GP00809N DARBY Iain.

The group is lead institution for an accepted experiment at REX-ISOLDE to perform single nucleon transfer reactions around 68Ni (IS469). This experiment will investigate the single particle properties of the neutron rich Ni isotopes in the mass region around 68Ni and subsequently later up to doubly-magic 78Ni. The goal of the study is to probe the changing behaviour of neutron magic numbers as one approaches 78Ni. In particular the region between 68Ni and 78Ni is of interest since here the single unique-parity 1g9/2 orbital is progressively filled allowing the observation of changes in shell effects from a single neutron orbital. However present experimental evidence has been unable to clarify the specific influence of this orbital. 68Ni is at present a somewhat anomalous nucleus with contradictory information describing its status as a "good" (or not) semi-magic or double-shell closure nucleus at the closed neutron harmonic oscillator sub-shell closure, N=40. 67Ni can, in terms of the extreme single particle shell model, be considered as a 68Ni core coupled to a neutron hole. A first step in this programme is to study the single particle character of the ground and first excited states of 67Ni thereby unambiguously determining the spins and parities of the first excited states and measuring the spectroscopic factors of those states as well as that of the ground state. This will allow us to verify whether the extreme single particle description for 67Ni is valid and give some indication as to the degree to which N=40 can be considered as a proper subshell closure. Experimentally this can be achieved by populating 67Ni via the transfer reaction 66Ni(d,p)67Ni. In this reaction a beam of 66Ni will enter the target chamber and impinge upon a CD2 target after which a single proton will be ejected and the excited 67Ni nucleus will de-excite in-flight via the emission of gamma-rays and subsequently leave the target chamber. The experimental setup consists of a CD2 target, an array of Silicon detectors to measure the ejected proton and an array of HPGe detectors (MINIBALL) to measure the emitted gamma-rays and a further downstream detector behind the primary beam dump to measure the purity of the beam. In order to obtain spectroscopic information for all the levels in 67Ni, the protons produced from the (d,p) reaction have to be detected either on their own (singles) or in coincidence with a corresponding gamma-ray(s) (a cleaner measurement). However for the low spin states in 67Ni the population of the ground state and the second excited state (microsecond lifetime) are not accompanied by prompt gamma-ray emission forcing a reliance on proton singles for these states, while other states and the first excited state (picosecond lifetime) can be measured in coincidence mode. In order to maximise the effectiveness of the experiment two targets have been proposed corresponding to the two modes (1) a thin target for singles measurements and (2) a thick target for coincidence events.However it would be desirable to maximise the yield of the experiment and ascertain the character of the second excited state by identifying those events where the second excited state was populated by looking at the 67Ni nucleus after it had left the target chamber and observing it de-populate the isomer via a characteristic gamma-ray cascade. These gamma-rays would then act a "tag" to identify the 67Ni nucleus and allow it to be associated with observed proton events providing information about the wavefunction of the isomeric state, which due to its long half life involve a change in the nuclear structure of the nucleus. In order to achieve this a detection setup around the beam dump is required which is able to observe the gamma-ray cascade from the microsecond isomer with a high efficiency and is able to cope with the large background and count rate due to its location at the beam dump. The development of such a detector system will be the Fellows primary task.

Keywords:radioactive ion beams, nuclear structure, acceleration, radiation detection