Target considerations

As outlined in section 1, the spectroscopic study of nuclei via transfer reactions in inverse kinematics imposes severe constraints on the target thicknesses that can be used. This is one of the key reasons for developing the TIARA array to take advantage of gamma-ray detection. The next problem is how to insert and replace targets when they are surrounded as much as possible by charged-particle and gamma-ray detectors. In the initial implementation of TIARA, the target materials are envisaged as being fairly easy to manipulate, i.e. natural or deuterated polyethylene foils for transfer, or nickel or lead foils for Coulomb excitation. In the longer term, the planning should allow for the transfer of targets that have been stored in vacuum such as ⁴⁰Ca, which could be employed for studies along the N=Z line with fusion-evaporation. When the target thickness is a key parameter, as it is for transfer studies, then the angle that it is mounted with respect to the beam must also be important. Also, for Coulomb excitation experiments, the range of scattering angles of interest will affect the choice of target angle. In some cases a perpendicular orientation would be best, but in some cases the target could be aligned for example with the angle at which the coverage of the annular detectors and the barrel detectors meet. Finally, it is never possible simply to put the beam down the beamline and do an experiment. It is always necessary to focus, and often to perform runs with a range of targets to characterise background reactions. The focussing can be achieved in this case using a small blank aperture together with PPAC tracking detectors and the zero degree telescope. Thus, the TIARA design has been chosen to include a simple and reliable target changer which operates with a rod. Space limitations mean that only one target can be mounted at a time, but the target can be withdrawn on the rod into a vacuum transfer cell, and replaced without breaking the vacuum in the target chamber. Again for simplicity and reliability, the target enters the detector barrel through a gap created by omitting one side of the octagonal barrel. This also provides sufficient room to insert targets mounted on specially designed tips for the rod, which allow any desired target angle to be employed.