Timing properties of silicon drift detectors for scintillation detection

In this work we have evaluated the timing properties offered by silicon drift detectors to be used as scintillation photodetectors in systems for medical imaging. The peculiar drifting mechanism or the charge created inside the SDD volume is responsible for a rise time of the signal at the output of the device when this is irradiated over its whole active area. Despite this effect, the rise time is in the order of 200 ns for a 5mm² device, therefore still comparable with the shaping time used for timing measurements. In the paper, the effect on the timing performances of SDDs due to the drifting mechanism is first theoretically evaluated. We have then carried out the experimental characterisation of the timing properties of a 5mm² SDD coupled to a GSO crystal, in coincidence with a NaI-PMT detector, using a ²²Na source. Despite the low conversion gain of the system (1240e-/MeV), due to the low light output of the crystal and the no-optimized quantum efficiency of the SDD, a timing resolution of 22 ns was measured for 511keV photons. This corresponds to a product resolution times number of collected electrons of about 13.9 × 10³ ns×e-h which is comparable to the one achieved with APDs of similar areas. By irradiating the SDD directly with laser pulses, a resolution better than I ns was achieved with more than 60.000 electrons, showing no relevant limitations due to possible jitters of the drift time.