Dynamic time-over-threshold method for multi-channel APD based gamma-ray detectors

Recently pixelated detectors with many channels are being applied to gamma-ray imaging application owing to advanced manufacturing technology. Pulse height information is important for gamma-ray measurement since we can avoid unnecessary events like scattered gamma-rays. However, as the number of channels increases, we need more electronics for processing each channel´s signal. As a result, its circuit size and power consumption cause problems. The Time-over-Threshold (ToT) [1] method is an effective signal processing method for those problems. However, ToT suffers from a poor linearity and its dynamic range is limited. We have proposed a new ToT method named dynamic Time-over-Threshold method (dToT) [2]. The dToT method has a dynamically changing threshold. Its threshold voltage starts sweeping after the detection of input signal. Both the sweeping pattern and the input pulse waveform affect the linearity of the system. Then we have proposed a new signal processing system with dToT and CR-RC shaping and showed a much better linearity than that of a conventional ToT. We have implemented a test circuit with GAGG scintillator and avalanche photodiode and measured pulse height spectra for a ¹³⁷CS and a ²²Na source, which showed good linearity. Encouraged by the test, we have designed a new ASIC for the multi-channel dToT system and measured spectra of ²²Na.