Pixelated Scintillator Arrays Research Articles

Characterization of EJ-270 and Ce-doped LiCAF scintillators for the development of high-rate neutron reflectometer detectors

The Second Target Station of the Spallation Neutron Source at Oak Ridge National Laboratory is anticipated to provide a neutron source with ∼20 times increase in peak brightness than the First Target Station. The neutron reflectometers currently in operation at the First Target Station need to be upgraded due to the increased neutron flux. A prototype neutron detector module based upon a pixelated scintillator array readout by silicon photomultipliers is being developed to address the high-rate challenge faced with future neutron reflectometer instruments at the Second Target Station. Two types of scintillator materials were considered for this detector development, i.e., 6Li-loaded EJ-270 plastic scintillator and Ce-doped LiCAF single crystal. This paper reports the scintillator characterization results, including light yield, pulse shape discrimination performance, capability to detect thermal neutrons in a high γ-ray field, and γ-ray sensitivity. The number of photons produced per neutron capture by EJ-270 and LiCAF:Ce was measured to be 2176 ± 91 and 2651 ± 108, respectively. EJ-270 demonstrated a good capability to discriminate between neutrons and γ-rays by employing the commonly used charge comparison method (figure-of-merit: 1.13 ± 0.01 for an energy cut of 292–426 keVee) and a reasonable performance when using the time-over-threshold techniques; however, no discrimination was observed from LiCAF:Ce regardless of the pulse shape discrimination approaches utilized, making pulse height discrimination necessary for LiCAF:Ce to differentiate between neutrons and γ-rays. Both EJ-270 and LiCAF:Ce exhibited an acceptable capacity to detect thermal neutrons at high exposure rates up to approximately 584 mR/h. The γ-ray sensitivities measured with a60Co source at an exposure rate of around 1145 mR/h were determined to be (6.11 ± 0.87) × 10−6 and (7.64 ± 1.08) × 10−7 for EJ-270 and LiCAF:Ce, respectively.

In-situ growth of Cs5Cu3Cl6I2 nanocrystals within AAO arrays for X-ray imaging

Lead-free halide perovskites have shown promising application for indirect X-ray detection as fascinating scintillator. The fabrication of pixelated scintillator films leveraging directional confinement allows high-resolution imaging, yet process complexity and the potential crystal structure damage pose challenges that compromises the corresponding scintillator performance. In this work, a facile in-situ strategy for growing scintillation perovskite of Cs5Cu3Cl6I2 within anodic aluminum oxide (AAO) membrane is developed as scintillator Cs5Cu3Cl6I2@AAO film. The as-fabricated pixelated scintillator array with an enhanced light directional confinement for high-resolution X-ray imaging is demonstrated. The corresponding film achieves a spatial resolution of approximately 20 lp mm−1 and a detection limit of 0.152 Gy·s−1. Moreover, the Cs5Cu3Cl6I2@AAO film performs excellent stability under long-term X-ray irradiation, making it a reliable option for cost-effective pixelated indirect X-ray imaging.

A Portable Three-Layer Compton Camera for Wide-Energy-Range Gamma-ray Imaging: Design, Simulation and Preliminary Testing

(1) Background: The imaging energy range of a typical Compton camera is limited due to the fact that scattered gamma photons are seldom fully absorbed when the incident energies are above 3 MeV. Further improving the upper energy limit of gamma-ray imaging has important application significance in the active interrogation of special nuclear materials and chemical warfare agents, as well as range verification of proton therapy. (2) Methods: To realize gamma-ray imaging in a wide energy range of 0.3~7 MeV, a principle prototype, named a portable three-layer Compton camera, is developed using the scintillation detector that consists of an silicon photomultiplier array coupled with a Gd3Al2Ga3O12:Ce pixelated scintillator array. Implemented in a list-mode maximum likelihood expectation maximization algorithm, a far-field energy-domain imaging method based on the two interaction events is applied to estimate the initial energy and spatial distribution of gamma-ray sources. The simulation model of the detectors is established based on the Monte Carlo simulation toolkit Geant4. The reconstructed images of a 133Ba, a 137Cs and a 60Co point-like sources have been successfully obtained with our prototype in laboratory tests and compared with simulation studies. (3) Results: The proportion of effective imaging events accounts for about 2%, which allows our prototype to realize the reconstruction of the distribution of a 0.05 μSv/h 137Cs source in 10 s. The angular resolution for resolving two 137Cs point-like sources is 15°. Additional simulated imaging of the 6.13 MeV gamma-rays from 14.1 MeV neutron scattering with water preliminarily demonstrates the imaging capability for high incident energy. (4) Conclusions: We conclude that the prototype has a good imaging performance in a wide energy range (0.3~7 MeV), which shows potential in several MeV gamma-ray imaging applications.

Compton Camera Arrangement With a Monolithic LaBr3(Ce) Scintillator and Pixelated GAGG Detector for Medical Imaging

The results presented in this manuscript are divided in two main parts. The aim of the presented work is to conduct feasibility tests on a Compton camera setup to be later scaled up and used for PET and triple coincidence γ-PET purposes. The first part is focused on laboratory characterization of a monolithic LaBr3(Ce) scintillator coupled to a 64-channel multi-anode photomultiplier (H8500C PMT from Hamamatsu) in combination with a well-established algorithm allowing to determine the interaction’s position resolution. The second part of the article is dedicated to the evaluation of a Compton camera detector arrangement in which the monolithic scintillator acts as an absorber, while a pixelated GAGG scintillator array, read out by a SiPM multi-pixel photon counter (MPPC), is used as a scatterer component. The accuracy achievable with this system for detecting γ rays from laboratory point sources is investigated. The Compton camera system has been tested with radioactive 137Cs and 60Co point sources. The signal readout and data acquisition system is based on individual spectroscopy electronics modules (NIM and VME), for the absorber and a customized electronics based on Anger logic for the GAGG scatter array. The raw data were analyzed to serve as input for the image reconstruction, performed using the MEGAlib toolkit software. With a spatial resolution of 1 mm for the scatterer and less than 3 mm for the absorber component, respectively, shifts of 2 mm of both radioactive sources (137Cs and 60Co) could be resolved with the whole Compton camera system.

Pulse shape discrimination using a stilbene scintillator array coupled to a large-area SiPM array for hand-held dual particle imager applications

A dual-particle imager (DPI) is configured in a hand-held form factor, then one can efficiently and conveniently deploy the DPI to detect the presence of special nuclear materials (SNM) and identify any isotopic variations that differ from their natural abundances. Here we show that by maximizing the areal coupling between a pixelated scintillator array and the partitioned photosensor readout such as a silicon photomultiplier (SiPM), the information utilization of the gamma-ray and neutron information in the radiation field can be enhanced, thus enabling one to rapidly acquire spatial maps of the distributions on gamma-ray and neutron emitters.

Technical note: Comparison of arithmetic mean and energy-weighted mean flood histogram generation methods for dual-ended readout positron emission tomography detectors.

Dual-ended readout pixelated scintillator array detectors can provide a suitable crystal resolvability and satisfactory depth of interaction (DOI), energy, and timing resolutions. Usually, the flood histogram measured by one-sided readout is depth dependent, and the flood histogram quality degrades as the distance between the interaction site and photodetector increases. Information measured by two photodetectors must be combined to obtain an improved flood histogram yielding a better PET scanner spatial resolution. Two flood histogram generation algorithms for dual-ended readout of pixelated scintillator array detectors were compared by theoretical calculations and experimental measurements. The first algorithm is the arithmetic mean (AM) algorithm, which assigns the same weight to the flood histograms measured by photodetectors 1 and 2. The second algorithm is the energy-weighted mean (EWM) algorithm, which assigns each flood histogram a certain weight proportional to the energy measured by the photodetector. Theoretical equations were derived to determine the quality of the flood histograms obtained with these two algorithms. Experimental measurements were performed with an 18 × 18 lutetium-yttrium oxyorthosilicate (LYSO) array with a crystal size of 0.62 × 0.62 × 20 mm3 read out by two multi-anode photomultiplier tubes at both ends. Flood histograms of the whole array and five specific depths were compared between the above two algorithms. The theoretical results indicated that the flood histograms obtained with the EWM method matched those obtained with the AM method at the middle detector depth and were better at other detector depths when the distance (S) between the locations of the same crystal in the flood histograms measured by photodetectors 1 and 2 reached 0. The advantage of the EWM method decreased with increasing S value since the crystal position in the flood histogram obtained with the EWM method varies with the depth when S does not equal 0. The advantage of the EWM method decreased with increasing S value. The experimental results generally agreed with the theoretical predictions. Compared to the AM method, the EWM method provided a similar flood histogram at a depth of 10 mm but generated a better flood histogram at depths of 2 and 18 mm. Although an inverse correlation between Q (a quality factor representing the advantage of the EWM method) and S was observed, the variation in Q given the same S value was high. The average Q value at the same S still agreed with the theoretical predictions. Theoretical equations were derived, and experimental measurements were performed to compare two flood histogram generation algorithms for dual-ended readout PET detectors. The results indicated that the EWM method based on inverse variance weighting theory could provide better flood histograms than those provided by the AM method.

Organic glass scintillator formulations and mold development towards scalable and cast-in-place pixelated fabrications

Herein we describe the development of organic glass scintillator (OGS) additives to enable the rapid and simple fabrication of form factors necessitated by pixelated neutron detection systems. These systems, developed for national security purposes, utilize arrays of many (O(100–1000))​ bar-shaped scintillators which are optically isolated and coupled to photodetectors. We describe two overall methods to address the construction of pixelated scintillator arrays at scale: individual bars cast directly in single bar molds, reducing the need to cut the bars to size, and casting directly into pixelated molds, eliminating both the need to cut the bars to size and to fabricate the array from individual bars. The mechanical blending of pre-polymerized plastic pellets of polystyrene and polycarbonate in OGS can be formulated up to 5% by weight into OGS to rigidify high aspect-ratio bars capable of withstanding the additional stress of being cast simultaneously side by side in a single pour with a variety of new mold materials. In addition, the inclusion of 15%–20% of “2-(p-tolyl)-1,3,2-dioxaborinane” (TDB) can enable direct, cast-in-place of OGS bars into aluminum or acetal sheet molds of arrays. We also describe the developments into the new mold materials and assembly to support the casting into these form factors. As a proof of concept, we cast a single pixel directly into a mold lined with 3M’s enhanced specular reflector (ESR) and characterize its performance for neutron detection applications.

On the Use of Pixelated Plastic Scintillator and Silicon Photomultipliers Array for Coded Aperture Gamma-Neutron Imaging

We report the investigations made on the use of pixelated plastic scintillator (PS) and silicon photomultipliers (SiPMs) array applied to coded aperture gamma-neutron imaging. Specifically, verification of the ability of a multiplexing readout to discriminate and localize neutron interactions was studied. In its intended configuration, the gamma-neutron imager design consists of a coded aperture aligned with a matrix of <inline-formula> <tex-math notation="LaTeX">$12\times12$ </tex-math></inline-formula> PS each coupled to a SiPM. The coded aperture is a rank 7 modified uniformly redundant array (MURA), composed of 1.2 cm of tungsten, with a surface area of 100.4 mm <inline-formula> <tex-math notation="LaTeX">$\times100.4$ </tex-math></inline-formula> mm and placed at 5 cm from the detector. The pixelated PS is composed of polystyrene and standard fluorophores (20 wt&#x0025; PPO, 0.03 wt&#x0025; POPOP) loaded with a lithium carboxylate (Li <inline-formula> <tex-math notation="LaTeX">$\alpha $ </tex-math></inline-formula>-valerate), which allows the triple discrimination between thermal neutrons, fast neutrons, and photons. Each pixel of PS has a dimension of 3.6 mm <inline-formula> <tex-math notation="LaTeX">$\times3.6$ </tex-math></inline-formula> mm <inline-formula> <tex-math notation="LaTeX">$\times3.6$ </tex-math></inline-formula> mm and they are separated from each other by 0.6 mm of polytetrafluoroethylene (PTFE). The photonic and electronic readout consists of the ArrayC-30035-144P SiPM from SensL, Cork, Ireland, connected to the diode coupled charge division readout from AiT. First, this neutron imager design was modeled and simulated using the MCNP6 Monte Carlo code. The encoding capability, field of view, and spatial resolution of the neutron imager were therefore evaluated by simulation. Then, we detailed the experimental setups implemented to demonstrate the feasibility of coupling pixelated PS to SiPM to localize radioactive sources and showed the results obtained. Finally, based on this position-sensitive gamma-neutron detector, a gamma-neutron imager was prototyped and tested.

Row–Column Readout Method to Mitigate Radiographic-Image Blurring From Multipixel Events in a Coded-Aperture Imaging System

In order to improve the image quality of a coded-aperture-based gamma-ray imaging apparatus, a method for removing Compton scattered and other multipixel events is presented that removes inaccurate position estimations from the data stream. In coded-aperture gamma-ray imaging detectors, scattering events that are shared between different partitions in the active medium can result in the inaccurate determination of the initial gamma-ray interaction position, the result of which is distortion in the angular reconstruction mapping. For coded-aperture system in which the rapid localization and identification of the emitting source is important, the distortion due to scattered radiation can increase the integration time of the system. We show, in both software and hardware implementations using a cerium-doped gadolinium aluminum gallium garnet pixelated scintillator array, that filtering the data to remove multipixel events can improve the image quality despite the efficiency loss associated with not utilizing all of the interactions within the detector.

Optimization based on Monte Carlo simulation of a pixelated scintillator array for megavolt X-ray flash radiography

Pixelated scintillators arrays are widely used in X-ray radiographic applications and provide the sensitivity and resolution of detectors, which are determined by the pixel size and thickness. The design of pixelated scintillator arrays is impeded by the absence of a suitable method to simulate and predict the performance. A method of design and optimization based on a Monte Carlo simulation of a pixelated lutetium yttrium oxyorthosilicate scintillator array for MeV X-ray flash radiography has been established. The variance of the gain and spatial resolution of scintillators with different structural parameters have been studied. The proposed method was subsequently validated using flash radiographic experiments. The results of both agree well. Furthermore, the results indicate that an array with a large pixel size and small thickness exhibits strong scintillator gain, sufficient spatial resolution, and excellent uniformity of response.

Experimental evaluation of depth-encoding absorber designs for prompt-gamma Compton imaging in proton therapy

Prompt-gamma Compton imaging (PGCI) has been presented as a promising in-vivo method for proton range verification. An accurate estimation of the Bragg-peak position can potentially be achieved by imaging high-energy prompt-gamma rays (in the range of several MeV). Therefore, scintillation detectors with thick and high Z crystals are mostly used as Compton absorbers for high-energy gamma-ray detection. However, an absorber using thick crystals degrades the angular resolution unless the absorber can provide continuous depth-of-interaction (DOI) measurement. The study investigated various detector configurations using thick crystals for developing a Compton absorber with DOI resolving capability without compromising energy performance. Two groups of lutetium yttrium oxyorthosilicate (LYSO) arrays with dimensions of 1.8 × 1.8 × 50 mm3 and 1.8 × 1.8 × 20 mm3, and each with four different surface treatments (combining crystal surface finishing and type of reflector coverage), were constructed for the study. The DOI detector utilized the dual-ended readout of pixelated scintillator arrays for depth encoding. The results revealed that the influences of type of the reflector coverage and crystal surface roughness on the performance of the DOI detectors for the 50- and 20-mm-thick LYSO differed greatly. Moreover, the combined use of the proposed partial inter-crystal reflector coverage and the unpolished surface finishing on the 50-mm-thick crystals were shown to improve both the DOI resolution and the energy performance but degraded the flood map quality. These results provide useful guidance for selecting an applicable Compton DOI absorber design for developing a high-performance PGCI system.

PET performance evaluation of the small-animal Hyperion IID PET/MRI insert based on the NEMA NU-4 standard

The Hyperion IID PET insert is the first scanner using fully digital silicon photomultipliers for simultaneous PET/MR imaging of small animals up to rabbit size. In this work, we evaluate the PET performance based on the National Eletrical Manufacturers Association (NEMA) NU 4-2008 standard, whose standardized measurement protocols allow comparison of different small-animal PET scanners. The Hyperion IID small-animal PET/MR insert comprises three rings of 20 detector stacks with pixelated scintillator arrays with a crystal pitch of 1 mm, read out with digital silicon photomultipliers. The scanner has a large ring diameter of 209.6 mm and an axial field of view of 96.7 mm. We evaluated the spatial resolution, energy resolution, time resolution and sensitivity by scanning a 22Na point source. The count rates and scatter fractions were measured for a wide range of 18F activity inside a mouse-sized scatter phantom. We evaluated the image quality using the mouse-sized image quality phantom specified in the NEMA NU4 standard, filled with 18F. Additionally, we verified the in-vivo imaging capabilities by performing a simultaneous PET/MRI scan of a mouse injected with 18F-FDG. We processed all measurement data with an energy window of 250 keV to 625 keV and a coincidence time window of 2 ns. The filtered-backprojection reconstruction of the point source has a full width at half maximum (FWHM) of 1.7 mm near the isocenter and degrades to 2.5 mm at a radial distance of 50 mm. The scanner’s average energy resolution is 12.7% (ΔE/E FWHM) and the coincidence resolution time is 609 ps. The peak absolute sensitivity is 4.0% and the true and noise-equivalent count rates reach their peak at an activity of 46 MBq with 483 kcps and 407 kcps, respectively, with a scatter fraction of 13%. The iterative reconstruction of the image quality phantom has a uniformity of 3.7%, and recovery coefficients from 0.29, 0.91 and 0.94 for rod diameters of 1 mm, 3 mm and 5 mm, respectively. After application of scatter and attenuation corrections, the air- and water-filled cold regions have spill-over ratios of 6.3% and 5.4%, respectively. The Hyperion IID PET/MR insert provides state-of-the-art PET performance while enabling simultaneous PET/MRI acquisition of small animals up to rabbit size.

Effects of Varying Matching Between Collimator Hole and Scintillator Pixel on Gamma Camera Image Resolution

A collimator is a key component of gamma cameras, which affects imaging performance significantly. Recently, pixelated scintillator arrays are widely applied in the development of highresolution gamma cameras, instead of monolithic scintillator blocks. Therefore, combining the collimator and the pixelated scintillator has been a crucial factor in achieving good imaging performance. In this study, we investigated the effects of the shape and position of a collimator hole on the image resolution of a gamma camera with a pixelated scintillator. We designed a pixel-matched collimator and a low-energy high-resolution (LEHR) collimator with hexagonal shaped holes that have the same collimator geometrical efficiency. Simulation experiments were performed with a Monte Carlo simulation package (GATE v7.0). For both types of collimator, camera sensitivity was measured for validating the collimator efficiency, and planar images were obtained to measure spatial resolution, with changing the line profile angle based on the septa direction. Camera sensitivities for both collimators were equivalent (difference = 1.76 ± 0.51%). In contrast to the case of the pixel-matched collimator, in the case of the LEHR collimator, as the angle of the line profile varied, the spatial resolution showed a difference at each profile angle. In the case when two collimator holes symmetrically matched with one scintillator pixel, the spatial resolution showed a 64.87% difference in its maximum and a position distortion of 26.06% from the original source position, according to the profile angle, compared to the other unmatched cases. These results showed that using a pixel-matched collimator in gamma camera systems employing a pixelated scintillator is essential for the exact delineation of small regions using high resolution gamma camera imaging

Development of depth encoding small animal PET detectors using dual-ended readout of pixelated scintillator arrays with SiPMs.

The performance of current small animal PET scanners is mainly limited by the detector performance and depth encoding detectors are required to develop PET scanner to simultaneously achieve high spatial resolution and high sensitivity. Among all depth encoding PET detector approaches, dual-ended readout detector has the advantage to achieve the highest depth of interaction (DOI) resolution and spatial resolution. Silicon photomultiplier (SiPM) is believed to be the photodetector of the future for PET detector due to its excellent properties as compared to the traditional photodetectors such as photomultiplier tube (PMT) and avalanche photodiode (APD). The purpose of this work is to develop high resolution depth encoding small animal PET detector using dual-ended readout of finely pixelated scintillator arrays with SiPMs. Four lutetium-yttrium oxyorthosilicate (LYSO) arrays with 11×11 crystals and 11.6×11.6×20mm3 outside dimension were made using ESR, Toray and BaSO4 reflectors. The LYSO arrays were read out with Hamamatsu 4×4 SiPM arrays from both ends. The SiPM array has a pixel size of 3×3mm2 , 0.2mm gap in between the pixels and a total active area of 12.6×12.6mm2 . The flood histograms, DOI resolution, energy resolution and timing resolution of the four detector modules were measured and compared. All crystals can be clearly resolved from the measured flood histograms of all four arrays. The BaSO4 arrays provide the best and the ESR array provides the worst flood histograms. The DOI resolution obtained from the DOI profiles of the individual crystals of the four array is from 2.1 to 2.35mm for events with E>350keV. The DOI ratio variation among crystals is bigger for the BaSO4 arrays as compared to both the ESR and Toray arrays. The BaSO4 arrays provide worse detector based DOI resolution. The photopeak amplitude of the Toray array had the maximum change with depth, it provides the worst energy resolution of 21.3%. The photopeak amplitude of the BaSO4 array with 80μm reflector almost doesn't change with depth, it provides the best energy resolution of 12.9%. A maximum timing shift of 1.37ns to 1.61ns among the corner and the center crystals in the four arrays was obtained due to the use of resistor network readout. A crystal based timing resolution of 0.68ns to 0.83ns and a detector based timing resolution of 1.26ns to 1.45ns were obtained for the four detector modules. Four high resolution depth encoding small animal PET detectors were developed using dual-ended readout of pixelated scintillator arrays with SiPMs. The performance results show that those detectors can be used to build a small animal PET scanner to simultaneously achieve uniform high spatial resolution and high sensitivity.

A phoswich detector design for improved spatial sampling in PET

Block detector designs, utilizing a pixelated scintillator array coupled to a photosensor array in a light-sharing design, are commonly used for positron emission tomography (PET) imaging applications. In practice, the spatial sampling of these designs is limited by the crystal pitch, which must be large enough for individual crystals to be resolved in the detector flood image. Replacing the conventional 2D scintillator array with an array of phoswich elements, each consisting of an optically coupled side-by-side scintillator pair, may improve spatial sampling in one direction of the array without requiring resolving smaller crystal elements. To test the feasibility of this design, a 4×4 phoswich array was constructed, with each phoswich element consisting of two optically coupled, 3.17×1.58×10mm3 LSO crystals co-doped with cerium and calcium. The amount of calcium doping was varied to create a ‘fast’ LSO crystal with decay time of 32.9 ns and a ‘slow’ LSO crystal with decay time of 41.2 ns. Using a Hamamatsu R8900U-00-C12 position-sensitive photomultiplier tube (PS-PMT) and a CAEN V1720 250 MS/s waveform digitizer, we were able to show effective discrimination of the fast and slow LSO crystals in the phoswich array. Although a side-by-side phoswich array is feasible, reflections at the crystal boundary due to a mismatch between the refractive index of the optical adhesive (n=1.5) and LSO (n=1.82) caused it to behave optically as an 8×4 array rather than a 4×4 array. Direct coupling of each phoswich element to individual photodetector elements may be necessary with the current phoswich array design. Alternatively, in order to implement this phoswich design with a conventional light sharing PET block detector, a high refractive index optical adhesive is necessary to closely match the refractive index of LSO.

Intercrystal Scatter Rejection for Pixelated PET Detectors

High-resolution positron emission tomography (PET) scanners often use pixelated scintillator arrays with lightsharing for the detection of gamma rays. The aim of this paper is to enhance the spatial resolution of such a pixelated scintillator detector by filtering out events with multiple interactions of gamma rays in the scintillator based on the measured light distributions. To develop and evaluate such enhancements in spatial resolution, we measure the point spread function (PSF) of our detector directly using a thinly collimated gamma ray beam setup, and then later verify their benefits on a full preclinical PET system with a hotrod phantom. The scintillator detector comprises a $30\times 30\times 12$ mm3 lutetium–yttrium oxyorthosilicate array with a pitch of 1 mm coupled to a digital silicon photomultiplier array via a 2-mm lightguide. We use a center of gravity algorithm for the crystal identification; however, the proposed filters are independent of the crystal identification algorithm. Investigating a single detector with our collimated gamma beam, we reject 15% of the events as multiple interaction while improving the crystal identification efficiency from 60.0% to 68.3% and the 90th-percentile diameter of the PSF from 7.88 to 3.98 mm. On system level, we analyze a line profile through two different rod sizes in a hotrod phantom. The filters reject 32% of the coincidences and increase the peak-to-valley ratio by 8% (0.9-mm rods) and by 18% (1.2-mm rods).

Position and time resolution measurements with a microchannel plate image intensifier: A comparison of monolithic and pixelated CeBr3 scintillators

To perform Four Dimensional Age Momentum Correlation measurements in the near future, where one obtains the positron lifetime in coincidence with the three dimensional momentum of the electron annihilating with the positron, we have investigated the time and position resolution of two CeBr3 scintillators (monolithic and an array of pixels) using a Photek IPD340/Q/BI/RS microchannel plate image intensifier. The microchannel plate image intensifier has an active diameter of 40mm and a stack of two microchannel plates in chevron configuration. The monolithic CeBr3 scintillator was cylindrically shaped with a diameter of 40mm and a height of 5mm. The pixelated scintillator array covered the whole active area of the microchannel plate image intensifier and the shape of each pixel was 2.5·2.5·8mm3 with a pixel pitch of 3.3mm. For the monolithic setup the measured mean single time resolution was 330ps (FWHM) at a gamma energy of 511keV. No significant dependence on the position was detected. The position resolution at the center of the monolithic scintillator was about 2.5mm (FWHM) at a gamma energy of 662keV. The single time resolution of the pixelated crystal setup reached 320ps (FWHM) in the region of the center of the active area of the microchannel plate image intensifier. The position resolution was limited by the cross-section of the pixels. The gamma energy for the pixel setup measurements was 511keV.

A piecewise-focused high DQE detector for MV imaging.

Electronic portal imagers (EPIDs) with high detective quantum efficiencies (DQEs) are sought to facilitate the use of the megavoltage (MV) radiotherapy treatment beam for image guidance. Potential advantages include high quality (treatment) beam's eye view imaging, and improved cone-beam computed tomography (CBCT) generating images with more accurate electron density maps with immunity to metal artifacts. One approach to increasing detector sensitivity is to couple a thick pixelated scintillator array to an active matrix flat panel imager (AMFPI) incorporating amorphous silicon thin film electronics. Cadmium tungstate (CWO) has many desirable scintillation properties including good light output, a high index of refraction, high optical transparency, and reasonable cost. However, due to the 0 1 0 cleave plane inherent in its crystalline structure, the difficulty of cutting and polishing CWO has, in part, limited its study relative to other scintillators such as cesium iodide and bismuth germanate (BGO). The goal of this work was to build and test a focused large-area pixelated "strip" CWO detector. A 361 × 52 mm scintillator assembly that contained a total of 28 072 pixels was constructed. The assembly comprised seven subarrays, each 15 mm thick. Six of the subarrays were fabricated from CWO with a pixel pitch of 0.784 mm, while one array was constructed from BGO for comparison. Focusing was achieved by coupling the arrays to the Varian AS1000 AMFPI through a piecewise linear arc-shaped fiber optic plate. Simulation and experimental studies of modulation transfer function (MTF) and DQE were undertaken using a 6 MV beam, and comparisons were made between the performance of the pixelated strip assembly and the most common EPID configuration comprising a 1 mm-thick copper build-up plate attached to a 133 mg/cm(2) gadolinium oxysulfide scintillator screen (Cu-GOS). Projection radiographs and CBCT images of phantoms were acquired. The work also introduces the use of a lightweight edge phantom to generate MTF measurements at MV energies and shows its functional equivalence to the more cumbersome slit-based method. Measured and simulated DQE(0)'s of the pixelated CWO detector were 22% and 26%, respectively. The average measured and simulated ratios of CWO DQE(f) to Cu-GOS DQE(f) across the frequency range of 0.0-0.62 mm(-1) were 23 and 29, respectively. 2D and 3D imaging studies confirmed the large dose efficiency improvement and that focus was maintained across the field of view. In the CWO CBCT images, the measured spatial resolution was 7 lp/cm. The contrast-to-noise ratio was dramatically improved reflecting a 22 × sensitivity increase relative to Cu-GOS. The CWO scintillator material showed significantly higher stability and light yield than the BGO material. An efficient piecewise-focused pixelated strip scintillator for MV imaging is described that offers more than a 20-fold dose efficiency improvement over Cu-GOS.

Performance of a new Electron-Tracking Compton Camera under intense radiations from a water target irradiated with a proton beam

We have developed an electron-tracking Compton camera (ETCC) for use in next-generation MeV gamma ray telescopes. An ETCC consists of a gaseous time projection chamber (TPC) and pixel scintillator arrays (PSAs). Since the TPC measures the three dimensional tracks of Compton-recoil electrons, the ETCC can completely reconstruct the incident gamma rays. Moreover, the ETCC demonstrates efficient background rejection power in Compton-kinematics tests, identifies particle from the energy deposit rate (dE/dX) registered in the TPC, and provides high quality imaging by completely reconstructing the Compton scattering process. We are planning the ``Sub-MeV gamma ray Imaging Loaded-on-balloon Experiment'' (SMILE) for our proposed all-sky survey satellite. Performance tests of a mid-sized (30 cm)3 ETCC, constructed for observing the Crab nebula, are ongoing. However, observations at balloon altitudes or satellite orbits are obstructed by radiation background from the atmosphere and the detector itself [1]. The background rejection power was checked using proton accelerator experiments conducted at the Research Center for Nuclear Physics, Osaka University. To create the intense radiation fields encountered in space, which comprise gamma rays, neutrons, protons, and other energetic entities, we irradiated a water target with a 140 MeV proton beam and placed a SMILE-II ETCC near the target. In this situation, the counting rate was five times than that expected at the balloon altitude. Nonetheless, the ETCC stably operated and identified particles sufficiently to obtain a clear gamma ray image of the checking source. Here, we report the performance of our detector and demonstrate its effective background rejection based in electron tracking experiments.

Optimized light sharing for high-resolution TOF PET detector based on digital silicon photomultipliers

The majority of current whole-body PET scanners are based on pixelated scintillator arrays with a transverse pixel size of 4 mm. However, recent studies have shown that decreasing the pixel size to 2 mm can significantly improve image spatial resolution. In this study, the performance of Digital Photon Counter (DPC) from Philips Digital Photon Counting (PDPC) was evaluated to determine their potential for high-resolution whole-body time of flight (TOF) PET scanners. Two detector configurations were evaluated. First, the DPC3200-44-22 DPC array was coupled to a LYSO block of 15 × 15 2 × 2 × 22 mm3 pixels through a 1 mm thick light guide. Due to light sharing among the dies neighbour logic of the DPC was used. In a second setup the same DPC was coupled directly to a scalable 4 × 4 LYSO matrix of 1.9 × 1.9 × 22 mm3 crystals with a dedicated reflector arrangement allowing for controlled light sharing patterns inside the matrix. With the first approach an average energy resolution of 14.5% and an average CRT of 376 ps were achieved. For the second configuration an average energy resolution of 11% and an average CRT of 295 ps were achieved. Our studies show that the DPC is a suitable photosensor for a high-resolution TOF-PET detector. The dedicated reflector arrangement allows one to achieve better performances than the light guide approach. The count loss, caused by dark counts, is overcome by fitting the matrix size to the size of DPC single die.