A silicon photomultiplier signal readout using transmission-line and waveform sampling for Positron Emission Tomography

We are investigating a new way of SiPM signal readout for Time-of-flight Positron Emission Tomography detector. Our new method adopts a transmission-line connected to multiple SiPMs in a row and high speed waveform sampling technology. The event information are retrieved from the digitized waveform measured at the two ends of the transmission-line; the interaction position along the strip direction is inferred from the arrival time difference at the two ends. With this approach, the number of readout channels can be efficiently reduced for larger detection area coverage by exploiting the fast time characteristic of SiPM while preserving the advantages of its compact size. We have built prototype transmission-line boards which holds up to eight SiPMs (3.5x3.5 mm² and 5.0 mm pitch) on the strip. The waveforms of signals from the strip were recorded using DRS4 evaluation board at 5 GHz sampling rate. From the tests using pulsed laser and light emitting diode, the position along the strip could be determined in ~1 mm FWHM from the measured time difference on the strip. The responses to 511 keV photon were measured also in a coincidence setup; two LYSO scintillators (3×3×10 mm³) were coupled to the SiPMs on each transmission-line board, and a 22Na source was placed in the middle of the two LYSO scintillators with the distance of 12 cm. Preliminary results show that the position resolution of ~3 mm along the strip-line is achieved with LYSO+SiPM signal in addition to ~15% energy resolution at 511 keV and ~560 ps FWHM of coincidence time resolution. The results indicate that the investigated approach is a promising way for SiPM signal readout for TOF PET.