Timing in silicon pad detectors for Compton cameras and high resolution PET

Silicon pad detectors and self-triggering readout electronics have been developed for use in Compton cameras and very high resolution PET applications. Events of interest in both techniques are registered when one or more detector interactions occur within a narrow time coincidence window. And in both methods poor time resolution leads directly to poor discrimination between true and random coincidences. Initial coincidence timing measurements between two silicon detectors each having 512 1.4 mmtimes1.4 mm pads in a 32times16 array have demonstrated a cusp-shaped spectrum of 80 ns FWHM with long tails. Ideally, for PET and Compton camera applications, time resolution would be less than 10 ns FWHM. This investigation demonstrates that the shape of the spectrum and to a large extent its width result from time-walk due to (1) the wide range of energies deposited in the silicon detectors from Compton interactions, (2) the leading-edge threshold trigger employed in the readout ASIC, and (3) the ~200 ns peaking time in the fast-channel of the readout ASIC. Although walk is a major component of the timing spectrum, it is not the only reason for broadening as determined by both simulations and measurements correlating pulse-height with threshold-crossing time