Initial Measurements with the PETsys TOFPET2 ASIC Evaluation Kit and a Characterization of the ASIC TDC

For a first characterization, we used the two KETEK-PM3325-WB SiPMs each equipped with a 3 mm x 3 mm x 5 mm LYSO scintillation crystal provided with the PETsys TOFPET2 ASIC evaluation kit. We changed the lower of two discriminator thresholds (D_T1) in the timing branch from vth_t1 = 5-30. The overvoltage was varied in a range of 1.25-7.25 V. The ambient temperature was kept at 16 °C. For all measurements, we performed an energy calibration including a correction for saturation. We evaluated the energy resolution, the coincidence resolving time (CRT) and the coincidence rate. At an overvoltage of 6 V, we obtained an energy resolution of about 10% full width at half maximum (FWHM), a CRT of approximately 210 ps FWHM and 400 ps full width at tenth maximum, the coincidence rate showed only small variations of about 5%. To investigate the influence of the ambient temperature, it was varied between 12 °C and 20 °C. At 12 °C and an overvoltage of 6.5 V, a CRT of approx. 195 ps FWHM and an energy resolution of about 9.5% FWHM could be measured. Observed satellite peaks in the time difference spectra were investigated in more detail. We could show that the location of the satellite peaks is correlated with a programmable delay element in the trigger circuit.