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Direct reaction models for exotic nuclei

Final Activity Report Summary - DREME (Direct Reaction Models for Exotic Nuclei)

Direct nuclear reactions, i.e. processes whereby a beam of one type of nucleus is fired at a target, usually composed of nuclei of a different type, in order to form a new nucleus of interest by the exchange of nuclear constituent particles, either protons or neutrons, between the beam and the target, are a valuable tool for the study of nuclear structure.

The interpretation of the actual measurable quantity in these reactions, the angular distribution of the differential cross section (essentially the probability of forming by a direct reaction the new nucleus of interest at a given angle with respect to the direction of the beam) and the extraction of the desired nuclear structure information required a reliable model of the reaction process or mechanism. Nuclei stable against ß-decay formed a small fraction of the total number of nuclides either known or expected to 'exist', i.e. to be stable against emission of neutrons, protons or a particles. The availability of accelerated beams of ß-unstable nuclei opened up new perspectives for our understanding of nuclear structure, enabling us to probe nuclear matter under conditions beyond the limited sample provided by the stable nuclei.

Direct nuclear reactions were an important source of nuclear structure information for these exotic nuclei; it was therefore necessary to develop a standardised analysis methodology for this reaction data if reliable conclusions were to be inferred from them. This project aimed to develop such a methodology by bringing together existing elements in such a way that a coherent picture of the whole reaction process could be obtained.

By the use of this methodology important information on the structure of the ground state of 8He was obtained from new data for the 8He(p,t)6He reaction. The necessity of modelling the reaction mechanism as accurately as possible for reactions involving exotic nuclei was well illustrated in this example, as was the need for complete data sets. A coherent picture of the ensemble of new plus existing data for this reaction was obtained, lending confidence to the nuclear structure information which was extracted from the analysis, for which using more approximate analysis techniques was not possible.

The second strand of this project was to investigate the mechanism of reactions induced by beams of the exotic 6He nucleus. The peculiar structure of this exotic isotope of Helium favoured breakup reactions, whereby the extra two neutrons compared to the usual stable isotope 4He were torn away, or transfer reactions where one or both of extra neutrons were removed from the 6He and placed in the target nucleus. The project investigated the effects of coupling to the transfer reactions on the fusion of 6He and its elastic scattering from a heavy target. These were found to be significant and, in a wider context, an important review article was produced in which the available data for fusion induced by light, weakly-bound exotic nuclei was viewed as a coherent whole in the light of transfer coupling effects. The available data was limited, therefore our conclusions remained preliminary, but a coherent picture seemed to emerge whereby neutron transfer reactions came to dominate at energies close to the Coulomb barrier for the class of light exotic nuclei that exhibited the neutron halo or skin phenomenon, whereby one or more 'active' neutrons orbited at relatively large distances from a compact core.