Scintillator Geometry Research Articles

Multi-objective scintillator shape optimization for increased photodetector light collection

Inorganic scintillators often use exotic, expensive materials to increase their light yield. Although material chemistry is a valid way to increase the light collection, these methods are expensive and limited to the material properties. As such, alternative methods such as the use of specific reflective coatings and crystal optical shapes are critical for the scintillator crystal design procedure. In this paper, we explore the modeling of a scintillator and silicon-photomultiplier (SiPM) assembly detector using GEANT4. GEANT4, an open-source software for particle–matter interaction based on ray-tracing, allows the modeling of a scintillator-based detector while offering methods to simplify and study the computational requirements for a precise calculation of the light collection. These studies incorporate two different geometries compatible with the barrel timing layer (BTL) particle detector that is being built for the compact muon solenoid (CME) experiment at CERN. Furthermore, the geometry of our model is parameterized using splines for smoother results and meshed using GMSH to perform genetic numerical optimization of the crystal shape through genetic algorithms, in particular non-dominated sorting genetic algorithm II (NGSAII). Using NSGA-II, we provide a series of optimized scintillator geometries and study the trade-offs of multiple possible objective functions including the light output, light collection, light collection per energy deposited, and track path length. The converged Pareto results according to the hypervolume indicator are compared to the original simplified design, and a recommendation towards the use of the light collection per energy deposition and track path length is given based on the results. The results provide increases in this objective of up to 18% for a constant volume for a geometry compatible with the current design of the BTL detector.

Performance simulation study of plastic scintillator arrays for stability monitoring applications

The characteristics of high energy cosmic-ray muons make them strongly penetrative in materials. It has great potential to monitor the stability of buildings by reconstructing the tracks of cosmic-ray muons. In this work, a buildings stability monitoring system based on plastic scintillator strips is proposed. The monitoring system consists of a single-plane upper telescope to record the incoming track of a muon, and a three-plane lower telescope to track its outgoing trajectory. Every detector plane is formed by orthogonally placed scintillator strips. The impacts of the scintillator geometry and spatial placement on the stability monitoring precision are carefully studied using Geant4 simulations. A sub-millimeter position resolution is achievable by selecting a proper data taking time. The triangular scintillator strips always outperform rectangular ones of the same size and placement. Two empirical formulas have been derived to quantitatively estimate the position resolution with respect to the abovementioned influential factors. In addition, the comprehensive effects of multiple parameters are also studied. Accordingly, triangular scintillator strips will be utilized in the construction of a real monitoring system with better performance and less detection channels. The simulation study in this work can provide good guidance for the manufacturing of triangular strips and their experimental configurations.

Conceptual design and simulations for the Detector Array for Energy Measurements Of Neutrons (DAEMON): Scintillator geometry for the full array

The use of large-scale neutron detector arrays with high energy resolution, especially those that can be coupled with highly efficient γ-ray spectrometers, is imperative in order to increase the understanding of the decay of β-delayed neutron emitters. The DAEMON array, currently being designed for the GRIFFIN decay station at TRIUMF, will improve the precision on the neutron kinetic energy measurement enhancing the current capabilities to study neutron rich nuclei at TRIUMF. The innovative design for DAEMON uses thin fast plastic scintillators placed directly on the front of the DESCANT liquid scintillator cans. To investigate the viability of this augmentation, Geant4 was used to simulate and optimize the experimental design. Two configurations were considered; a geometry using vertical bars spanning the inner arc of DESCANT, and individual tiles matching the geometry of the DESCANT scintillator cans. The latter was found to be the most optimum design, enabling the project to enter a prototype testing phase.

Water Phantom Characterization of a Novel Optical Fiber Sensor for LDR Brachytherapy

This work considers the feasibility of using a novel optical fiber-based sensor, employing a terbium-doped gadolinium oxysulfide (Gd2O2S:Tb) inorganic scintillator, as a real-time in vivo dosimetry solution for applications in low-dose-rate (LDR) prostate brachytherapy (BT). This study specifically considers the influence of scintillator geometry (hemisphere tip versus cylindrical cavity), polymethyl methacrylate (PMMA) fiber core diameter (0.5 versus 1.0 mm), and sensor housing material (stainless steel versus plastic) on the measured scintillation signal. Characterization measurements were performed using a silicon photon-multiplier (SiPM) detector and a commercial water phantom system, integrated with custom 3-D printed components to allow for precise positioning of the LDR BT radiation source with respect to the optical fiber sensor (OFS). Significant differences in the rate of fall-off in the scintillation signal, with distance from the source, were observed between the different scintillator geometries considered. The hemisphere tip geometry was shown to be the most accurate, tracking with the expected fall-off in dose-rate, within measurement uncertainty. Reducing the fiber core diameter from 1.0 to 0.5 mm resulted in a sixfold reduction in the detected scintillation signal. A further 57% reduction was observed when housing the 0.5-mm fiber within a stainless steel LDR BT needle applicator. Initial results demonstrate the feasibility of employing an OFS, for applications in LDR BT, given the excellent agreement of measurements with theoretical expectations. Furthermore, a calibration process has been described for converting the detected scintillation signal into absorbed dose/dose rate, using our water phantom-based experimental setup.

A new optical photon transport model for application to high aspect ratio scintillation pillars

A new Monte Carlo model of optical photon transport has been developed for application to scintillation pillars with side surfaces modeled using their measured 3D topography. The model addresses deficiencies of existing models specifically pertinent to high aspect ratio scintillator geometries. Inaccuracies with these models’ description of the reflectance of scintillation photons at media boundaries are exacerbated by the high number of surface interactions in such geometries. These models rely either on (a) user-specified parameters that are challenging to set, or (b) pre-computed look-up tables that assume geometrically unconstrained photon incidence on uniform surface finishes, an assumption that is invalid in high aspect ratio geometries. Alternatively, the new model reconstructs the side surfaces of a pillar from 3D topographic measurements and uses first-principles physics of reflection and transmission to transport photons over the surfaces’ microscopic features and throughout the entire pillar volume. It takes advantage of the recent availability of open-source software capable of complex surface modeling and ray-tracing, as well as high performance computing. The model was validated against measurements of the number of photoelectrons (PEs) generated in two silicon photomultipliers (SiPMs) located at the ends of 5 mm × 5 mm × 200 mm EJ-204 scintillation pillars with different surface finishes. It was also validated against the relative amplitudes and arrival time difference of the SiPMs’ signals. The model accurately predicts the measurements as function of the scintillation position along the pillars’ length with 2% mean-absolute-percentage error (MAPE) in the number of PEs, and 16% in the relative amplitudes. The root-mean-squared error (RMSE) in the arrival time difference is 0.114 ns. The model also predicts the experimentally observed higher scintillation light collection in pillars with smoother side surfaces. A proof-of-concept code of the model is available as open source.

Investigation of a SiPM-GGAG:Ce,B scintillator detector for environmental gamma radiation monitoring

A study of a silicon photomultiplier (SiPM) coupled with a GGAG:Ce,B scintillator detector for environmental gamma radiation monitoring is presented in this paper. FLUKA based Monte Carlo simulations were carried out for the optimization of scintillator material and geometry for this application. Based on the simulation study, the dimensions of GGAG:Ce,B scintillator crystal were finalized and the scintillator was prepared. The detector was assembled by optically coupling to a commercially available SiPM. The analog output pulses of the SiPM were amplified by a voltage sensitive amplifier and subsequently counted using a microcontroller. The linearity of the detector response with gamma dose rate was verified using 137Cs standard gamma source. The radiation response of the detector for different gamma energies was investigated using standard 137Cs, 241Am and 60Co sources. The variation of detector sensitivity with SiPM overvoltage was measured for choosing an optimal overvoltage for the SiPM operation. The experimentally observed response of the SiPM-GGAG:Ce,B detector was very linear in the measurement range of 3.5–300 μGy/h with a sensitivity of 2.44 cps/μGy/h. The gamma response study of the detector confirm the suitability of the developed detector for making a compact and low power gamma monitor for environmental gamma radiation monitoring network.

Measuring small field profiles and output factors with a stemless plastic scintillator array.

To fabricate a 1D stemless plastic scintillation detector (SPSD) array using organic photodiodes and to use the detector to measure small field profiles and output factors. An organic photodiode array was fabricated by spin coating a mixture of P3HT and PCBM organic semiconductors onto an ITO-coated glass substrate and depositing aluminum top contacts. Four bulk scintillators of various dimensions were placed on top of the photodiode array. A fifth scintillator was used that had been segmented by laser etching and the septa filled with black paint. Each detector array was first calibrated using a reference field of 95cm SSD, 5cm depth, and 10×10 cm2 field size for a 6 MV photon beam. After calibration, profiles were measured for three small field sizes: 0.5×0.5 cm2 , 1×1 cm2 , and 2×2 cm2 . Using the central pixel of the array, output factors were measured for field sizes of 0.5×0.5 cm2 to 25×25 cm2 . Small field profiles were compared to film measurements and output factors compared to ion chamber measurements. The segmented scintillator measured profiles that were in good agreement with film for all three field sizes. Output factors agreed to within 1.2% of ion chamber over the field size range of 1×1 cm2 to 25×25 cm2 . At 0.5×0.5 cm2 the segmented scintillator underestimated the output factor compared to film and a microDiamond detector. Bulk scintillators failed to produce a good agreement with film for measured profiles and deviations from ion chamber for output factors were apparent at field sizes below 5×5 cm2 . In comparison to a bulk scintillator of dimensions 5×5×0.5 cm3 the etched scintillator saw a reduction of 5.1, 7.1, and 10.5 times the signal for field sizes of 0.5×0.5 cm2 , 1×1 cm2 , and 2×2 cm2 , respectively. The reduction of signal comes from reduced cross-talk that was present in all of the bulk scintillator geometries to various degrees. A 1D SPSD array was demonstrated with various scintillator designs. The etched scintillator array demonstrated excellent small field profile measurements when compared to film and output factors (down to 1×1cm2 field size) when compared to micro ion chamber and diamond detector measurements.

Pulse shape simulations for organic scintillation detectors using Geant4

The accurate simulation of the temporal pulse shapes from organic scintillation detectors capable of pulse shape discrimination (PSD) presents the opportunity to assess the pulse shape discrimination of these detectors prior to fabrication. The aim of this study is the simulation of the temporal pulse shapes from EJ-276, a PSD-capable plastic scintillator developed by Eljen Technologies. PSD plastic scintillators are increasingly replacing organic liquid scintillators for the detection of neutrons in the presence of mixed radiation fields for nuclear security applications. Plastics are inexpensive, robust and can be fabricated in a variety of shapes and sizes. They offer a solid-state alternative to liquid scintillators which can be difficult to transport due to the risk of leakage. However, the PSD performance of plastic scintillators has been observed to decrease due to various factors which combine to influence the overall shape of the pulse. The Monte Carlo toolkit Geant4 has been used to simulate the temporal pulse shapes from an EJ-276 plastic scintillator coupled to a photomultiplier tube (PMT). All three decay time components of EJ-276 have been modelled, utilising new methods available in the latest version of Geant4, for two different scintillator geometries. The simulated n/γ pulse shapes reproduce the features important for PSD. Future work will involve integrating the temporal response of the PMT with existing pulse shape simulations. Simulated data will then be compared with experimental measurements.

Evaluating the energy dependence of various polystyrene based plastic scintillators

Plastic scintillation detectors (PSDs) have been extensively tested as in-vivo dosimeters for brachytherapy dosimetry, but they are known to suffer from an energy dependence in low energy photon beams, an unwanted characteristic of any dosimeter. It has been demonstrated that the base material of the plastic scintillator can have a significant impact on the PSDs energy dependent behavior in x-ray beam voltages from 25 to 100kVP (Peralta and Rêgo, 2014). In this work, a comprehensive range of polystyrene based PSDs are studied to determine if the scintillator composition and geometry has any impact on their energy dependent behavior in superficial x-ray beams in the 30 to 150 kVP range. The relative response of each scintillator tested was found to vary by up to 77% across the 0.014–0.061 MeV effective energy range, with a maximum variation in relative response of 13.1% between the different scintillators. These results demonstrate the PSD diameter, formulation and cladding do not significantly impact the scintillator performance in the studied energy region.

Improved neutron–gammadiscrimination at low-light output events using conical trans-stilbene

Scintillator geometry affects light-collection efficiency (LCE) and, as a consequence, detector-performance metrics that depend on LCE, such as particle-type identification. This work compares the neutron–gammapulse-shape discrimination (PSD) performance of two trans-stilbene crystals of different geometries: a cylinder and cone. Both geometries have a base diameter and height of 50 mm. Results demonstrate that the cone achieved a higher figure of merit over the cylinder by approximately 22.6% for pulses ranging from 25 keVee to 100 keVee. This improvement can be especially helpful for applied detection systems to better resolve neutrons and gamma rays at low-light output events (<100 keVee), where PSD can suffer.

Detector block performance based on a monolithic LYSO crystal using a novel signal multiplexing method

Organ dedicated PET devices provide improved imaging performance when compared to whole body systems. The present study summarizes the test carried out to study a new detector block designed for an organ dedicated PET system. This block includes three novel components namely the scintillator geometry and a retroreflector layer coupled to the entrance face, the photosensor and the readout electronics. We used arrays of 12 × 12 SiPM photosensors with 3 × 3 mm2 active area each and a pitch of 4.2 mm. We are proposing a new readout electronics that permits to reduce the 12 row and columns signals to only 8 without significant detector performance degradation. This approach also allows for resolving radioactive sources in the whole volume of the proposed crystal, significantly reducing the edge effect that typically rejects these events.An overall spatial resolution of about 1.8 mm FWHM is obtained for the whole scintillation volume, with an average energy resolution of 13% FWHM and a photon depth of interaction resolution (FWHM) of 3.7 mm.

A scintillator geometry suitable for very small PET gantries

In this work we are describing a novel approach to the scintillator crystal configuration as used in nuclear medicine imaging. Our design is related to the coupling in one PET module of the two separate crystal configurations used so far there: monolithic and crystal arrays. The particular design we have studied is based on a two-layer scintillator approach (hybrid) composed of a monolithic LYSO crystal (5–6 mm thickness) and a LYSO crystal array with 4–5 mm height (0.8 and 1 mm pixels). We show here the detector block performance, in terms of spatial, energy and DOI information, to be used as a module in the design of PET scanners. The design we propose allows one to achieve accurate three-dimensional spatial resolution (including DOI information) while assuring high detection efficiency at reasonable cost. Moreover, the proposed design improves the spatial response uniformity across the whole detector module, and especially at the edge region. The crystal arrays are mounted in the front and were well resolved. The monolithic crystal inserted between crystal array and the photosensor, provided measured FWHM resolution as good as 1.5–1.7 mm including the 1 mm source size. The monolithic block achieved a DOI resolution (FWHM) nearing 3 mm. We compared these results with an approach in which we use a single monolithic block with total volume equals to the hybrid approach. In general, comparable performances were obtained.

Performance evaluation of novel LaBr3(Ce) scintillator geometries for fast-timing applications

We evaluate the performance of two LaBr3(Ce) crystals that were produced with special geometries, aimed at enhancing the scintillation light collection and thus the time resolution. Their design was motivated by the construction of high-performance fast-timing arrays like the FAst TIMing array for DESPEC (FATIMA), which demands a high packing factor in addition to good time and energy resolutions.Energy resolution and efficiency were measured using standard calibration sources. Timing measurements were performed at 60Co and 22Na γ-energies against a fast BaF2 reference detector. The time resolution was optimized by the choice of the photomultiplier bias voltage and the fine tuning of the constant fraction discriminator parameters. Monte Carlo simulations using the Geant4 toolkit were performed in order to achieve a better understanding of how the new geometries affect the light transport and thus the performance of the crystals. It is found that the conical-shaped LaBr3(Ce) crystals are optimal for fast-timing applications and for the construction of arrays such as FATIMA.

Impact of geometry on light collection efficiency of scintillation detectors for cryogenic rare event searches

Simulations of photon propagation in scintillation detectors were performed with the aim to find the optimal scintillator geometry, surface treatment, and shape of external reflector in order to achieve maximum light collection efficiency for detector configurations that avoid direct optical coupling, a situation that is commonly found in cryogenic scintillating bolometers in experimental searches for double beta decay and dark matter. To evaluate the light collection efficiency of various geometrical configurations we used the ZEMAX ray-tracing software. It was found that scintillators in the shape of a triangular prism with an external mirror shaped as truncated cone gives the highest light collection efficiency. The results of the simulations were confirmed by carrying out measurements of the light collection efficiencies of CaWO4 crystal scintillators. A comparison of simulated and measured values of light output shows good agreement.

BC404 scintillators as gamma locators studied via Geant4 simulations

In many applications in industry and academia, an accurate determination of the direction from where gamma rays are emitted is either needed or desirable. Ion-beam therapy treatments, the search for orphan sources, and homeland security applications are examples of fields that can benefit from directional sensitivity to gamma-radiation. Scintillation detectors are a good option for these types of applications as they have relatively low cost, are easy to handle and can be produced in a large range of different sizes. In this work a Geant4 simulation was developed to study thedirectional sensitivity of different BC404 scintillator geometries andarrangements. The simulation includes all the physical processesrelevant for gamma detection in a scintillator. In particular, thecreation and propagation of optical photons inside the scintillatorwas included. A simplified photomultiplier tube model was alsosimulated.The physical principle exploited is the angular dependence of the shape of the energy spectrum obtained from thin scintillator layers when irradiated from different angles. After an experimental confirmation of the working principle of the device and a check of the simulation, the possibilities and limitations of directional sensitivity to gamma radiation using scintillator layers was tested. For this purpose, point-like sources of typical energies expected in ion-beam therapy were used. Optimal scintillator thicknesses for different energies were determined and the setup efficiencies calculated. The use of arrays of scintillators to reconstruct the direction of incoming gamma rays was also studied. For this case, a spherical source emitting Bremsstrahlung radiation was used together with a setup consisting of scintillator layers. The capability of this setup to identify the center of the extended source was studied together with its angular resolution.

A New Scintillator Geometry for Positronium Time of Flight Measurement

The positronium time of flight (Ps-TOF) spectroscopy is a novel technique for studying the microstructure in porous materials, in which the high-efficient detection of γ rays generated from ortho-positronium annihilation is the key point for the apparatus setup. In this paper, a new geometry of the scintillator was proposed to improve the γ ray detection efficiency which was indicated by the 22.1% promotion using the simulation results.

Use of internal scintillator radioactivity to calibrate DOI function of a PET detector with a dual-ended-scintillator readout.

Positron emission tomography (PET) detectors that use a dual-ended-scintillator readout to measure depth-of-interaction (DOI) must have an accurate DOI function to provide the relationship between DOI and signal ratios to be used for detector calibration and recalibration. In a previous study, the authors used a novel and simple method to accurately and quickly measure DOI function by irradiating the detector with an external uniform flood source; however, as a practical concern, implementing external uniform flood sources in an assembled PET system is technically challenging and expensive. In the current study, therefore, the authors investigated whether the same method could be used to acquire DOI function from scintillator-generated (i.e., internal) radiation. The authors also developed a method for calibrating the energy scale necessary to select the events within the desired energy window. The authors measured the DOI function of a PET detector with lutetium yttrium orthosilicate (LYSO) scintillators. Radiation events originating from the scintillators' internal Lu-176 beta decay were used to measure DOI functions which were then compared with those measured from both an external uniform flood source and an electronically collimated external point source. The authors conducted these studies with several scintillators of differing geometries (1.5 × 1.5 and 2.0 × 2.0 mm(2) cross-section area and 20, 30, and 40 mm length) and various surface finishes (mirror-finishing, saw-cut rough, and other finishes in between), and in a prototype array. All measured results using internal and external radiation sources showed excellent agreement in DOI function measurement. The mean difference among DOI values for all scintillators measured from internal and external radiation sources was less than 1.0 mm for different scintillator geometries and various surface finishes. The internal radioactivity of LYSO scintillators can be used to accurately measure DOI function in PET detectors, regardless of scintillator geometry or surface finish. Because an external radiation source is not needed, this method of DOI function measurement can be practically applied to individual PET detectors as well as assembled systems.

SU‐C‐220‐05: Investigation of Novel, Focused, Segmented Scintillator Geometries for High DQE Megavoltage Active Matrix Imagers

Purpose: Megavoltage active matrix, flat‐panel imagers (AMFPIs) have become ubiquitous in modern radiotherapy environments. However, their imaging performance is severely constrained by very low DQE (∼1%). While significant performance improvements have been demonstrated through introduction of thick, precision‐machined matrices of optically‐ isolated crystalline detector elements, the benefits are constrained by geometric beam divergence. It is therefore of interest to examine the potential performance of focused, segmented scintillator geometries specifically designed to circumvent DQE losses caused by divergence. Methods: The potential detection efficiency and DQE performance of hypothetical detector geometries was examined by means of Monte Carlo simulation of radiation transport. The choice of geometries examined was governed by considerations including: maximization of DQE through geometrically focused elements; manufacturability of the focused matrix; and feasibility of the underlying active matrix of indirect detection imaging pixels. Calculations were performed for a variety of candidate scintillator materials up to 6 cm thick. Results: The most promising detector geometries identified in the study consisted of matrices of scintillator elements located along the surface of a plane, as well as along the surface of a spherical cap. For cap detectors, elements arranged in a warped‐rectangular matrix, concentric rings, and a geodesic are of interest — and geometries employing a single element shape are particularly favored, although at the cost of fill factor. While focused planar detectors would be compatible with conventional AMFPI arrays, cap detectors would involve active matrix arrays in the shape of an approximately spherically curved surface created through cut and bend techniques. Overall, depending upon detector geometry, scintillator material and scintillator thickness, calculations indicate that DQE improvements of up to a factor of ∼50 are conceivable. Conclusions: This initial theoretical study suggests that the promise of very high DQE performance from thick, segmented scintillators could be realized through novel, focused detector geometries. Work supported by NIH grant R01‐CA051397

Monte Carlo investigations of the effect of beam divergence on thick, segmented crystalline scintillators for radiotherapy imaging

The use of thick, segmented scintillators in electronic portal imagers offers the potential for significant improvement in x-ray detection efficiency compared to conventional phosphor screens. Such improvement substantially increases the detective quantum efficiency (DQE), leading to the possibility of achieving soft-tissue visualization at clinically practical (i.e. low) doses using megavoltage (MV) cone-beam computed tomography. While these DQE increases are greatest at zero spatial frequency, they are diminished at higher frequencies as a result of degradation of spatial resolution due to lateral spreading of secondary radiation within the scintillator—an effect that is more pronounced for thicker scintillators. The extent of this spreading is even more accentuated for radiation impinging the scintillator at oblique angles of incidence due to beam divergence. In this paper, Monte Carlo simulations of radiation transport, performed to investigate and quantify the effects of beam divergence on the imaging performance of MV imagers based on two promising scintillators (BGO and CsI:Tl), are reported. In these studies, 10–40 mm thick scintillators, incorporating low-density polymer, or high-density tungsten septal walls, were examined for incident angles corresponding to that encountered at locations up to ∼15 cm from the central beam axis (for an imager located 130 cm from a radiotherapy x-ray source). The simulations demonstrate progressively more severe spatial resolution degradation (quantified in terms of the effect on the modulation transfer function) as a function of increasing angle of incidence (as well as of the scintillator thickness). Since the noise power behavior was found to be largely independent of the incident angle, the dependence of the DQE on the incident angle is therefore primarily determined by the spatial resolution. The observed DQE degradation suggests that 10 mm thick scintillators are not strongly affected by beam divergence for detector areas up to ∼30 × 30 cm2. For thicker scintillators, the area that is relatively unaffected is significantly reduced, requiring a focused scintillator geometry in order to preserve spatial resolution, and thus DQE.

Light collection optimization in scintillator-based gamma-ray spectrometers

Scintillator-based gamma-ray detectors are being actively pursued for homeland security applications. A key property of such detectors is their energy resolution which enables faster detection and more precise identification of gamma-ray sources. In order to obtain the best energy resolution with a given scintillator material, it is crucial to collect the largest fraction possible of the light emitted after gamma-ray absorption. Different techniques to maximize the light collection efficiency were investigated and tested experimentally. In particular, the effect of the scintillator geometry has been simulated with Detect2000. Also, a number of wrapping materials have been tested for their reflectivity and their performance in terms of improving the energy resolution in a BGO-based gamma-ray detector. The best results were obtained with a tapered cylinder geometry and the GORE DRP tape.