Timing properties of silicon drift detectors for scintillation detection

In this paper, we have evaluated the timing properties offered by silicon drift detectors to be used as scintillation photodetectors in systems for medical imaging. The peculiar drift mechanism of 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 5 mm² device, therefore, still comparable with the shaping time used for timing measurements. In this paper, the effect on the timing performances of SDDs due to the drift mechanism is first theoretically evaluated. We have then carried out the experimental characterization of the timing properties of a 5-mm² SDD coupled to a GSO crystal, in coincidence with a NaI-PMT detector, using a ²² Na source. Despite the low conversion gain of the system (2.5 e-/keV), due to the low light output of the crystal and the nonoptimized quantum efficiency of the SDD, a timing resolution of 22 ns was measured for 511 keV 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 1 ns was achieved with more than 60.000 electrons, showing no relevant limitations due to possible jitters of the drift time.