SITH: An SiPM Readout ASIC for Prompt Gamma-Ray Detection in Hadrontherapy Applications

We present the design and experimental characterization of spectroscopy imaging timing hadrontherapy (SITH) ASIC, a custom-designed integrated circuit (IC) 16-channel ASIC for the readout of arrays of silicon photomultipliers (SiPMs) coupled to pixelated scintillators. This IC measures both the energy and time-of-arrival (ToA) of the detected gamma photons, and it is tailored for the specific application of prompt gamma imaging (PGI) for particle range verification in hadrontherapy treatments. The SITH ASIC features a low input impedance ( $\lt 10~\Omega $ ), an input dynamic range close to 80 dB with a full-scale range of 5nC, multiple triggering logic (internal/external), power dissipation of 14 mW/ch, and the flexibility for being used with both pixelated and monolithic scintillation crystals. Moreover, the input stage of each channel can process high input currents (in the tens of milliampere range) and can be coupled to large-area photodetectors (up to tens of nanofarad). The coincidence resolving time (CRT) of annihilation events measured with $3\times 3$ mm2 SiPMs coupled with $3\times 3\times 5$ mm3 LYSO scintillators is close to 200-ps FWHM. The reported experimental measurements were conducted with a 16-channel prototype of a prompt gamma-ray detection module, which provides a real-time measurement of the gamma-ray hit position, energy, and ToA through 16 time-to-digital converters (TDCs) implemented in FPGA. The maximum count rate per channel is approximately 3 Mc/s.