Annular detectors

For the annular detectors, in order to cover completely the forward angle range out to 35 degrees in the laboratory frame, there is a choice of 5-inch and 6-inch wafer technology. Both are available via commercial manufacturers. The 6-inch technology is preferred since it gives more scope for optimisation of the inner hole diameter compared to the outer diameter of the active area and it also allows the detector to be placed further from the target. Note that, whilst the strip pitch on an annular strip detector can be scaled down to very small values, there are two other important quantities that cannot be scaled with the size of the annular detector. Firstly, the achievable beam spot size (or reconstructed hit resolution) in experiments with radioactive beams is limited to of order 1 mm, and secondly there are unavoidable dead areas required for passivation at the edges of detectors and drilling of the central hole and these also amount to at least 1 mm at the crucial most forward angles. The 6-inch (150 mm) diameter annular detector is mounted at a distance of 86 mm from the target, with annular strips of 1 mm pitch which is matched to the beam spot resolution. At this distance, there is no geometrical problem imposed by the gamma-ray detectors, as the segmented clover detectors of Exogam have an overall radius of less than 70 mm and hence the close-packed cube geometry does not conflict with the larger diameter of the silicon detector. The 1 mm wide annular strips will give the scattering angle of reaction products to the required accuracy without an unacceptable overhead in the number of channels. Alternatives using resistive division were considered in the design of the array, but these introduced potential difficulties (such as the variable resistances between different segments of the detector) which were deemed to outweigh any possible benefits. Having decided to use simple non-resistive strips, it is then possible to consider segmenting the other side of the detector also, to provide azimuthal angle information. This would be useful for correlating different outgoing particles for an individual event, which would produce cleaner event selection, and furthermore would allow for the possibility of multiple hits in the annular detector to be analysed. A sensible choice for the radial segmentation is 32 segments, to match the angular spacing of the barrel segments.