Transfer reactions on neutron-rich nuclei at REX-ISOLDE

Light-ion induced particle transfer reactions are a well-established method to investigate the single-particle structure of nuclei. For radioactive isotopes, such reactions have to be performed in inverse kinematics using light targets. Our approach to study neutron-rich nuclei by neutron transfer reactions makes use of radioactive beams obtained from the REX-ISOLDE facility (CERN). Target-like fragments (p,d,t,α) are detected and identified by a new array of position-sensitive Si detectors covering a large angular range [1]. In coincidence, the γ-rays are measured with the MINIBALL spectrometer. As first physics case, the shore of the island of inversion has been investigated with the reactions d(30Mg, 31Mg)p and t(30Mg, 32Mg)p. Lying between the nearly spherical 30Mg and the deformed 32Mg, 31Mg is expected to be right at the point where the inversion occurs. Its ground state properties are consistent with a large intruder contribution in the wave function [2,3]. For the excited states, spin assignments and single-particle configuration will be deduced from the angular distributions and the relative spectroscopic factors, respectively, determined in our experiment. The aim of the second experiment is to discover the excited spherical 02+ state in 32Mg, the analogue to the ground state in 30Mg. Since the particle-hole structure of both is similar, its population in a two-neutron transfer reaction starting from the nearly spherical ground state of 30Mg is favoured. Both experiments will shed new light on the underlying physics relevant for the formation of the island of inversion. Already approved or submitted are proposals to study the region around 68,78Ni with (d,p)-reactions. The planned upgrade of the facility to HIE-ISOLDE will allow to extend such studies towards heavier nuclei. In particular, the successful demonstration that two-neutron transfer experiments utilising a tritium target are feasible at REX-ISOLDE opens a wide field for investigations of shape coexistence and pairing correlations in exotic nuclei. The status of the programme and future plans will be presented.(author)