Online program Overview Session: NSS-03

Abstract

Independent readout of highly pixelated organic scintillator detectors is needed for next-generation associated particle imaging, neutron radiography, and single-volume scatter cameras. Field-programmable gate arrays (FPGAs) are an attractive solution as they have hundreds to thousands of IO pins and can be programmed for a variety of applications while minimizing cost as compared to ASICs. We detail a dual time-over-threshold (ToT) method for measuring the deposited energy and pulse width of gamma and neutron interactions in scintillator using a Time-to-Digital Converter (TDC) realized with an FPGA and minimal external components. We compare a simple DC threshold and an active feedback rising edge signal for PSD and energy estimation. Simulated ToT results using digitized data are presented, with hardware details for an upcoming data campaign. For each circuit, the performance of energy resolution and PSD capability is weighed against the complexity and power requirements of the external analog components. In this work we present each of these approaches for SiPM readout, detail current progress, and experimental results.

Abstract

This work, conducted within the framework of the AIDER project, presents the design and preliminary characterization of an electronics readout architecture for SiPM-based Compton cameras (CCs) in nuclear applications. The system includes two front-end units, serving the absorber and the scatterer layers. Each module integrates four GAMMA ASICs to interface with up to 64 SiPMs per layer, performing individual charge integration while providing an aggregated sum-current output. To mitigate amplitude-walk effects over the broad energy spectra typical of Compton imaging, time stamps are extracted by a custom designed Constant Fraction Discriminator (CFD) circuit. A central FPGA with embedded Time-to-Digital Converters (TDCs) synchronizes energy and timing data across both layers. A pre-trained Artificial Neural Network (ANN) is embedded directly into the FPGA logic to provide real-time estimation of the interaction position, reducing data throughput. SiPM are biased by a custom module that dynamically compensates for temperature fluctuations. The electronics, coupled with a 25 mm x 25 mm x 5 mm LaBr3 scintillator and a 16 SiPMs tile, yielded an energy resolution of 3.6% FWHM at 662 keV. Preliminary characterization of the CFD circuit with current pulses injected into the ASICs yielded a 140 ps resolution at 511 keV maintaining a sub nanosecond resolution down to 45 keV. The amplitude walk on such dynamic range was limited to 2.15 ns.

Abstract

In this work, we present a scalable high-performance readout system for LYSO-coupled SiPMs. The project aims to enable real-time beam range monitoring in proton therapy through Prompt Gamma Imaging (PGI), detecting prompt gamma rays emitted during proton–tissue interactions [1]. This application requires operation over a wide energy range, from 3 MeV to 10 MeV, together with high count-rate capability up to 1 Mcps/channel.

The resulting platform is suitable for a broad range of gamma-ray detection applications. A compact DAQ board has been developed integrating all the electronics required for gamma event acquisition, processing, and data transmission without the need for external units [2]. The system is able to detect gamma rays with energies above 10 MeV using 12-bit energy digitization and 300 ps (FWHM) timing resolution.

The module is connected to LYSO-coupled SiPMs (Hamamatsu S14160-6015PS) featuring a 6 × 6 mm² photosensitive area and 15 μm pixel pitch. Tests with calibration sources show a 10.2% FWHM resolution at 511 keV and 6.2% at 1275 keV.

A custom ASIC specifically developed for high-energy gamma-ray detection is used for the readout of SiPM currents. With a minimum integration and readout time of 350 ns, the ASIC supports output count rates above 1 Mcps/channel.

While each module supports 32 SiPM channels, multiple identical modules can be connected in a daisy-chain configuration to support larger detector arrays. The PCB mechanical design allows multiple boards to be stacked without inactive regions between modules, with the only limitation being the available 10 Gbps transmission bandwidth.

Abstract

Photon-number-resolving (PNR) detection based on Silicon Photomultipliers (SiPMs) is emerging as a promising approach for high-rate quantum optics and quantum communication experiments, particularly for the possible implementation of hybrid schemes. We present a fully digital SiPM readout platform combining a 14-bit, 1 GS/s waveform digitizer with a real-time FPGA-based signal-processing chain optimized for fast charge extraction and photon-number reconstruction. The system is implemented on an open-FPGA architecture programmable through the Sci-Compiler framework, enabling custom firmware development without dedicated HDL design.

The firmware performs real-time baseline restoration, digital pole-zero compensation, synchronous triggering, and charge integration entirely inside the FPGA pipeline at full sampling rate. The system supports synchronous operation with externally or internally triggered laser sources and enables deterministic acquisition windows with nanosecond-scale integration gates.

Different Hamamatsu SiPM models were characterized using coherent and twin-beam optical states to evaluate the impact of detector gain, pixel pitch, pile-up, and photon detection efficiency on photon-number reconstruction. Measurements demonstrate reliable operation from mean photon numbers well below 1 up to more than 50 detected photons per pulse at repetition rates up to 1 MHz, with operation extendable toward 10–20 MHz depending on the analog front-end configuration. The digital processing chain significantly improves peak separation, dynamic range, and noise performance compared with conventional boxcar-based acquisition systems.

The presented architecture demonstrates how modern FPGA-based real-time digital processing can transform SiPM detectors into scalable, high-speed PNR systems suitable for next-generation quantum optics instrumentation and quantum communication protocols.

Abstract

Direct positron emission imaging (dPEI) enables imaging of positron-emitting radionuclide distributions without tomographic reconstruction by directly localizing annihilation events using ultrafast time-of-flight measurements. This modality was first demonstrated experimentally in 2021 using two Cherenkov-radiator-integrated microchannel plate photomultiplier tubes (CR-IMPs) read out by an oscilloscope. However, to scale to a larger number of channels for clinical applications, the oscilloscope must be replaced with an embedded front-end and digitizer. In this study, a dPEI acquisition chain for multi-channel readout was designed using two CR-IMPs, a FastIC+ front-end, and a tapped delay line time-to-digital converter implemented inside an UltraScale+ FPGA. A single-shot RMS precision of 4.0 ps was achieved by the front-end and the TDC when measuring pulses from a function generator. This timing precision is sufficient to digitize CR-IMP signals while preserving the timing performance required to approach the best coincidence timing resolution (CTR) currently measured using an oscilloscope. After evaluating the combined FastIC+ and TDC system, a complete readout chain from two CR-IMPs through the FastIC+ to the TDC was successfully configured, and coincidence events from a Na-22 source were successfully acquired during preliminary operation. We are currently optimizing the front-end parameters to achieve the best CTR between the two CR-IMPs. This study highlights the capability of the FastIC+ to read out CR-IMP signals and the ability of the combined FastIC+ and FPGA-based TDC system to digitize events with sufficient precision for dPEI.

Abstract

TEMPOROC3 is a 64-channel mixed-signal SiPM read-out application-specific integrated circuit (ASIC) designed in TSMC130 technology. The ASIC aims at measuring the energy with a 10-bit analogue-to-digital converter (ADC), time-of-arrival (TOA) and time-over-threshold (TOT) with a 40-ps bin time-to-digital converter (TDC). The analogue front-end offers a dynamic range up to 500 pC. The dual discriminator allows for measuring either two TOA at different threshold levels or the time between a rising and falling edge, giving a TOT. The dual-gain charge measurement integrates the input signal over a tuneable range from 20 to 300 ns. The 64 channels are divided into 4 clusters, each outputting data at 320 Mbps. The clusters can be synchronised to stream all channel data whenever a channel from the matrix has triggered. The number of triggered channels (per cluster) required to launch the data serialisation is programmable, as well as the window duration for accepting new triggers. At 450 kEvents/s per cluster (with 1 TDC + 1 ADC per channel), the power consumption is about 8 mW per channel. The trigger scheme for the monolithic matrix readout was successfully validated by acquiring a gamma spectrum of an ¹¹Na source with a large LYSO(Ce) crystal coupled to a 61-channel SiPM. Event filtering using a charge discriminator was validated, allowing for data rate reduction in the ASIC. Temporal characteristics were evaluated using external trigger, charge injection, laser pulses with SiPM, and coincident gamma from an ¹¹Na source. Measurements in coincidence between 2 channels from 2 ASICs have shown TDC coincidence time resolution (CTR) as low as 60 ps FWHM. Coincident annihilation gamma measurement using LYSO(Ce) crystals with SiPM demonstrated spatial sensitivity to the source displacement, providing CTR as low as 139 ps FWHM. TEMPOROC3 has been shown to be a versatile read-out ASIC with numerous configuration parameters that can be applied in ToF-PET, Compton cameras, LIDAR.