Timing in Thick Silicon Pad Detectors

Our group is developing a Compton camera [Clinthorne, N, 2006] for medical imaging. Most camera incarnations envision the scatterer as a set of thick silicon pad detectors, and the absorber as a conventional NaI scintillator camera. High single detector countrates are expected in both sub-detectors, requiring good timing resolution in both detectors. For this paper the timing properties of currently used pad detectors associated with readout electronics were tested. The silicon pad detectors used were 1 mm thick, with a pad size of 1.4 by 1.4 mm
²
. The depletion voltage was 140 V, and the range of biases set were up to 300 V. The detectors are coupled to VATAGP3 ASIC; each channel consists of a charge-sensitive amplifier coupled to a pair of CR-RC shapers (200 ns, 2-5 mus). Trigger is given by a leading edge discriminator on the fast shaped pulse. The results were encouraging, showing 15-19 ns FWHM timing resolution with no time-walk correction for interaction electron energies above 10-times the threshold set on the triggering circuit. The present simulation studies predicted even a slightly worse resolution and the discrepancy will be resolved in further studies.