Single-particle structure and nucleon-transfer reactions

The introduction of the spin-orbit interaction by Goepper-Maier and Jensen led to an understanding of the observed shell gaps and magic numbers in near-stable nuclei. The appearance of these ideas in undergraduate textbooks gives the impression of solidity and permanence to the well-known sequence of magic numbers. Recent observations, however, have challenged this basic assumption by suggesting that the sequence of single-particle states observed near stability is actually quite fragile; studies of nuclei far from the line of beta stability have begun to indicate that the familiar shell gaps do not persist in exotic systems. Instead, shifts in the sequence of single-particle levels conspire to give gaps, which change with changing nucleon number, fundamentally reshaping the basis of nuclear structure and producing new and unexpected phenomena. The reasons for these fundamental alterations to one of the basic tenets of nuclear physics are currently being debated and are of paramount interest in the development of the understanding of atomic nuclei. Single-nucleon transfer offers a suitable probe of the single particle-characteristics via the spectroscopic factor (SF), measuring the overlap of the wave function of a state with simple single-particle configurations. Being subject to sum rules, SFs allow access to the occupancies of underlying single-particle orbits. Studying the evolution of single-particle structure is important in highlighting which aspects of the nuclear force drives shell evolution.

Members of the Nuclear Group at Manchester are leading the future scientific exploitation of a new spectrometer currently being installed at ISOLDE, CERN - the ISOL Solenoidal Spectrometer (ISS). The ISS has been designed to study the single-particle properties of nuclei using short-lived radioactive ion beams. This enables the studies of the evolution of single-particle structure to be extended away from stability in to exotic regions of the nuclear chart.