Multi-pixel Photon Counter Research Articles

First imaging demonstration of a crosshair light-sharing PET detector

The crosshair light-sharing (CLS) PET detector is our original depth-of-interaction (DOI) detector, which is based on a single-ended readout scheme with quadrisected crystals comparable in size to a photo-sensor. In this work, we developed 32 CLS PET detectors, each of which consisted of a multi-pixel photon counter (MPPC) array and gadolinium fine aluminum garnet (GFAG) crystals, and we developed a benchtop prototype of a small animal size PET. Each GFAG crystal was 1.45 × 1.45 × 15 mm3. The MPPC had a surface area of 3.0 × 3.0 mm2. The benchtop prototype had two detector rings of 16 detector blocks. The ring diameter and axial field-of-view were 14.2 cm and 4.9 cm, respectively. The data acquisition system used was the PETsys silicon photomultiplier readout system. The continuous DOI information was binned into three DOI layers by applying a look-up-table to a 2D position histogram. Also, energy and timing information was corrected using DOI information. After the calibration procedure, the energy resolution and the coincidence time resolution were 14.6% and 531 ps, respectively. Imaging test results of a small rod phantom obtained by an iterative reconstruction method showed clear separation of 1.6 mm rods with the help of DOI information.

Two dimensional array of MPPC and CsI(Tl) for radiation monitoring prototype

An effcient radiation and position monitoring system is useful for preliminary security inspection and radiation emergency preparedness and response. A high sensitivity and effective monitoring system is needed for these applications. In this study, two position-sensitive prototypes were proposed by using of 8 Multi Pixel Photon Counters (MPPC) and coupled to the cesium iodide with Thallium (CsI(Tl)) scintillator. This inorganic scintillator was chosen for its output wavelength compatibility with MPPC, slightly hygroscopic, high photon transmission, and input energy. For both prototypes, eight MPPCs with di_erent con_gurations were used to obtain active areas of 2.0 × 2.4 cm2 and 2.5 × 4.5cm2, respectively. The prototype has been tested with radioactive (Tl-204 and Co-60). The intensity received by each MPPC’s readout will be recorded using Extended Analogue SiPM Integrated ReadOut Chip (EASIROC) module. Our test showed that both prototypes were sensitive to both beta and gamma sources and were able to reliably determine the sources’ positions. It was shown that the first prototype had greater resolution and performance than the second prototype. This studies may help to improve radiation detection system which is suitable for homeland security, medical field and scanning system.

A layered single-side readout depth of interaction time-of-flight-PET detector

We are exploring a scintillator-based PET detector with potential of high sensitivity, depth of interaction (DOI) capability, and timing resolution, with single-side readout. Our design combines two previous concepts: (1) multiple scintillator arrays stacked with relative offset, yielding inherent DOI information, but good timing performance has not been demonstrated with conventional light sharing readout. (2) Single crystal array with one-to-one coupling to the photodetector, showing superior timing performance compared to its light sharing counterparts, but lacks DOI. The combination, where the first layer of a staggered design is coupled one-to-one to a photodetector array, may provide both DOI and timing resolution and this concept is here evaluated through light transport simulations. Results show that: (1) unpolished crystal pixels in the staggered configuration yield better performance across all metrics compared to polished pixels, regardless of readout scheme. (2) One-to-one readout of the first layer allows for accurate DOI extraction using a single threshold. The number of multi pixel photon counter (MPPC) pixels with signal amplitudes exceeding the threshold corresponds to the interaction layer. This approach was not possible with conventional light sharing readout. (3) With a threshold of 2 optical photons, the layered approach with one-to-one coupled first layer improves timing close to the MPPC compared to the conventional one-to-one coupling non-DOI detector, due to effectively reduced crystal thickness. Single detector timing resolution values of 91, 127, 151 and 164 ps were observed per layer in the 4-layer design, to be compared to 148 ps for the single array with one-to-one coupling. (4) For the layered design with light sharing readout, timing improves with increased MPPC pixel size due to higher signal per channel. In conclusion, the combination of straightforward DOI determination, good timing performance, and relatively simple design makes the proposed concept promising for DOI-Time-of-Flight PET detectors.

Mid-infrared photon counting and resolving via efficient frequency upconversion

Optical detectors with single-photon sensitivity and large dynamic range would facilitate a variety of applications. Specifically, the capability of extending operation wavelengths into the mid-infrared region is highly attractive. Here we implement a mid-infrared frequency upconversion detector for counting and resolving photons at 3 μm. Thanks to the spectrotemporal engineering of the involved optical fields, the mid-infrared photons could be spectrally translated into the visible band with a conversion efficiency of 80%. In combination with a silicon avalanche photodiode, we obtained unprecedented performance with a high overall detection efficiency of 37% and a low noise equivalent power of 1.8 × 10 − 17 W / Hz 1 / 2 . Furthermore, photon-number-resolving detection at mid-infrared wavelengths was demonstrated, for the first time to our knowledge, with a multipixel photon counter. The implemented upconversion detector exhibited a maximal resolving photon number up to 9 with a noise probability per pulse of 0.14% at the peak detection efficiency. The achieved photon counting and resolving performance might open up new possibilities in trace molecule spectroscopy, sensitive biochemical sensing, and free-space communications, among others.

Improving the spatial resolution of a pixelated LaBr3(Ce) scintillator coupled with a multi-pixel photon counter array for boron neutron capture therapy

Fundamental preclinical research with mice to improve boron neutron capture therapy requires a prompt gamma-ray imaging detector that can chart the real-time accumulation of 10B in tumors. This study aimed to improve the spatial resolution of our previous detector, which was based on a slab-shape LaBr3(Ce) scintillator, by changing to a pixelated 8 × 8 array scintillator.The difference between the scintillation-light detection processes, such as Compton scattering and the photoelectron effect, affects energy resolution. This was experimentally revealed and evaluated with Monte Carlo simulations. The events caused by Compton scattering were eliminated, and our detector obtained the desired energy resolution for 511 keV gamma rays at 5.0 ± 0.4 %. The spatial resolution was evaluated using collimated gamma rays and was found to be better than that of the previous system because the scintillation light did not spread over the pixelated scintillator. As a result, the proposed detection system had the energy resolution to discriminate between 478- and 511-keV gamma rays to obtain the 10B concentration. The lateral and vertical spatial resolutions FWHM of the improved system were 5.9 mm and 5.2 mm, respectively, which were better than the value of 8.0 mm provided by the previous system to visualize the two-dimensional distribution in real time.

Optical Pulse Position Modulation Synchronization and Data Recovery Based on Multipixel Photon Counter

Optical Pulse Position Modulation Synchronization and Data Recovery Based on Multipixel Photon Counter

GAGG–MPPC detector with optimized light guide thickness for combined Compton-PET applications

Silicon photomultipliers (SiPMs) have become a standard photodetector to be coupled with scintillators in PET, but the SiPM saturation is limiting the performance achievable with the detectors employed, in particular the high energy resolution which is necessary for whole gamma imaging (WGI). The concept of WGI combines PET and a Compton camera by inserting a scatterer detector ring into a PET ring. Not only typical SPECT radionuclides such as 99mTc (140 keV), but also unusual positron emitters such as 89Zr (909 keV) and 44Sc (1157 keV) can be imaging targets. For better spatial resolution in Compton imaging, the scatterer detector requires better energy resolution for a wide range of deposited energies. The use of bright scintillators such as GAGG is essential, but the SiPM saturation may prevent full use being made for such bright scintillators. We expected that inserting a thick light guide between GAGG and SiPM could spread scintillation photons to surrounding SiPMs and eliminate the saturation effect. On the other hand, the thicker the light guide becomes, the greater the number of scintillation photons that may be absorbed. Therefore, in this paper, we investigated the relationship between the light guide thickness and the energy resolution. A 22 × 22 array of GAGG crystals (0.9 ×0.9× 6 mm3 each) was optically coupled to the 8 x 8 multi-pixel photon-counter (MPPC) array (3 × 3 mm2 pixel, 50 ×50μm2 sub-pixel) via a light guide, for which thickness was changed from 0 (i.e., without the light guide) to 8 mm. Using point sources with different energies (133Ba, 22Na and 137Cs), we compared crystal identification performance, linearity of the output signal and energy resolution. Increasing the light guide thickness gradually degraded crystal identification performance but improved linearity of the output signals. Energy resolution at 81 keV constantly deteriorated with increasing light guide thickness. Energy resolutions at 356, 511 and 662 keV were improved with increasing light guide thickness to a certain value after which they deteriorated; the thicknesses at which deterioration started were 2.0 mm, 3.0 mm and 4.0 mm, respectively, for the energy resolutions at 356, 511 and 662 keV. We found that the optimum light guide thickness for the target energy range was 2.0 mm, and for this thickness, energy resolution values were 22.0% at 81 keV, 7.6% at 356 keV, 8.3% at 511 keV and 8.2% at 662 keV.

Pulse Shape Discrimination of CsI(Tl) With a Photomultiplier Tube and Multipixel Photon Counters

In this study, we evaluate and compare the pulse shape discrimination (PSD) performance of multipixel photon counters (MPPCs, also known as silicon photomultiphers - SiPMs) with that of a typical photomultiplier tube (PMT) when testing using CsI(Tl) scintillators. We use the charge comparison method, whereby we discriminate different types of particles by the ratio of charges integrated within two time-gates (the delayed part and the entire digitized waveform). For a satisfactory PSD performance, a setup should generate many photoelectrons (p.e.) and collect their charges efficiently. The PMT setup generates more p.e. than the MPPC setup does. With the same digitizer and the same long time-gate (the entire digitized waveform), the PMT setup is also better in charge collection. Therefore, the PMT setup demonstrates better PSD performance. We subsequently test the MPPC setup using a new data acquisition (DAQ) system. Using this new DAQ, the long time-gate is extended by nearly four times the length when using the previous digitizer. With this longer time-gate, we collect more p.e. at the tail part of the pulse and almost all the charges of the total collected p.e. Thus, the PSD performance of the MPPC setup is improved significantly. This study also provides an estimation of the short time-gate (the delayed part of the digitized waveform) that can give a satisfactory PSD performance without an extensive analysis to optimize this gate.

Silicon photomultiplier (Si-PM) comparisons for low-energy gamma ray readouts with BGO and CsI (Tl) scintillators

We studied herein the low-energy performances of three multi-pixel photon counters (MPPCs, silicon photomultipliers, Si-PMs), namely, S13360-6050VE, S13360-6050CS, and S14160-6050HS, provided by Hamamatsu K.K. to read 1 cm3 BGO and CsI (Tl) scintillators. S13360s had 1/4 lower dark currents, while S14160-6050HS had 25% higher quantum efficiency and 1.5× higher gain with a lower operational voltage. S13360-6050VE and S14160-6050HS were used for surface mounting, while S13360-6050CS was in a black ceramic case. We changed the overvoltage and found that the best performances are achieved at overvoltages of +4 V and +2.5 V for S13360s and S14160-6050HS, respectively. In the case of the BGO scintillator, S13360-6050VE obtained a lower threshold of 6 keV compared to 9 keV of S14160-6050HS. Both surface-mounting Si-PMs achieved an energy resolution of ∼35% at 59.5 keV of the 241Am source. The threshold and the energy resolution of S13360-6050CS were the worst, because the black case absorbed approximately half of the scintillation photons. The CsI (Tl) scintillator yielded a number of scintillation photons that was 6× higher than the BGO. The same trends were measured for the three MPPCs. This result is attributable to the lower threshold determined by the dark current, and the energy resolution mainly dominated by the statistical fluctuation of the scintillation photons.

A digital phoswich detector using time-over-threshold for depth of interaction in PET

We present the performance of a digital phoswich positron emission tomography (PET) detector, composed by layers of pixilated scintillator arrays, read out by solid state light detectors and an application specific integrated circuit (ASIC). We investigated the use of integrated charge from the scintillation pulses along with time-over-threshold (ToT) to determine the layer of interaction (DOI) in the scintillator. Simulations were performed to assess the effectiveness of the ToT measurements for separating the scintillator events and identifying cross-layer-crystal-scatter (CLCS) events. These simulations indicate that ToT and charge integration from such a detector provide sufficient information to determine the layer of interaction. To demonstrate this in practice, we used a pair of prototype LYSO/BGO detectors. One detector consisted of a 19 × 19 array of 7 mm long LYSO crystals (1.36 mm pitch) coupled to a 16 × 16 array of 8 mm long BGO crystals (1.63 mm pitch). The other detector was similar except the LYSO crystal pitch was 1.63 mm. These detectors were coupled to an 8 × 8 multi-pixel photon counter mounted on a PETsys TOFPET2 ASIC. This high performance ASIC provided digital readout of the integrated charge and ToT from these detectors. We present a method to separate the events from the two scintillator layers using the ToT, and also investigate the performance of this detector. All the crystals within the proposed detector were clearly resolved, and the peak to valley ratio was 11.8 ± 4.0 and 10.1 ± 2.9 for the LYSO and BGO flood images. The measured energy resolution was 9.9% ± 1.3% and 28.5% ± 5.0% respectively for the LYSO and BGO crystals in the phoswich layers. The timing resolution between the LYSO–LYSO, LYSO–BGO and BGO–BGO coincidences was 468 ps, 1.33 ns and 2.14 ns respectively. Results show ToT can be used to identify the crystal layer where events occurred and also identify and reject the majority of CLCS events between layers.

Development of an optical photon-counting imager with a monolithic Geiger Avalanche Photodiode array

Abstract We have developed a sensor system based on an optical photon-counting imager with high timing resolution, aiming for highly time-variable astronomical phenomena. The detector is a monolithic Geiger-mode avalanche photodiode array customized in a Multi-Pixel Photon Counter with a response time on the order of nanoseconds. This paper evaluates the basic performance of the sensor and confirms the gain linearity, uniformity, and low dark count. We demonstrate the system’s ability to detect the period of a flashing light-emitting diode, using a data acquisition system developed to obtain the light curve with a time bin of 100 μs. The Crab pulsar was observed using a 35-cm telescope without cooling, and the equipment detected optical pulses with a period consistent with the data from the radio ephemeris. Although improvements to the system will be necessary for more reliability, the system has been proven to be a promising device for exploring the time-domain optical astronomy.

Development and characterization of thermal helium beam diagnostic with four helium lines for RFX-mod2 experiment.

Thermal Helium Beam (THB) diagnostic is widely used for measuring the electron density and temperature in the boundary region of fusion plasmas, edges, and scrape-off layers. In its standard configuration, it measures three HeI lines (667.8 nm, 706.5 nm, and 728.1 nm) and, by using a collisional-radiative model, evaluates ne and Te from the ratios of their intensities. At large neutral He density (n0 ≳ 1017 m-3), radiation re-absorption is not negligible and it has to be taken into account; it can be estimated by measuring the intensity of the fourth HeI line, λ = 501.6 nm. The original THB diagnostic of the RFX-mod experiment has been upgraded, setting up the fourth line intensity acquisition. A Czerny-Turner spectrograph separates the lines, and the old multichannel photomultiplier (PMT) detectors are replaced with the new Multi-Pixel Photon Counter (MPPC). Two 16-channel MPPC array modules allow the observation of 32 signals (4 lines × 8 spatial points). Since the MPPCs are not sensitive to the magnetic field, the whole system can be installed near the experimental device, allowing a large reduction in the optical fibers' length with a gain in the collected signal intensity. This new THB will be installed in the new experiment RFX-mod2, the upgrade of the RFX-mod device. The RFX-mod2 will operate as both reversed field pinch and tokamak, and the goal of the THB is the evaluation of the edge electron density and temperature profiles in the two magnetic configurations, in D or H plasmas. This paper describes the system, the performance of the MPPC compared with the PMTs, the alignment, and the calibration.

Long-reach underwater wireless optical communication with relaxed link alignment enabled by optical combination and arrayed sensitive receivers.

To extend the transmission distance and relax the strict alignment requirement of underwater wireless optical communication ((UWOC), we design and implement a UWOC system using a 3×1 fiber combiner and a high-sensitive multi-pixel photon counter (MPPC). The 50-m and 100-m transmission distances (corresponding to 24 attenuation lengths) are experimentally achieved with the data rates of 16.78 Mbps and 8.39 Mbps, respectively, in a 50-m standard swimming pool. Moreover, we also investigate and optimize the performance of misalignment tolerance of this system using two MPPCs as the detectors together with different diversity reception technologies. At the 50-m transmission distance, the maximum offset between the MPPC array and the light spot center can be extended to 9 m using the maximal ratio combining (MRC), while the maximum offset is 6 m when using single MPPC.

A Crosshair Light Sharing PET Detector With DOI and TOF Capabilities Using Four-to-One Coupling and Single-Ended Readout

To improve positron emission tomography (PET) image quality, depth-of-interaction (DOI) information and time-of-flight (TOF) information are key technologies. In this work, we developed the DOI-TOF detector based on our original single-ended readout scheme with the continuous layered gadolinium fine aluminum garnet (GFAG) array. The size of each GFAG crystal is 1.45×1.45×20 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> . The multipixel photon counter (MPPC) used as a photo sensor has a surface area of 3.0×3.0 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . One pair of crystals coupled with a partial optical window is arranged across two MPPCs. Boundaries without the partial optical window are covered with optical reflectors. Each pair is coupled with paired MPPCs of different patterns. Crystal identification is obtained from paired MPPCs of different patterns and output rates. By limiting the spread of scintillation light, the proposed DOI detector based on the local centroid calculation was expected to improve crystal response, including the edge. For performance comparison, we prepared three discrete layers of the GFAG array with the same total thickness. For the continuous layer, the DOI resolution, the energy resolution and the timing resolution of the pair of detectors are 4.7 mm, 14%, and 402 ps, respectively. Their respective values are 29%, 20%, and 33% better than those with the three discrete layers.

Study on ZnS(Ag) for alpha spectrometer using silicon based photo sensor

The measuring of alpha particles is essential in many research fields such as radiation safety, health science, high energy, and nuclear physics. One of the most hazardous background radiation sources, radon, emits alpha particles, and damages lungs. Thus, the measurement of alpha particles in air correlates with the amount of radon and indicates when to ventilate the air. To detect alpha particles decayed from radon, a silicon photon multiplier (SiPM) in the shape of a multipixel photon counter (MPPC) and a scintillator optimized for zinc sulfide phosphor [ZnS(Ag)] alpha measurement were used. In this study, a dedicated amplifier was designed. The change in the pulse shape owing to operational amplifier, feedback resistor, and capacitor affect were studied and compared to that of a photo multiplier tube (PMT).

Annealing of proton radiation damages in Si-PM at room temperature

We report the radiation damage results of two new types of Multi-Pixel Photon Counter (MPPC) under the 200 MeV proton beam, for cosmic gamma-ray observations with CubeSats. The new MPPCs, S13360-6050CS and S14160-6050HS, have recently become commercially available by Hamamatsu Photonics K.K. (HPK). After 100 rad irradiation, the dark current increased by a factor of ∼100 and the threshold energy of gamma-ray detection by CsI scintillator increased, but S14160-6050HS was less degraded. We then confirmed the annealing effects of the dark current and threshold energy for both. The decrease rate of the dark current in both MPPCs were similar to those in the previous works. At 20 °C, the threshold energies of both MPPCs reduced by several tens of percent after irradiation, but below 0 °C, the threshold energy of S14160-6050HS was not significantly lowered. We also confirmed that annealing began before one month post-irradiation.

64-channel photon-counting computed tomography using a new MPPC-CT system

X-ray computed tomography (CT) is a widely used diagnostic tool to visualize the interior of the human body. However, the exposure dose of conventional CT in a single scan is large, typically 10 mSv, and therefore, it is necessary to find ways to reduce the radiation dose. Furthermore, conventional CT does not contain the energy information of individual X-ray photons because the X-ray signals are read out as an integrated form. This causes misidentification of materials. To resolve this issue, we propose a novel photon counting CT (PC-CT) system consisting of multi-pixel photon counters (MPPCs) coupled with high speed scintillators. The system has a 64-channel MPPC array that improves energy information and wide-area imaging. By fine energy adjustment and increasing the number of energy thresholds to six, which were newly implemented in the 64-channel PC-CT system, we succeeded in accurately estimating the concentrations of contrast agents such as iodine and gadolinium. Moreover, for mixed phantoms of iodine and gadolinium, we demonstrate discrimination between them, and estimate the concentrations individually, which cannot be done by conventional CTs. This shows great potential in expanding the applications of X-ray CTs.

245 ps-TOF brain-dedicated PET prototype with a hemispherical detector arrangement

Brain PET, which has led research in molecular imaging and diagnosis of brain cancer, epilepsy and neurodegenerative disorders, is being spotlighted again to promote earlier diagnosis of dementia with the advent of amyloid and tau tracers. To meet this demand, in this paper, we developed a brain-dedicated PET imaging device with a hemispherical detector arrangement, which provides comparable sensitivity with fewer detectors than conventional cylindrical geometries. The introduction of the time-of-flight (TOF) measurement capability was a key point for the development to get a gain in the image signal-to-noise ratio. Currently, whole-body PET scanners with around 200–400 ps coincidence resolving time (CRT) are commercially available. In order to obtain the same TOF gain which can be obtained with 400 ps CRT for a 30 cm diameter object, 267 ps CRT will be required for a 20 cm diameter object such as the human head. In this work, therefore, we aimed at developing a TOF brain-dedicated PET prototype with the hemisphere detector arrangement and the CRT faster than 267 ps. The detector was composed of a 12 × 12 lutetium fine silicate (LFS) array coupled with a 12 × 12 multi-pixel photon counter (MPPC) array. Each LFS crystal with a size of 4.14 × 4.14 × 10 mm3 was individually coupled to a separate MPPC. Singles list-mode data from each detector were stored, and coincidences were identified using a coincidence-detection software algorithm. The CRT of 245 ps was finally achieved as the system average after a fine timing correction. For image reconstruction, we implemented the list-mode TOF-OSEM. For a small rod phantom, rods of 3 mm diameter were clearly separated. Also, images of the 3D Hoffman brain phantom, which demonstrated clear contrast between gray and white matter, supported the effect of TOF information.

Development of Miniaturized Electrochemiluminescence Instrument using Multi‐pixel Photon Counter as the Optical Detector

AbstractWe developed a miniaturized electrochemiluminescence (ECL) instrument coupled with a light‐emitting diode‐based bipolar electrochemical sensor (LED‐BPES). This instrument composes of a microcontroller circuit, a power supply circuit, a potentiostat, an optical detecting circuit, and a communication circuit. The multi‐pixel photon counter (MPPC), which is low‐cost, small‐size, and wide‐range in optical measurements, is chosen as the optical detector. The LED‐BPES composes of a disposable screen‐printed carbon electrode (SPCE) and a surface‐mount red LED. Depended on the closed bipolar electrode (C‐BPE) structure, the LED‐BPES not only avoids the employment of unstable and complex ECL reactions but also offers a cost‐effective alternative for the over‐priced ECL reagents by using a mini‐size commercial LED as the luminescent producer. The combination of MPPC and LED‐BPES helps to set up the simplified and downsized instrument system with low price and high efficiency. The presented instrument coupled with LED‐BPES works excellent in electroactive molecules detection and has great potential in the application of heavy metal ions detection.

Advanced characterization of an optical fibre sensor system based on an MPPC detector for measurement of X-ray radiation in clinical linacs

A reliable, accurate and in-vivo dosimetry system for measuring the radiation dose and profiling the X-ray beam during radiotherapy is reported. Its dynamic range is investigated using an accurately controlled pulsed light emitting diode (LED) system. Highly resolved temporal analog and digital signals were captured from the analog and digital outputs of a multi-pixel photon counter (MPPC) detector when exposed to the LED system. The photon distribution of a low intensity pulsed LED light source was observed and is found to obey a Poisson distribution with changing light intensity. The average number of photons was obtained using the digital MPPC output signals which in turn allowed the appropriate intensity of the light source to be determined for the correct light exposure conditions for the detector. The average analog output voltage over a single 3 μs pulse is determined to indicate the intensity of the detected light. The MPPC detector output analog signal is limited to a narrow range (0.6 V–1.4 V) to ensure adequate signal detection level (the lower limit) and prevention of entry into saturation (the upper limit) which also corresponds to a digital output signal range (in counts). An average photon number range of 3–7 for the digital output signal is established, which leads to the establishment of a unique and constant photon number to average output voltage ratio of 4.64 ± 0.10. Experimental results show that the establishment of this ratio is significant as adherence to it ensures the correct exposure conditions of the MPPC and speeds up the measurement cycle in the clinical setting.