Radiation hardness of Ce-doped sol-gel silica fibers for high energy physics applications.

The results of irradiation tests on Ce-doped sol-gel silica using x- and γ-rays up to 10 kGy are reported in order to investigate the radiation hardness of this material for high-energy physics applications. Sol-gel silica fibers with Ce concentrations of 0.0125 and 0.05 mol. % are characterized by means of optical absorption and attenuation length measurements before and after irradiation. The two different techniques give comparable results, evidencing the formation of a main broad radiation-induced absorption band, peaking at about 2.2 eV, related to radiation-induced color centers. The results are compared with those obtained on bulk silica. This study reveals that an improvement of the radiation hardness of Ce-doped silica fibers can be achieved by reducing Ce content inside the fiber core, paving the way for further material development.

Preparation and luminescence properties of ZnO:Ga - polystyrene composite scintillator.

Highly luminescent ZnO:Ga-polystyrene composite (ZnO:Ga-PS) with ultrafast subnanosecond decay was prepared by homogeneous embedding the ZnO:Ga scintillating powder into the scintillating organic matrix. The powder was prepared by photo-induced precipitation with subsequent calcination in air and Ar/H2 atmospheres. The composite was subsequently prepared by mixing the ZnO:Ga powder into the polystyrene (10 wt% fraction of ZnO:Ga) and press compacted to the 1 mm thick pellet. Luminescent spectral and kinetic characteristics of ZnO:Ga were preserved. Radioluminescence spectra corresponded purely to the ZnO:Ga scintillating phase and emission of polystyrene at 300-350 nm was absent. These features suggest the presence of non-radiative energy transfer from polystyrene host towards the ZnO:Ga scintillating phase which is confirmed by the measurement of X-ray excited scintillation decay with picosecond time resolution. It shows an ultrafast rise time below the time resolution of the experiment (18 ps) and a single-exponential decay with the decay time around 500 ps.

Performance evaluation of the FastIC readout ASIC with emphasis on Cherenkov emission in TOF-PET.

Objective.The efficient usage of prompt photons like Cherenkov emission is of great interest for the design of the next generation, cost-effective, and ultra-high-sensitivity time-of-flight positron emission tomography (TOF-PET) scanners. With custom, high power consuming, readout electronics and fast digitization the prospect of sub-300 ps FWHM with PET-sized BGO crystals have been shown. However, these results are not scalable to a full system consisting of thousands of detector elements.Approach.To pave the way toward a full TOF-PET scanner, we examine the performance of the FastIC ASIC with Cherenkov-emitting scintillators (BGO), together with one of the most recent SiPM detector developments based on metal trenching from FBK. The FastIC is a highly configurable ASIC with 8 input channels, a power consumption of 12 mW ch-1and excellent linearity on the energy measurement. To put the timing performance of the FastIC into perspective, comparison measurements with high-power consuming readout electronics are performed.Main results.We achieve a best CTR FWHM of 330 ps for 2 × 2 × 3 mm3and 490 ps for 2 × 2 × 20 mm3BGO crystals with the FastIC. In addition, using 20 mm long LSO:Ce:Ca crystals, CTR values of 129 ps FWHM have been measured with the FastIC, only slightly worse to the state-of-the-art of 95 ps obtained with discrete HF electronics.Significance.For the first time, the timing capability of BGO with a scalable ASIC has been evaluated. The findings underscore the potential of the FastIC ASIC in the development of cost-effective TOF-PET scanners with excellent timing characteristics.

Image Reconstruction Analysis for Positron Emission Tomography With Heterostructured Scintillators.

The concept of structure engineering has been proposed for exploring the next generation of radiation detectors with improved performance. A TOF-PET geometry with heterostructured scintillators with a pixel size of 3.0 × 3.1 × 15 mm3 was simulated using Monte Carlo. The heterostructures consisted of alternating layers of BGO as a dense material with high stopping power and plastic (EJ232) as a fast light emitter. The detector time resolution was calculated as a function of the deposited and shared energy in both materials on an event-by-event basis. While sensitivity was reduced to 32% for 100-µm thick plastic layers and 52% for 50 µm, the coincidence time resolution (CTR) distribution improved to 204 ± 49 and 220 ± 41 ps, respectively, compared to 276 ps that we considered for bulk BGO. The complex distribution of timing resolutions was accounted for in the reconstruction. We divided the events into three groups based on their CTR and modeled them with different Gaussian TOF kernels. On an NEMA IQ phantom, the heterostructures had better contrast recovery in early iterations. On the other hand, BGO achieved a better contrast-to-noise ratio (CNR) after the 15th iteration due to the higher sensitivity. The developed simulation and reconstruction methods constitute new tools for evaluating different detector designs with complex time responses.

A2Bn-1PbnI3n+1 (A = BA, PEA; B = MA; n = 1, 2): Engineering Quantum-Well Crystals for High Mass Density and Fast Scintillators.

Quantum-well (QW) hybrid organic-inorganic perovskite (HOIP) crystals, e.g., A2PbX4 (A = BA, PEA; X = Br, I), demonstrated significant potentials as scintillating materials for wide energy radiation detection compared to their individual three-dimensional (3D) counterparts, e.g., BPbX3 (B = MA). Inserting 3D into QW structures resulted in new structures, namely A2BPb2X7 perovskite crystals, and they may have promising optical and scintillation properties toward higher mass density and fast timing scintillators. In this article, we investigate the crystal structure as well as optical and scintillation properties of iodide-based QW HOIP crystals, A2PbI4 and A2MAPb2I7. A2PbI4 crystals exhibit green and red emission with the fastest PL decay time <1 ns, while A2MAPb2I7 crystals exhibit a high mass density of >3.0 g/cm3 and tunable smaller bandgaps <2.1 eV resulting from quantum and dielectric confinement. We observe that A2PbI4 and PEA2MAPb2I7 show emission under X- and γ-ray excitations. We further observe that some QW HOIP iodide scintillators exhibit shorter radiation absorption lengths (∼3 cm at 511 keV) and faster scintillation decay time components (∼0.5 ns) compared to those of QW HOIP bromide scintillators. Finally, we investigate the light yields of iodide-based QW HOIP crystals at 10 K (∼10 photons/keV), while at room temperature they still show pulse height spectra with light yields between 1 and 2 photons/keV, which is still >5 times lower than those for bromides. The lower light yields can be the drawbacks of iodide-based QW HOIP scintillators, but the promising high mass density and decay time results of our study can provide the right pathway for further improvements toward fast-timing applications.

Ultrafast and Radiation-Hard Lead Halide Perovskite Nanocomposite Scintillators.

The use of scintillators for the detection of ionizing radiation is a critical aspect in many fields, including medicine, nuclear monitoring, and homeland security. Recently, lead halide perovskite nanocrystals (LHP-NCs) have emerged as promising scintillator materials. However, the difficulty of affordably upscaling synthesis to the multigram level and embedding NCs in optical-grade nanocomposites without compromising their optical properties still limits their widespread use. In addition, fundamental aspects of the scintillation mechanisms are not fully understood, leaving the scientific community without suitable fabrication protocols and rational guidelines for the full exploitation of their potential. In this work, we realize large polyacrylate nanocomposite scintillators based on CsPbBr3 NCs, which are synthesized via a novel room temperature, low waste turbo-emulsification approach, followed by their in situ transformation during the mass polymerization process. The interaction between NCs and polymer chains strengthens the scintillator structure, homogenizes the particle size distribution and passivates NC defects, resulting in nanocomposite prototypes with luminescence efficiency >90%, exceptional radiation hardness, 4800 ph/MeV scintillation yield even at low NC loading, and ultrafast response time, with over 30% of scintillation occurring in the first 80 ps, promising for fast-time applications in precision medicine and high-energy physics. Ultrafast radioluminescence and optical spectroscopy experiments using pulsed synchrotron light further disambiguate the origin of the scintillation kinetics as the result of charged-exciton and multiexciton recombination formed under ionizing excitation. This highlights the role of nonradiative Auger decay, whose potential impact on fast timing applications we anticipate via a kinetic model.

Advances in heterostructured scintillators: toward a new generation of detectors for TOF-PET.

Objective.Time-of-flight-positron emission tomography would highly benefit from a coincidence time resolution (CTR) below 100 ps: improvement in image quality and patient workflow, and reduction of delivered dose are among them. This achievement proved to be quite challenging, and many approaches have been proposed and are being investigated for this scope. One of the most recent consists in combining different materials with complementary properties (e.g. high stopping power for 511 keVγ-ray and fast timing) in a so-calledheterostructure,metascintillatorormetapixel. By exploiting a mechanism of energy sharing between the two materials, it is possible to obtain a fraction of fast events which significantly improves the overall time resolution of the system.Approach.In this work, we present the progress on this innovative technology. After a simulation study using the Geant4 toolkit, aimed at understanding the optimal configuration in terms of energy sharing, we assembled four heterostructures with alternating plates of BGO and EJ232 plastic scintillator. We fabricated heterostructures of two different sizes (3 × 3 × 3 mm3and  3 × 3 × 15 mm3), each made up of plates with two different thicknesses of plastic plates. We compared the timing of these pixels with a standard bulk BGO crystal and a structure made of only BGO plates (layeredBGO).Main results.CTR values of 239 ± 12 ps and 197 ± 10 ps FWHM were obtained for the 15 mm long heterostructures with 100µm and 200µm thick EJ232 plates (both with 100µm thick BGO plates), compared to 271 ± 14 ps and 303 ± 15 ps CTR for bulk and layered BGO, respectively.Significance.Significant improvements in timing compared to standard bulk BGO were obtained for all the configurations tested. Moreover, for the long pixels, depth of interaction (DOI) collimated measurements were also performed, allowing to validate a simple model describing light transport inside the heterostructure.

Concept development of an on-chip PET system.

BACKGROUND: Organs-on-Chips (OOCs), microdevices mimicking in vivo organs, find growing applications in disease modeling and drug discovery. With the increasing number of uses comes a strong demand for imaging capabilities of OOCs as monitoring physiologic processes within OOCs is vital for the continuous improvement of this technology. Positron Emission Tomography (PET) would be ideal for OOC imaging, however, current PET systems are insufficient for this task due to their inadequate spatial resolution. In this work, we propose the concept of an On-Chip PET system capable of imaging OOCs and optimize its design using a Monte Carlo Simulation (MCS). MATERIAL AND METHODS: The proposed system consists of four detectors arranged around the OOC device. Each detector is made of two monolithic LYSO crystals and covered with Silicon photomultipliers (SiPMs) on multiple surfaces. We use a Convolutional Neural Network (CNN) trained with data from a MCS to predict the first gamma-ray interaction position inside the detector from the light patterns that are recorded by the SiPMs on the detector's surfaces. RESULTS: The CNN achieves a mean average prediction error of 0.80 mm in the best configuration. The proposed system achieves a sensitivity of 34.81% for 13 mm thick crystals and does not show a prediction degradation near the boundaries of the detector. We use the trained network to reconstruct an image of a grid of 21 point sources spread across the field-of-view and obtain a mean spatial resolution of 0.55 mm. We show that 25,000 Line of Responses (LORs) are needed to reconstruct a realistic OOC phantom with adequate image quality. CONCLUSIONS: We demonstrate that it is possible to achieve a spatial resolution of almost 0.5 mm in a PET system made of multiple monolithic LYSO crystals by directly predicting the scintillation position from light patterns created with SiPMs. We observe that a thinner crystal performs better than a thicker one, that increasing the SiPM size from 3 mm to 6 mm only slightly decreases the prediction performance, and that certain surfaces encode significantly more information for the scintillation-point prediction than others.

Design rules for time of flight Positron Emission Tomography (ToF-PET) heterostructure radiation detectors.

Despite the clinical acceptance of ToF-PET, there is still a gap between the technology's performance and the end-user's needs. Core to bridging this gap is the ability to develop radiation detectors combining a short attenuation length and a sub-nanosecond time response. Currently, the detector of choice, Lu2SiO5:Ce3+ single crystal, is not selected for its ability to answer the performance needs, but as a trade-off between requirements and availability. To bypass the current performance limitations, in particular restricted time response, the concept of the heterostructured detector has been proposed. The concept aims at splitting the scintillation mechanisms across two materials, one acting primarily as an absorber and one as an ultra-fast emitter. If the concept has attracted the interest of the medical and material communities, little has been shown in terms of the benefits/limitations of the approach. Based on Monte Carlo simulations, we present a survey of heterostructure performance versus detector design. The data allow for a clear understanding of the design/performance relationship. This, in turn, enables the establishment of design rules toward the development and optimization of heterostructured detectors that could supersede the current detector technology in the medical imaging field but also across multiple sectors (e.g. high-energy physics, security).

A roadmap for sole Cherenkov radiators with SiPMs in TOF-PET.

Time of flight positron emission tomography can strongly benefit from a very accurate time estimator given by Cherenkov radiation, which is produced upon a 511 keV positron-electron annihilation gamma interaction in heavy inorganic scintillators. While time resolution in the order of 30 ps full width at half maximum (FWHM) has been reported using MCP-PMTs and black painted Cherenkov radiators, such solutions have several disadvantages, like high cost and low detection efficiency of nowadays available MCP-PMTs. On the other hand, silicon photomultipliers (SiPMs) are not limited by those obstacles and provide high photon detection efficiency with a decent time response. Timing performance of PbF2crystals of various lengths and surface conditions coupled to SiPMs was evaluated against a reference detector with an optimized test setup using high-frequency readout and novel time walk correction, with special attention on the intrinsic limits for one detected Cherenkov photon only. The average number of detected Cherenkov photons largely depends on the crystal surface state, resulting in a tradeoff between low photon time spread, thus good timing performance, and sensitivity. An intrinsic Cherenkov photon yield of 16.5 ± 3.3 was calculated for 2 × 2 × 3 mm3sized PbF2crystals upon 511 keVγ-deposition. After time walk correction based on the slew rate of the signal, assuming two identical detector arms in coincidence, and using all events, a time resolution of 215 ps FWHM (142 ps FWHM) was obtained for 2 × 2 × 20 mm3(2 × 2 × 3 mm3) sized PbF2crystals, compared to 261 ps (190 ps) without correction. Selecting on one detected photon only, a single photon coincidence time resolution of 113 ps FWHM for black painted and 166 ps for Teflon wrapped crystals was measured for 3 mm length, compared to 145 ps (black) and 263 ps (Teflon) for 20 mm length.

DOI estimation through signal arrival time distribution: a theoretical description including proof of concept measurements.

The challenge to reach 10 ps coincidence time resolution (CTR) in time-of-flight positron emission tomography (TOF-PET) is triggering major efforts worldwide, but timing improvements of scintillation detectors will remain elusive without depth-of-interaction (DOI) correction in long crystals. Nonetheless, this momentum opportunely brings up the prospect of a fully time-based DOI estimation since fast timing signals intrinsically carry DOI information, even with a traditional single-ended readout. Consequently, extracting features of the detected signal time distribution could uncover the spatial origin of the interaction and in return, provide enhancement on the timing precision of detectors. We demonstrate the validity of a time-based DOI estimation concept in two steps. First, experimental measurements were carried out with current LSO:Ce:Ca crystals coupled to FBK NUV-HD SiPMs read out by fast high-frequency electronics to provide new evidence of a distinct DOI effect on CTR not observable before with slower electronics. Using this detector, a DOI discrimination using a double-threshold scheme on the analog timing signal together with the signal intensity information was also developed without any complex readout or detector modification. As a second step, we explored by simulation the anticipated performance requirements of future detectors to efficiently estimate the DOI and we proposed four estimators that exploit either more generic or more precise features of the DOI-dependent timestamp distribution. A simple estimator using the time difference between two timestamps provided enhanced CTR. Additional improvements were achieved with estimators using multiple timestamps (e.g. kernel density estimation and neural network) converging to the Cramér-Rao lower bound developed in this work for a time-based DOI estimation. This two-step study provides insights on current and future possibilities in exploiting the timing signal features for DOI estimation aiming at ultra-fast CTR while maintaining detection efficiency for TOF PET.

Pushing Cherenkov PET with BGO via coincidence time resolution classification and correction.

Bismuth germanate (BGO) shows good properties for positron emission tomography (PET) applications, but was substituted by the development of faster crystals like lutetium oxyorthosilicate (LSO) for time-of-flight PET (TOF-PET). Recent improvements in silicon photomultipliers (SiPMs) and fast readout electronics make it possible to access the Cherenkov photon signal produced upon 511 keV interaction, which makes BGO a cost-effective candidate for TOF-PET. Tails in the time-delay distribution, however, remain a challenge. These are mainly caused by the high statistical fluctuation on the Cherenkov photons detected. To select fast events with a high detected Cherenkov photon number, the signal rise time of the SiPM was used for discrimination. The charge, time delay and signal rise time was measured for two different lengths of BGO crystals coupled to FBK NUV-HD SiPMs and high frequency readout in a coincidence time resolution setup. The recorded events were divided into 5 × 5 categories based on the signal rise time, and time resolutions of 200 ± 3 ps for 2 × 2 × 20 mm3 and 117 ± 3 ps for 2 × 2 × 3 mm3 were measured for the fastest 20% of the events (4% in coincidence). These good timing events can provide additional information for the image reconstruction in order to increase the SNR significantly, without spoiling the detector sensitivity. Putting all photopeak events together and correcting for the time bias introduced by different numbers of Cherenkov photons detected, time resolutions of 259 ± 3 ps for 20 mm long and 151 ± 3 ps for 3 mm long crystals were measured. For a small fraction of events sub-100 ps coincidence time resolution with BGO was reached for a 3 mm short pixel.

TOF-PET Image Reconstruction With Multiple Timing Kernels Applied on Cherenkov Radiation in BGO.

Today Time-of-Flight (TOF), in PET scanners, assumes a single, well-defined timing resolution for all events. However, recent BGO-Cherenkov detectors, combining prompt Cherenkov emission and the typical BGO scintillation, can sort events into multiple timing kernels, best described by the Gaussian mixture models. The number of Cherenkov photons detected per event impacts directly the detector time resolution and signal rise time, which can later be used to improve the coincidence timing resolution. This work presents a simulation toolkit which applies multiple timing spreads on the coincident events and an image reconstruction that incorporates this information. A full cylindrical BGO-Cherenkov PET model was compared, in terms of contrast recovery and contrast-to-noise ratio, against an LYSO model with a time resolution of 213 ps. Two reconstruction approaches for the mixture kernels were tested: 1) mixture Gaussian and 2) decomposed simple Gaussian kernels. The decomposed model used the exact mixture component applied during the simulation. Images reconstructed using mixture kernels provided similar mean value and less noise than the decomposed. However, typically, more iterations were needed. Similarly, the LYSO model, with a single TOF kernel, converged faster than the BGO-Cherenkov with multiple kernels. The results indicate that the model complexity slows down convergence. However, due to the higher sensitivity, the contrast-to-noise ratio was 26.4% better for the BGO model.

Improving depth-of-interaction resolution in pixellated PET detectors using neural networks.

Parallax error is a common issue in high-resolution preclinical positron emission tomography (PET) scanners as well as in clinical scanners that have a long axial field of view (FOV), which increases estimation uncertainty of the annihilation position and therefore degrades the spatial resolution. A way to address this issue is depth-of-interaction (DOI) estimation. In this work we propose two machine learning-based algorithms, a dense and a convolutional neural network (NN), as well as a multiple linear regression (MLR)-based method to estimate DOI in depolished PET detector arrays with single-sided readout. The algorithms were tested on an 8× 8 array of 1.53× 1.53× 15 mm3 crystals and a 4× 4 array of 3.1× 3.1× 15 mm3 crystals, both made of Ce:LYSO scintillators and coupled to a 4× 4 array of 3× 3 mm3 silicon photomultipliers (SiPMs). Using the conventional linear DOI estimation method resulted in an average DOI resolution of 3.76 mm and 3.51 mm FWHM for the 8× 8 and the 4× 4 arrays, respectively. Application of MLR outperformed the conventional method with average DOI resolutions of 3.25 mm and 3.33 mm FWHM, respectively. Using the machine learning approaches further improved the DOI resolution, to an average DOI resolution of 2.99 mm and 3.14 mm FWHM, respectively, and additionally improved the uniformity of the DOI resolution in both arrays. Lastly, preliminary results obtained by using only a section of the crystal array for training showed that the NN-based methods could be used to reduce the number of calibration steps required for each detector array.

Experimental time resolution limits of modern SiPMs and TOF-PET detectors exploring different scintillators and Cherenkov emission.

Solid state photodetectors like silicon photomultipliers (SiPMs) are playing an important role in several fields of medical imaging, life sciences and high energy physics. They are able to sense optical photons with a single photon detection time precision below 100 ps, making them ideal candidates to read the photons generated by fast scintillators in time of flight positron emission tomography (TOF-PET). By implementing novel high-frequency readout electronics, it is possible to perform a completely new evaluation of the best timing performance achievable with state-of-the-art analog-SiPMs and scintillation materials. The intrinsic SiPM single photon time resolution (SPTR) was measured with Ketek, HPK, FBK, SensL and Broadcom devices. Also, the best achieved coincidence time resolution (CTR) for these devices was measured with LSO:Ce:Ca of [Formula: see text] mm3 and [Formula: see text] mm3 size crystals. The intrinsic SPTR for all devices ranges between 70 ps and 135 ps FWHM when illuminating the entire [Formula: see text] mm2 or [Formula: see text] mm2 area. The obtained CTR with LSO:Ce:Ca of [Formula: see text] mm3 size ranges between 58 ps and 76 ps FWHM for the SiPMs evaluated. Bismuth Germanate (BGO), read out with state of-the-art NUV-HD SiPMs from FBK, achieved a CTR of 158 [Formula: see text] ps and 277 [Formula: see text] ps FWHM for [Formula: see text] mm3 and [Formula: see text] mm3 crystals, respectively. Other BGO geometries yielded 167 [Formula: see text] 3 ps FWHM for [Formula: see text] mm3 and 235 [Formula: see text] 5 ps FWHM for [Formula: see text] mm3 also coupled with Meltmount (n = 1.582) and wrapped in Teflon. Additionally, the average number of Cherenkov photons produced by BGO in each 511 keV event was measured to be 17 [Formula: see text] 3 photons. Based on this measurement, we predict the limits of BGO for ultrafast timing in TOF-PET with Monte Carlo simulations. Plastic scintillators (BC422, BC418), BaF2, GAGG:Ce codoped with Mg and CsI:undoped were also tested for TOF performance. Indeed, BC422 can achieve a CTR of 35 [Formula: see text] 2 ps FWHM using only Compton interactions in the detector with a maximum deposited energy of 340 keV. BaF2 with its fast cross-luminescence enables a CTR of 51 [Formula: see text] 5 ps FWHM when coupled to VUV-HD SiPMs from FBK, with only ∼22% photon detection efficiency (PDE). We summarize the measured CTR of the various scintillators and discuss their intrinsic timing performance.

Roadmap toward the 10 ps time-of-flight PET challenge.

Since the seventies, positron emission tomography (PET) has become an invaluable medical molecular imaging modality with an unprecedented sensitivity at the picomolar level, especially for cancer diagnosis and the monitoring of its response to therapy. More recently, its combination with x-ray computed tomography (CT) or magnetic resonance (MR) has added high precision anatomic information in fused PET/CT and PET/MR images, thus compensating for the modest intrinsic spatial resolution of PET. Nevertheless, a number of medical challenges call for further improvements in PET sensitivity. These concern in particular new treatment opportunities in the context personalized (also called precision) medicine, such as the need to dynamically track a small number of cells in cancer immunotherapy or stem cells for tissue repair procedures. A better signal-to-noise ratio (SNR) in the image would allow detecting smaller size tumours together with a better staging of the patients, thus increasing the chances of putting cancer in complete remission. Moreover, there is an increasing demand for reducing the radioactive doses injected to the patients without impairing image quality. There are three ways to improve PET scanner sensitivity: improving detector efficiency, increasing geometrical acceptance of the imaging device and pushing the timing performance of the detectors. Currently, some pre-localization of the electron-positron annihilation along a line-of-response (LOR) given by the detection of a pair of annihilation photons is provided by the detection of the time difference between the two photons, also known as the time-of-flight (TOF) difference of the photons, whose accuracy is given by the coincidence time resolution (CTR). A CTR of about 10 picoseconds FWHM will ultimately allow to obtain a direct 3D volume representation of the activity distribution of a positron emitting radiopharmaceutical, at the millimetre level, thus introducing a quantum leap in PET imaging and quantification and fostering more frequent use of 11C radiopharmaceuticals. The present roadmap article toward the advent of 10 ps TOF-PET addresses the status and current/future challenges along the development of TOF-PET with the objective to reach this mythic 10 ps frontier that will open the door to real-time volume imaging virtually without tomographic inversion. The medical impact and prospects to achieve this technological revolution from the detection and image reconstruction point-of-views, together with a few perspectives beyond the TOF-PET application are discussed.

High-frequency SiPM readout advances measured coincidence time resolution limits in TOF-PET.

Scintillator based radiation detectors readout by SiPMs successively break records in their reached time resolution. Nevertheless, new challenges in time of flight positron emission tomography (TOF-PET) and high energy physics are setting unmatched goals in the 10 ps range. Recently it was shown that high frequency (HF) readout of SiPMs significantly improves the measured single photon time resolution (SPTR), allowing to evaluate the intrinsic performance of large area devices; e.g. FBK NUV-HD SiPMs of [Formula: see text] mm2 area and 40 [Formula: see text]m single photon avalanche diode (SPAD) size achieve 90 ps FWHM. In TOF-PET such readout allows to lower the leading edge detection threshold, so that the fastest photons produced in the crystal can be utilized. This is of utmost importance if a high SPTR and prompt Cherenkov light generated by the hot-recoil electron upon 511 keV photo-absorption should improve timing. This paper shows that high-frequency bipolar transistor readout of state-of-the-art SiPMs coupled to high-performance scintillators can substantially improve the best achievable coincidence time resolution (CTR) in TOF-PET. In this context a CTR of 158 [Formula: see text] 3 ps FWHM with [Formula: see text] mm3 BGO crystals coupled to FBK SiPMs is achieved. This faint Cherenkov signal is as well present in standard LSO scintillators, which together with low SPTR values (<90 ps FWHM) improves the CTR of [Formula: see text] mm3 LSO:Ce:Ca coupled to FBK NUV-HD [Formula: see text] mm2 with 25 [Formula: see text]m SPAD size to 61 [Formula: see text] 2 ps FWHM using HF-electronics, as compared to 73 [Formula: see text] 2 ps when readout by the NINO front-end ASIC. When coupling the LSO:Ce:Ca crystals to FBK NUV-HD SiPMs of [Formula: see text] mm2 and 40 [Formula: see text]m SPAD size, using HF-electronics, a CTR of even 58 [Formula: see text] 3 ps for [Formula: see text] mm3 and 98 [Formula: see text] 3 ps for [Formula: see text] mm3 is achieved. This new experimental data will allow to further discuss the timing limits in scintillator-based detectors.

Core-shell ZnO:Ga-SiO2 nanocrystals: limiting particle agglomeration and increasing luminescence via surface defect passivation.

Heat treatment is needed to increase the luminescence intensity of ZnO:Ga particles, but it comes at the cost of higher particle agglomeration. Higher agglomeration results in low transparency of scintillating powder when embedded in a matrix and constitutes one of the biggest disadvantages, besides low light yield and low stopping power, of ZnO:Ga powder. Limiting ZnO:Ga particle size is therefore a key step in order to prepare highly luminescent and transparent composites with prospects for optical applications. In this work, SiO2 coating was successfully used to improve luminescence intensity or limitation of crystallite size growth during further annealing. Furthermore, ZnO:Ga and ZnO:Ga-SiO2 core-shells were embedded in a polystyrene matrix.

Improved single photon time resolution for analog SiPMs with front end readout that reduces influence of electronic noise.

A key step to improve the coincidence time resolution of positron emission tomography detectors that exploit small populations of promptly emitted photons is improving the single photon time resolution (SPTR) of silicon photomultipliers (SiPMs). The influence of electronic noise has previously been identified as the dominant factor affecting SPTR for large area, analog SiPMs. In this work, we measure the achievable SPTR with front end electronic readout that minimizes the influence of electronic noise. With this readout circuit, the SPTR measured for one FBK NUV single avalanche photodiode (SPAD) was also achieved with a [Formula: see text] mm2 FBK NUV SiPM. SPTR for large area devices was also significantly improved. The measured SPTRs for [Formula: see text] mm2 Hamamatsu and SensL SiPMs were [Formula: see text]150 ps FWHM, and SPTR [Formula: see text]100 ps FWHM was measured for [Formula: see text] mm2 and [Formula: see text] mm2 FBK NUV and NUV-HD SiPMs. We also explore additional factors affecting the achievable SPTR for large area, analog SiPMs when the contribution of electronic noise is minimized and pinpoint potential areas of improvement to further reduce the SPTR of large area sensors towards that achievable for a single SPAD.

Measurement of intrinsic rise times for various L(Y)SO and LuAG scintillators with a general study of prompt photons to achieve 10 ps in TOF-PET.

The coincidence time resolution (CTR) of scintillator based detectors commonly used in positron emission tomography is well known to be dependent on the scintillation decay time (τd) and the number of photons detected (n'), i.e. CTR proportional variant √τd/n'. However, it is still an open question to what extent the scintillation rise time (τr) and other fast or prompt photons, e.g. Cherenkov photons, at the beginning of the scintillation process influence the CTR. This paper presents measurements of the scintillation emission rate for different LSO type crystals, i.e. LSO:Ce, LYSO:Ce, LSO:Ce codoped Ca and LGSO:Ce. For the various LSO-type samples measured we find an average value of 70 ps for the scintillation rise time, although some crystals like LSO:Ce codoped Ca seem to have a much faster rise time in the order of 20 ps. Additional measurements for LuAG:Ce and LuAG:Pr show a rise time of 535 ps and 251 ps, respectively. For these crystals, prompt photons (Cherenkov) can be observed at the beginning of the scintillation event. Furthermore a significantly lower rise time value is observed when codoping with calcium. To quantitatively investigate the influence of the rise time to the time resolution we measured the CTR with the same L(Y)SO samples and compared the values to Monte Carlo simulations. Using the measured relative light yields, rise- and decay times of the scintillators we are able to quantitatively understand the measured CTRs in our simulations. Although the rise time is important to fully explain the CTR variation for the different samples tested we determined its influence on the CTR to be in the order of a few percent only. This result is surprising because, if only photonstatistics of the scintillation process is considered, the CTR would be proportional to the square root of the rise time. The unexpected small rise time influence on the CTR can be explained by the convolution of the scintillation rate with the single photon time resolution (SPTR) of the photodetector and the photon travel spread (PTS) in the crystal. The timing benefits of prompt photons at the beginning of the scintillation process (Cherenkov etc) are further studied, which leads to the conclusion that the scintillation rise time, SPTR and PTS have to be lowered simultaneously to fully profit from these fast photons in order to improve the CTR significantly.