Single-particle structure in neutron-rich isotopes

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A fundamental way of understanding the structure of the atomic nucleus is to consider the motion of its constituent particles, protons and neutrons, under the influence of the individual interactions between them. These so-called ab-initio calculations are difficult in all but the lightest nuclei due to increasing complexity as the number of constituents gets larger. Approaches that consider the motion of individual particles in the average field generated by all the other particles have enjoyed some success in near stable nuclei, when corrections or residual interactions are included. Such success is actually rather limited as the number of stable systems is very small in comparison with the total number of bound isotopes, but despite this such methods appear frequently even in student textbooks, giving an air of permanence and solidity to the theories. However, with increasing experimental sophistication, new effects have been found that only become apparent when studying single-particle structures over a wide range of neutron excess. Such changes in single-particle structures are surprising within the context of these models; for example, in some cases they disturb even the 'well-known' sequence of magic numbers dervived for stable systems. But they also have deeper consequences since the underlying single-particle nature of a nucleus dramatically effects other properties such as the nuclear shape and the existence and type of other excitation modes like collective vibrations and rotations of the whole nucleus. This grant proposal aims to study two aspects of single-particle structure. Firstly to investigate the mechanisms which may be responsible for the changes that are being uncovered. These can be related directly back to the force between two nucleons. It has been suggested that some of these changes are due to the tensor component of this force; this is particularly interesting if substantiated as there are only a few direct manifestations of this component in nuclear structure. Secondly the proposal aims probe a region of exotic nuclei where calculations based on the single-particle structure extrapolated rather crudely from stable systems predict some interesting new phenomena. Such calculations are based on single-particle levels extrapolated from stability, which need to be questioned given the dramatic changes in shell structure which are being uncovered recently. The plan is to measure single-particle orbitals using transfer in the A~100 region to tie down calculations in that region and to make extrapolations to the more exotic systems more reliable. The experiments will use reactions involving the transfer of single particles in collisions initiated with radioactive beams. These will be supplied by the CARIBU facility at Argonne National Laboratory which performs isotopic selection of fission fragments using a method largely independent of chemical effects and accelerates them to the required energies. The chemical independency is important as it allows beams of refractory elements to be produced. This is a unique feature which is essential for studying the A~100, Sr-Zr-Mo region. The experiments will also employ another unique device, the novel HELIOS spectrometer. This is a new concept in spectrometer design which uses a superconducting solenoid to analyse ejectile ions from transfer reactions initiated with heavy beams on light targets. It has considerable advantages over traditional methods in terms of acceptance, energy resolution and ease of particle identification. The proposal requests funds to build an essential part of this spectrometer, a device to detect the heavy recoiling ions.