GAMMASPHERE. Overview of detector and signal processing system

Gammasphere consists of a close-packed spherical array of 110 coaxial germanium detectors about 65 mm in diameter and 80 mm long each of which is in the middle of a 7-element bismuth germanate scintillator acting as a Compton suppression shield to result in a very large peak to Compton ratio in gamma-ray spectra. The need to maintain excellent energy resolution in such a complex system and to cope with high counting rates (up to 20 kHz in each detector) imposes major challenges in system design and its implementation is the main theme here. A significant new departure in detector technology is the use of longitudinal segmentation (essentially splitting a coaxial detector into two D-shaped elements) to reduce the angle subtended by a detector element at the target. This reduces the signal spread produced by the Doppler effect in gamma rays emitted from nuclei which are generally recoiling at high velocity after the impact of a high-energy incident beam particle. This feature is also covered.