Design Considerations for Double-Sided Silicon Strip Detectors Applicable to PET Imaging

One can overcome low stopping power of silicon detectors by dense stacking of multiple sensor layers. This leads to a growing number of the readout channels. In a double-sided strip geometry the number of the readout channels grows only as 2 times (size/w) compared to (size/w)² for individual cell readout, where w is the desired spatial resolution and size accounts for the multiple layers of detectors used. We performed a simulation to determine the viability of strip detectors for PET applications. GEANT4 [Agostinelli, S, et al., 2003] was used to generate the recoil electron tracks produced by the interaction of the annihilation photons. A single layer 1 mm thick silicon detector with high resistivity bulk (n-type) was assumed (rho = 24 kOmegacm, FDV ap 140 V), with p⁺- and n⁺-strips of a 400 mum width and a pitch of 500 mum. The electric (bias set to 300 V) and weighting field in the sensor were calculated and used to track the ionized charge and form raw induced signals on the electrodes. A virtual electronics in a typical arrangement (a pre-amplifier, a leading-edge trigger on a CR-RC shaper with a shaping time of 75 ns) was used to generate the timing references from the raw signals. Assuming the high count-rates in a PET setup the ability to resolve multiple hits was tested. The timing tests showed that a timing resolution on the order of 10 ns FWHM relative to original event is realistic to expect. To collect 95% of all events, triggers from both sides should be less than 50-100 ns apart, depending on the readout scheme. The matching of the hits on both sides of the detectors based on the energies collected in each strip is very efficient; even assuming a 10 keV energy resolution, 90% of all double and 70% of triple hits can be perfectly resolved. This allows operation of the sensor at the count-rates similar to the inverse of the coincident window.