Two-nucleon structures in the most exotic nuclei

Today nuclear physicists are able to produce exotic nuclei with very abnormal neutron and proton combinations by using dedicated, laboratory-based accelerators. Although produced only rarely and fleetingly in our earth-based laboratories, these exotic nuclei are the raw materials for the synthesis of the heavier elements in the Universe and are of considerable importance in nuclear astrophysics. It is a scientific challenge both to calculate and measure their novel properties, structures and stability. One very successful applicable nuclear structure theory is the shell model. Originally a model involving independent nucleon motions, modern shell-model structure calculations are also able to include the effects of residual interactions between the pairs of (otherwise independent) nucleons, using forces that reproduce the measured masses, charge radii and low-lying excited states of a large number of well studied nuclei. These result in detailed predictions also for the exotic nuclei in which the motions of pairs of nucleons are now correlated and where it is energetically favourable for two nucleons to 'pair' to states with total spin of zero. Testing and improving the accuracy of the theoretical predictions for these completely new nuclei is a long term goal to which this research project will contribute. The interrogation (spectroscopy) of these quantum mechanical shell model states, of individual nucleons and of pairs of nucleons in very short-lived exotic nuclei, can be achieved using suitable direct nuclear reactions that probe just one or two nucleons leaving the rest, ideally, undisturbed. Two-nucleon knockout, two-nucleon transfer and two-nucleon decay are all processes that are sensitive to the wave function of pairs of nucleons, although they are expected to have somewhat different sensitivities to the wave function components in different spin states and spatial regions. The proposed research brings together shell model structure and nuclear reactions experts to begin to clarify such sensitivities. We propose to study to what extent different direct reactions could be used to probe correlated nucleon motions and pairing effects, including any novel nucleon pairing effects that may arise when there is a large excess of one type of nucleon: e.g. di-neutron pairs in very neutron-rich nuclei. These studies will be carried out for systems in which one: (a) removes a pair of like-nucleons that are tightly bound - as occurs in two-proton removal from a neutron rich nucleus - which is a rather clean direct process, and (b) removes a pair of like nucleons that are very weakly bound - as occurs in two-neutron removal from a neutron-rich nucleus - and which will involve both direct and indirect (two-step) reaction paths. The research is expected to guide future experimental proposals to test the theoretical predictions.