff Tostevin for further information about theoretical nuclear physics research at Surrey.