The Exogam Array

The Exogam array will consist of high-resolution germanium detectors. These will be arranged to give a high photopeak efficiency, 28% for 0.662 MeV gamma rays in the simple ``gamma-cube'' geometry (50.3mm from the target) envisaged for use with TIARA. Very high efficiency is required since the physics interest will naturally concentrate on the study of the most exotic species and hence will require the weakest beams. Segmented clover Ge detectors will be used in the array to give the optimum solid angle coverage and optimum performance in a radioactive beam environment. Segmentation is required to provide the optimum performance in terms of efficiency, energy resolution for gamma rays from scattered reaction products, and minimisation of multiple-hit events. These Ge detectors will be supplied with suppression shields made from bismuth germanate BGO and CsI(Na). They will also optionally have passive heavy metal collimators.

  

Figure 3: The EXOGAM spectrometer with 16 segmented clover detectors. TIARA will be able to use the eight 90 degree detectors, but in many applications it is intended to employ just the top, bottom and two side detectors placed as close as possible to the target. This is known as the ``Gamma-Cube'' geometry.

The assembly of the complete Exogam array is shown in fig. 3 to indicate the geometrical arrangement. In operation with the TIARA array, the emphasis is on the octagonal collection of detectors in the ring around the beamline at $90^\circ$. The option will be provided whereby this complete $90^\circ$ ring can be operated with TIARA. However, in most cases it will be advantageous to remove a further four detectors and to adjust the detectors on the top, bottom and side to be as close to the target as possible.