16-channel SiPM high-frequency readout with time-over-threshold discrimination for ultrafast time-of-flight applications

Abstract

Background: Over the past five years, ultrafast high-frequency (HF) readout concepts have advanced the timing performance of silicon photomultipliers (SiPMs). The shown impact in time-of-flight (TOF) techniques can further push the limits in light detection and ranging (LiDAR), time-of-flight positron-emission tomography (TOF-PET), time-of-flight computed tomography (TOF-CT) or high-energy physics (HEP). However, upscaling these electronics to a system-applicable, multi-channel readout, has remained a challenging task, posed by the use of discrete components and a high power consumption. To this day, there are no means to exploit the high TOF resolution of these electronics on system scale or to measure the actual timing performance limits of a full detector block.

Methods: In this work, we present a 16-channel HF readout board, including leading-edge discrimination and a linearized time-over-threshold (TOT) method, which is fully compatible with a high-precision time-to-digital converters (TDCs), such as the picoTDC developed at CERN. The discrete implementation allows ideal adaptation of this readout to a broad range of detection tasks. As a first step, the functionality of the circuit has been tested using the TOFPET2 ASIC as back-end electronics to emulate the TDC, also in view of its properties as a highly scalable data acquisition solution.

Results: The produced board is able to mitigate influences of baseline shifts in the TOFPET2 front end, which has been shown in experiments with a pulsed laser, increasing the achievable intrinsic coincidence timing resolution (CTR) of the TOFPET2 readout electronics from 70 ps (FWHM) to 62 ps (FWHM). Single-channel coincidence experiments including a [Formula: see text]-source, 2[Formula: see text]2[Formula: see text]3 mm[Formula: see text] LYSO:Ce,Ca crystals and Broadcom NUV-MT SiPMs resulted in a CTR of 118 ps (FWHM). For a 4[Formula: see text]4 matrix of 3.88[Formula: see text]3.88[Formula: see text]19 mm[Formula: see text] LYSO:Ce,Ca crystals one-to-one coupled to a 4[Formula: see text]4 array of Broadcom NUV-MT SiPMs, an average CTR of 223 ps (FWHM) was obtained.

Conclusion: The implemented 16-channel HF electronics are fully functionall and have a negligible influence on the timing performance of the back-end electronics used, here the TOFPET2 ASIC. The ongoing integration of the picoTDC with the 16-channel HF board is expected to further set the path toward sub-100 ps TOF-PET and sub-30ps TOF resolution for single-photon detection.

Keywords: CTR; Fast timing; High-frequency readout; PET; TOFPET2 ASIC; TOT discrimination; Time-of-flight.

Fig. 1

A detector block consisting of a $4\times 4$ Broadcom NUV-MT SiPM array, one-to-one coupled to a $4\times 4$ LYSO:Ce,Ca crystal matrix, where crystals were isolated by a 150-$\mathrm{\mu }$m layer of ESR-OCA-ESR sheets

Fig. 2

This figure illustrates the transition between setups and circuits used in this study. a Single-channel HF readout as developed by [1] and modified by [15]. b Single-channel HF readout including a TLV3801 for pulse discrimination. c Schematic circuit of a single channel of the 16-channel HF readout including the TLV3801 and establishing a connection to the TOFPET2 ASIC

Fig. 3

Signal waveforms at the output of the TLV3801 (blue) and the analog debug output of the TOFPET2 ASIC (red). The applied TOFPET2 discriminator threshold is drawn in green. These waveforms were acquired with a reduced transimpedance amplifier gain ${\mathsf{G}}_{\mathsf{T}}$=375 $\mathrm{\Omega }$ for the FEB/A of the TOFPET2 ASIC evaluation kit

Fig. 4

Manufactured PCB of the 16-channel HF readout including TOT discrimination. The track of one signal is shown in yellow

Fig. 5

Different SiPM and ASIC channel combinations realized to investigate the contribution of the electronic front end in measurements with SiPM irradiation via scintillation emission and optical photons generated by a picosecond pulsed laser (PILAS). a and b One SiPM channel coupled to two ASIC channels of the same ASIC. c Two out of 16 SiPM channels each coupled to one ASIC channel of the same ASIC and optically illuminated

Fig. 6

16-channel HF readout board including TOT discrimination and set up in a coincidence experiment with a reference detector using a ${}^{22}$Na source

Fig. 7

Computed COG of a detector block consisting of a $4\times 4$ Broadcom NUV-MT SiPM array one-to-one coupled to a $4\times 4$ LYSO:Ce,Ca crystal matrix at a bias voltage of 41.5 V (overvoltage 9 V) and a discriminator threshold of $\mathsf{vth}\_\mathsf{t}\mathsf{1}$=50. The COG was computed only considering the neighboring channels of the hottest pixel. Gaps in horizontal orientation correspond to the trenches for the bond wires of the NUV-MT SiPM array [44], opposed to an exactly symmetric 4$\times$4 crystal matrix

Fig. 8

CTR of individual channels of a detector block consisting of a $4\times 4$ Broadcom NUV-MT SiPM arrays one-to-one coupled to a $4\times 4$ LYSO:Ce,Ca crystal matrix at a bias voltage of 41.5 V and a discriminator threshold of $\mathsf{vth}\_\mathsf{t}\mathsf{1}$=  50. a DC readout at default TOFPET2 configuration with R${}_{\mathsf{in}}$ = 30 $\mathrm{\Omega }$. b DC readout with R${}_{\mathsf{in}}$ = 11 $\mathrm{\Omega }$. c Upscaled HF readout including TOT discrimination via a TLV3801 with a threshold of 90 mV

Fig. 9

Channel raw energy value spectrum for a 2$\times$2 $\times$3 mm${}^3$ LYSO:Ce,Ca crystal read out via one Broadcom NUV-MT SiPM channel of a 4 $\times$ 4 SiPM array connected to the 16-channel HF readout board at an overvoltage of 6 V and 9 V. Raw energy values were digitized via the TOFPET2 ASIC operated in qdc mode

Fig. 10

Multi-photon coincidence time resolution of the different SiPM-ASIC channel combinations illuminated with optical photons at 406 nm and at different frequencies. Data were acquired using Broadcom NUV-MT SiPMs and the TOFPET2 ASIC with an ASIC threshold of $\mathsf{vth}\_\mathsf{t}\mathsf{1}=20$. a One SiPM channel coupled to two ASIC channels of the same ASIC. b Two SiPM channels on one SiPM array each coupled to one ASIC channel of the same ASIC. c Comparison between different combinations and readout electronics at an overvoltage of 9 V

Fig. 11

Coincidence multi-photon time resolution of the different SiPM-ASIC channel combinations illuminated with optical photons at 406 nm and a pulse frequency of 10 kHz and triggering at different TOFPET2 ASIC thresholds $\mathsf{vth}\_\mathsf{t}\mathsf{1}$. Data were acquired using Broadcom NUV-MT SiPMs and a DC connection to the TOFPET2 ASIC

Fig. 12

Channel coincidence time difference spectrum for the detector block read out via the TOFPET2 ASIC (left) and for the detector block read out via the 16-channel HF readout board including TOT discrimination with the TOFPET2 ASIC emulating the TDC (right) at an overvoltage of 9 V

Fig. 13

TOFPET2 performance of a detector block consisting of a $4\times 4$ Broadcom NUV-MT SiPM arrays one-to-one coupled to a $4\times 4$ LYSO:Ce,Ca crystal matrix in comparison with exposing the SiPMs to laser pulses (406 nm) of an SiPM array. The performance with default configuration and DC readout can be optimized (1), (2), while laser pulses reveal timing limitations of the front end (3). Reducing the applied bias leads to a better intrinsic resolution (4), which is why baseline shifts are thought to be the cause of the problem. Pulse amplification and discrimination via a TLV3801, i.e., 16-channel HF electronics, are employed (5), delivering the same intrinsic timing resolution at higher overvoltages. Switching back to a full detector block with a scintillator, the performance is still limited by a high TLV threshold, which is needed due to unstable baselines at lower thresholds. (6). Adapting signal filters in the timing and energy signal paths reduces this noise and improve the CTR to DC level (7), which shows that the adapted HF readout board does not worsen the CTR and, hence, could be a good candidate for readout with the picoTDC