Timing in thick silicon detectors for a compton camera

In the scope of construction of a PET apparatus based on detection of Compton scattering in silicon (Compton camera), timing properties of 1-mm-thick silicon pad and double-sided microstrip detectors are studied. Timing in pad detectors is also investigated for 140.5 keV [sup.99m]Tc and 364.5 keV [sup.131]I gamma rays in view of a SPECT application. Compton scattering and energy loss of the Compton electron in silicon detector are simulated using the GEANT package. Electric field in the detector is calculated numerically for a fully depleted detector in the abrupt junction approximation, taking into account the geometry and varying the reverse voltage. Signal formation is studied using Ramo's theorem and pulse shaping properties of the trigger circuit. A time-walk cut is seen to be directly corresponding to a deposited-energy cut. At 10 keV threshold in 1-mm-thick detectors, 10 ns timing windows are shown to reject a significant portion of events, degrading efficiency or limiting the angular range in a prohibitive way. More involved techniques are therefore suggested, either in the electronics circuit or in later stages of the trigger. Index Terms--Charge collection, Compton camera, PET, silicon detectors, simulation, SPECT, timing.