Simulations of possible Ge arrays for the DESPEC setup at FAIR

At the Low Energy Branch of the future FAIR facility, the fragmented beams from the Super-FRS are slowed down in energy or completely stopped. One of the experiments proposed at this branch is DESPEC (decay spectroscopy) where studies of the decay of short-lived nuclei, either in their ground state or in an isomeric state are planned. The ions at some tens of MeV/u will be deeply implanted in an active stopper; a highly pixellated Si detector which allows correlating in time and space the signal from the implanted ion with the subsequent signal produced by the β in case of β-decay. This detector together with gammaray and neutron detectors arranged in a compact geometry around the stopper are the heart of the experimental setup. The present report deals with the simulations for the Ge array. For the moment the collaboration is considering two possibilities. The first one is to use an array of planar Ge detectors specifically designed to fulfill the DESPEC requirements. The other alternative is to use more standard segmented Ge detectors of EXOGAM [1] or TIGRESS [2] type. The R&D phase requires the realization of Monte Carlo simulations in order to determine the optimal and most cost-effective setup for the future facility. The ”planar setup” consists of an array of 24 composite planar detectors (see Fig. 1 for a schematic view). The detector unit is a stack of three planar Ge detectors with dimensions 72× 72× 22 mm. Each planar has a guard ring of 2 mm, which leaves an active Ge volume of 68×68×22 mm. The distance between the planars is 3 mm and the stack is encapsulated in an Al capsule of 1.5 mm thickness. They are situated around a focal plane of 24× 8 cm surface.