Evaluating plastic scintillator performance as a substitute of LYSO in SiPM based animal PET scanners: a GEANT4 simulation analysis

We have developed a sensor head for a time-of-flight (TOF) PET scanner using plastic scintillators that have a very fast timing property. Given the very small cross section of photoelectric absorption in plastic scintillators at 511 keV, we use Compton scattering in order to compensate for detection efficiency. The detector will consist of two layers of scatterers and absorbers which are made of plastic and inorganic scintillators such as GAGG:Ce, respectively. Signals are read by monolithic Multi Pixel Photon Counters, and with energy deposits and interaction time stamps are being acquired. The scintillators are built to be capable of resolving interaction position in three dimensions, so that our system has also a function of depth-of-interaction (DOI) PET scanners. TOF resolution of ∼ 200 ps (FWHM) is achieved in both cases of using the leading-edge discriminator and time-walk correction and using a configuration sensitive to DOI. Both the position resolution and spectroscopy are demonstrated using the prototype data acquisition system, with Compton scattering events subsequently being obtained. We also demonstrated that the background rejection technique using the Compton cone constraint could be valid with our system.

The Plastic Scintillator Detector (PSD) is one of the main sub-detectors in the DArk Matter Particle Explorer (DAMPE) project. It will be operated over a large temperature range from −10 to 30 °C, so the temperature effect of the whole detection system should be studied in detail. The temperature dependence of the PSD system is mainly contributed by the three parts: the plastic scintillator bar, the photomultiplier tube (PMT), and the Front End Electronics (FEE). These three parts have been studied in detail and the contribution of each part has been obtained and discussed. The temperature coefficient of the PMT is −0.320(±0.033)%/°C, and the coefficient of the plastic scintillator bar is −0.036(±0.038)%/°C. This result means that after subtracting the FEE pedestal, the variation of the signal amplitude of the PMT-scintillator system due to temperature mainly comes from the PMT, and the plastic scintillator bar is not sensitive to temperature over the operating range. Since the temperature effect cannot be ignored, the temperature dependence of the whole PSD has been also studied and a correction has been made to minimize this effect. The correction result shows that the effect of temperature on the signal amplitude of the PSD system can be suppressed.

Measurement of gamma quantum interaction point in plastic scintillator with WLS strips

Evaluation of an instrument to improve PET timing alignment

The main thrust for this work is the investigation and design of a positron emission tomography (PET) scanner based on new Lanthanum Halide scintillators. In three-dimensional (3-D) PET the major limitations are scanner dead-time and ability to reject randoms and scatter. Therefore, to reach the full potential of 3-D PET requires a scintillator with good timing resolution and good energy resolution. The new Lanthanum Halide scintillators have very fast decay and very high light output which leads to timing resolution and energy resolution that are both superlative. For application to PET, the authors have constructed pixels with dimensions 4 /spl times/ 4 /spl times/ 30 mm/sup 3/ and have measured energy resolution of 4.6% (fwhm) at 662 keV and a timing resolution (fwhm) of 350 ps, in coincidence with a plastic scintillator. Using a detector based on LaBr/sub 3/, a 3-D PET scanner with 90 cm diameter and 25 cm axial extent is predicted to achieve a sensitivity of 1400 kc/s//spl mu/Ci/cc and a peak NEC count-rate of 120 kc/s using the NEMA NU2-2001 standard. Further, the excellent timing resolution opens the possibility of measuring time-of-flight with sufficient accuracy to reduce the noise propagation during image reconstruction, thus leading to a significant gain in signal-to-noise. Assuming a system timing resolution of 500 ps, one can expect the effective NEC to increase by a factor of 3 for a thin patient (20 cm diameter) and a factor 6 for a very heavy patient (40 cm diameter). Thus, even with lower stoping power than other PET scanners, the combination of excellent energy resolution and timing resolution of LaBr/sub 3/ can potentially lead to a very significant improvement in PET performance.

Development of the front-end electronics for a cost-effective PET-like detector system

The Polarized Gamma-ray Observer (PoGOLite) is a balloon-borne polarimeter based on measuring anisotropy in the azimuthal scattering angle distribution of photons in the energy range 25-80 keV. This is achieved through coincident detection of Compton scattering and photoelectric absorption within a close-packed array of phoswich detector cells (PDCs). Each PDC contains a plastic scintillator rod (main detector component), a plastic scintillator tube (active collimator) and a BGO crystal (anticoincidence shield). A significant in-flight background is expected from atmospheric neutrons as well as from neutrons produced by interactions of cosmic rays with mechanical structures surrounding the instrument. Although this background can be reduced by introducing suitable shielding materials such as polyethylene, the shield geometry must be optimized through simulations in order to yield sufficient shielding with an acceptable increase in weight. Geant4-based Monte Carlo simulations have shown that a 10 cm thick polyethylene shield surrounding the PoGOLite instrument is required to sufficiently reduce the background, i.e. fake polarization events, from atmospheric neutrons. In order to validate these simulations, a beam test was carried out, at which 14 MeV neutrons were used to irradiate a simple detector array with four plastic scintillators and three BGO crystals. The array was configured to mimic the PoGOLite detector geometry and also featured a polyethylene neutron shield. Here, we present details of the neutron beam test and our simulation thereof, which demonstrate that the treatment of neutron interactions within the Geant4 framework is reliable. Such simulations can therefore be used to assess in-flight neutron background in balloon-borne instruments, such as the PoGOLite polarimeter.

We propose and evaluate the performance of an improved preclinical positron emission tomography (PET) scanner design, referred to as Polaroid-PET, consisting of a detector equipped with a layer of horizontal Polaroid to filter scintillation photons with vertical polarization. This makes it possible to improve the spatial resolution of PET scanners based on monolithic crystals. First, a detector module based on a lutetium-yttrium orthosilicate monolithic crystal with 10 mm thickness and silicon photomultipliers (SiPMs) was implemented in the GEANT4 Monte Carlo toolkit. Subsequently, a layer of Polaroid was inserted between the crystal and the SiPMs. Two preclinical PET scanners based on ten detector modules with and without Polaroid were simulated. The performance of the proposed detector modules and corresponding PET scanner for the two configurations (with and without Polaroid) was assessed using standard performance parameters, including spatial resolution, sensitivity, optical photon ratio detected for positioning, and image quality. The detector module fitted with Polaroid led to higher spatial resolution (1.05 mm FWHM) in comparison with a detector without Polaroid (1.30 mm FHWM) for a point source located at the center of the detector module. From 100% of optical photons produced in the scintillator crystal, 65% and 66% were used for positioning in the detectors without and with Polaroid, respectively. Polaroid-PET resulted in higher axial spatial resolution (0.83 mm FWHM) compared to the scanner without Polaroid (1.01 mm FWHM) for a point source at the center of the field of view (CFOV). The absolute sensitivity at the CFOV was 4.37% and 4.31% for regular and Polaroid-PET, respectively. Planar images of a grid phantom demonstrated the potential of the detector with a Polaroid in distinguishing point sources located at close distances. Our results indicated that Polaroid-PET may improve spatial resolution by filtering the reflected optical photons according to their polarization state, while retaining the high sensitivity expected with monolithic crystal detector blocks.

The purpose of the presented research is estimation of the performance characteristics of the economic total-body Jagiellonian-PET system (TB-J-PET) constructed from plastic scintillators. The characteristics are estimated according to the NEMA NU-2-2018 standards utilizing the GATE package. The simulated detector consists of 24 modules, each built out of 32 plastic scintillator strips (each with cross section of 6 mm times 30 mm and length of 140 or 200 cm) arranged in two layers in regular 24-sided polygon circumscribing a circle with the diameter of 78.6 cm. For the TB-J-PET with an axial field-of-view (AFOV) of 200 cm, a spatial resolutions (SRs) of 3.7 mm (transversal) and 4.9 mm (axial) are achieved. The noise equivalent count rate (NECR) peak of 630 kcps is expected at 30 kBq cc−1. Activity concentration and the sensitivity at the center amounts to 38 cps kBq−1. The scatter fraction (SF) is estimated to 36.2 %. The values of SF and SR are comparable to those obtained for the state-of-the-art clinical PET scanners and the first total-body tomographs: uExplorer and PennPET. With respect to the standard PET systems with AFOV in the range from 16 to 26 cm, the TB-J-PET is characterized by an increase in NECR approximately by factor of 4 and by the increase of the whole-body sensitivity by factor of 12.6 to 38. The time-of-flight resolution for the TB-J-PET is expected to be at the level of CRT = 240 ps full width at half maximum. For the TB-J-PET with an AFOV of 140 cm, an image quality of the reconstructed images of a NEMA IEC phantom was presented with a contrast recovery coefficient and a background variability parameters. The increase of the whole-body sensitivity and NECR estimated for the TB-J-PET with respect to current commercial PET systems makes the TB-J-PET a promising cost-effective solution for the broad clinical applications of total-body PET scanners. TB-J-PET may constitute an economic alternative for the crystal TB-PET scanners, since plastic scintillators are much cheaper than BGO or LYSO crystals and axial arrangement of the strips significantly reduces the costs of readout electronics and SiPMs.

This thesis documents a proof-of-concept study of a novel, scintillator based, coded-aperture approach to neutron detection. Developments presented in this document suggest that coded-aperture approach, previously mainly associated with photon detectors, can be adapted for a small scale neutron detector. This work represents an innovative, scintillator based approach for small scale radiation detector aimed at nuclear decommissioning applications. A novel pixelated plastic scintillator was designed and built in this work. Scintillator cells 2.8 x 2.8 x 15 mm each), build of EJ-299-34 plastic were manufactured and arranged into a 13 x 13 array. The plastic scintillator which was used in this research was sensitive to both gamma and neutron fields. Experimental data were obtained for various solid scintillator samples and a comparison of a number of pulse shape discrimination techniques was performed. Prior to the experimental work, a simulation based study identified potential candidates for the scintillation material, as well as characterised the mixed-field environment, provided by 252Cf at Lancaster University, UK. Suitable coded-aperture materials were also computationally identified, and were subsequently used to manufacture a tungsten coded aperture, based on modified uniformly redundant array design technique. Pixelated nature of the coded-aperture based approach to radiation imaging allows the lateral resolution of the image to be improved, without affecting the signal-to-noise ratio. The focal point of this technique is located in the coded-aperture design and the scintillator. Modulation properties of the rank-7 coded aperture, made of tungsten using additive manufacturing techniques, were investigated. The experiment was performed using 137Cs gamma-ray calibration source at Lancaster University. Data obtained were subsequently used to perform the localisation of the point source used in this study. The idea of using tungsten coded aperture for dual-particle imaging was also simulated using Monte Carlo techniques (MCNPX) prior to the experimental work. The pulse shape discrimination performance of the pixelated organic plastic scintillator was investigated. The scintillator was exposed to a mixed-field environment provided by 252Cf and its performance was compared to that of a cylindrical plastic sample. Tests were also carried out in moderated neutron and gamma-ray fields of 252Cf. Suitable pixelated photodetectors, together with associated readout electronics circuitry, were also identified.

Timing alignment of the detectors in a PET Scanner is important in order to reduce the random counts with a major source of image noise in 3D whole body scans used in the evaluation of cancer patients. A new technique is described here and being tested on individual PET detectors at the Montreal Neurological Institute. A timing alignment probe manufactured by Scanwell Systems (Montreal, QC) uses a small quantity of Na‐22 embedded in plastic scintillator, which can detect each positron decay. The difference between the time of decay and time of gamma ray detection can be used to establish a time offset for each crystal in a PET Scanner. This paper reports the results from isolated PET detectors, since no PET Scanners presently have an input for the timing probe. We found that in fast detectors the timing alignment could be made with an accuracy of 120 ps in a ten minute scan for each crystal. Variations of up to 1.9 ns were observed from individual crystals from typical block detectors in conventional PET Scanners.

The ALICE Collaboration is planning to construct a new detector (ALICE 3) aiming at exploiting the potential of the high-luminosity Large Hadron Collider (LHC). The new detector will allow ALICE to participate in LHC Run 5 scheduled from 2036 to 2041. The muon-identifier subsystem (MID) is part of the ALICE 3 reference detector layout. The MID will consist of a standard magnetic iron absorber (≈4 nuclear interaction lengths) followed by muon chambers. The baseline option for the MID chambers considers plastic scintillation bars equipped with wave-length shifting fibers and readout with silicon photomultipliers. This paper reports on the performance of a MID chamber prototype using 3 GeV/c pion- and muon-enriched beams delivered by the CERN Proton Synchrotron (PS). The prototype was built using extruded plastic scintillator produced by FNAL-NICADD (Fermi National Accelerator Laboratory - Northern Illinois Center for Accelerator and Detector Development). The prototype was experimentally evaluated using varying absorber thicknesses (60, 70, 80, 90, and 100 cm) to assess its performance. The analysis was performed using Machine Learning techniques and the performance was validated with GEANT 4 simulations. Potential improvements in both hardware and data analysis are discussed.

Low-cost extruded plastic scintillator

The HEXTE, part of the X-Ray Timing Explorer (XTE), is designed to make high sensitivity temporal and spectral measurements of X-rays with energies between 15 and 250 keV using NaI/CsI phoswich scintillation counters. To achieve the required sensitivity it is necessary to provide anticoincidence of charged cosmic ray particles incident upon the instrument, some of which interact to produce background X-rays. The proposed cosmic ray particle anticoincidence shield detector for HEXTE uses a novel design based on plastic scintillators and wavelength-shifter bars. It consists of five segments, each with a 7 mm thick plastic scintillator, roughly 50 cm x 50 cm in size, coupled to two wavelength-shifter bars viewed by 1/2 inch photomultiplier tubes. These segments are configured into a five-sided, box-like structure around the main detector system. Results of laboratory testing of a model segment, and calculations of the expected performance of the flight segments and particle anticoincidence detector system are presented to demonstrate that the above anticoincidence detector system satisfies its scientific requirements.

CTOF@CLAS12 plastic scintillator counter was tested with proton beam at Korea Institute of Radiological and Medical Sciences (KIRAMS). The counter have been designed and constructed as a plastic scintillation bar coupled with the 1.5 and the 2 inches of diameter ne mesh photomultiplier tubes (FM-PMTs). The dependence of the TOF resolution on the light output has dened. Obtained results of the 1.5 00 , 2 00 FM-PMTs’ timing and gain performances in magnetic eld conrm their suitability for precise particle identication in the CLAS12 future detector.