Dejun Han

This study presents a high-performance photon-counting detector (PCD) for photon-counting computed tomography (PCCT) applications, addressing the critical challenge of pulse pile-up at high X-ray photon fluxes. The PCD is composed of a fast Lu1.8Y0.2SiO5:Ce (LYSO) scintillator with an epitaxial quenching resistor type silicon photomultiplier (EQR SiPM), which features a small microcell pitch of 10 μm, a small active area of 0.52 × 0.52 mm2 and high fill factor of 49%. It exhibits a rapid recovery time of 1.85 ns and high photon detection efficiency (PDE) greater than 30%, thus is able to achieve a high photon counting rate while retaining adequate high energy resolution. In order to further mitigate pile-up effects, a pole-zero cancellation (PZC) circuit was implemented to narrow pulse width from 46.1 ns to 19.5 ns (FWHM) along with a Pileup Recovery Algorithm (PRA) for pileup pulse discrimination. Experimental results demonstrate an energy resolution of 37.7%(FWHM) at 59.5 keV (241Am, 24 kBq) and a maximum count rate of 102.7 Mcps/mm2 under 80kV X-ray irradiation. This count rate was twice as high as that obtained without using PRA methods. These preliminary results indicate the feasibility of EQR-SiPM based scintillation PCDs in clinical PCCT. The results demonstrate the PPR method’s efficacy in improving spectral performance under high flux conditions in a laboratory setting, though its clinical scalability remains challenging

The characteristics of one-dimensional (1D) position sensitive SiPM with an intrinsic continuous cap resistive layer (CRL) and its performance in acquiring two-dimensional (2D) radiation imaging are reported in this paper. The device, with an active size of 6.14 mm × 6.14 mm, demonstrated the position measurement error of 32.0 ± 24.0 µm without correction, which accounted for 0.5% of the length of the SiPM at a mean response photoelectron number around 230 and the reverse bias voltage of 40 V. Under the same condition, it demonstrated a position resolution of 128.7 ± 20.1 µm and a time resolution of 96.0 ± 33.2 ps. A 3D scintillation detector for acquiring 2D radiation imaging with depth information of interaction (DOI) is proposed by implementing two 1D CRL SiPMs in a perpendicular orientation on the two ends of a scintillator. Using this configuration, the 2D imaging of a pixelated 10 × 10 lutetium yttrium orthosilicate (LYSO) array, with each element having a volume of 0.52 mm × 0.52 mm × 20 mm, was successfully resolved by utilizing the natural radioactive radiation of lutetium element in LYSO without distortions at the edges and corners.

The performance of an active-quenching single-photon avalanche diode (SPAD) array that is based on the tri-state gates of a field programmable gate array (FPGA) is presented. The array is implemented by stacking a bare 4 × 4 N-on-P SPAD array on a bare FPGA die, and the electrodes of the SPAD pixels and the I/O ports of the FPGA are connected through wire bonding within the same package. The active quenching action on each SPAD pixel is performed by using the properties of the tri-state gates of the FPGA. Digital signal processing, such as pulse counters, data encoders, and command interactions, is also performed by using the same FPGA. The breakdown voltage of the SPAD pixels, with an active area of 60 μm × 60 μm, is 47.2–48.0 V. When the device is reverse biased at a voltage of ~50.4 V, a response delay of ~50 ns, a dead time of 157 ns, a dark count rate of 2.44 kHz, and an afterpulsing probability of 6.9% are obtained. Its peak photon detection probability (PDP) reaches 17.0% at a peak wavelength of 760 nm and remains above 10% at 900 nm. This hybrid integrated SPAD array is reconfigurable and cost effective.

Low-gain avalanche detectors (LGADs) are promising candidates for four-dimensional (4D) tracking detectors in future high-energy physics (HEP) experiments, offering high position and time resolution. However, conventional pixel array LGADs suffer from large dead areas between pixels and require a large number of readout channels, resulting in a poor spatial resolution and high cost. In this paper, we present a one-dimensional position-sensitive LGAD (1D PS-LGAD) based on DC coupling and a resistive charge division mechanism. The 1D PS-LGAD features two cathodes and one anode, with two-dimensional position coordinates obtained by cross-placing two 1D PS-LGADs. With an active thickness of 50 μm and an active area of 2.7 mm × 2.7 mm, a 1D PS-LGAD demonstrated an average position measurement error (PME) of 32.2 ±35.4 μm, accounting for 1.2% of the side length of the active area with a 100% fill factor. The coincidence time resolution and position resolution were 8.8 ps and 22.5 μm, respectively, at the center of the device for incident 532 nm light with a spot diameter of approximately 25 μm and a mean photoelectron number (MPEN) of 6000 when the reverse voltage was 620 V.

The thickness of the high-resistivity layer (active layer) is a crucial factor affecting the timing resolution of low gain avalanche diodes (LGADs). We studied the potential improvement of the timing resolution of charge particles based on LGADs with different active layer thicknesses. The Institute of High Energy Physics (IHEP) and the Novel Device Laboratory (NDL) jointly designed two types of LGAD sensors with active layer thicknesses of 33 and 50 $\mu \text{m}$ . The timing resolution is given by contributions from the time walk, jitter, and nonuniform charge deposition (Landau term). The jitter and the Landau term contributions in the timing resolution are extracted by the combination of the ${^{90}{Sr}}$ beta source test and the picosecond laser test. The Landau contribution is found to be the dominant contribution for both types of LGAD sensors, and the Landau contribution of the 33 $\mu \text{m}$ sensor is lower than that of the 50 $\mu \text{m}$ sensor. Based on the experimental results, we perform the simulation for thinner active layer LGAD with the WeightField2 program. The timing resolution can be further improved by reducing the active layer thickness from 33 to 20 $\mu \text{m}$ .

A two-dimensional (2D) square-bordered position-sensitive (PS) silicon photomultiplier (SiPM) toward distortion-free performance with high spatial resolution is reported in this letter. The device, based on SiPM technology of an epitaxial quenching resistor, contains a central cap resistive region enclosed by a narrow square-shaped metal ring contact located at the surface periphery. Four small output metal pads are connected to the metal ring at each corner to read out signal charges. With an active area of 6.14 mm × 6.14 mm, the PS-SiPM demonstrated an average position measurement error (PME) of 86.9 μm without correction, which accounted for 1.4% of the side length of the active area. The average position resolutions were demonstrated to be ~130.7 μm and ~135.6 μm in the X and Y directions, respectively, for incident light with a spot diameter of 80 μm and a mean photoelectron number (MPEN) of ~600. In addition, an intrinsic radioactive-light image of a pixelated 12×12 lutetium-yttrium oxyorthosilicate (LYSO) array with each element volume of 0.475 × 0.475 × 6 mm3 was clearly resolved by the 2D square-bordered PS-SiPM.