Inorganic scintillators are widely used in various applications of gamma spectroscopy such as nuclear nonproliferation and safeguards, medical applications, space applications, and astronomy. This is ...due to good energy resolution, stable performance, somewhat low cost, and relatively high detection efficiency. However, many inorganic scintillators have high refractive indices and suffer significant light losses due to total internal reflection (TIR). This project proposes using optimized periodic nanostructures called photonic crystals to recover some of the light originally lost due to TIR. Photonic crystals provide an optical bridge (constructive interference) between the scintillator and the photosensor for the trapped light photons. Improving the light extraction can improve the energy and time resolutions of the scintillator, allowing for a wider range of research and industry applications. Photonic crystals can be optimized in terms of their dimensions, shapes, and materials to maximize the light extraction. Preliminary optimization tests were performed using a LYSO scintillator coupled with Si3N4 photonic crystals. First, a realistic light input source is obtained by simulating the scintillation process in Monte Carlo code Geant4. The simulated scintillation photons are collected at the LYSO-PMT boundary to obtain their energy and angular distributions. In the next step, a deterministic code OptiFDTD is used to simulate light interactions with different nanostructures. Currently, the simulations are limited to 2-D block nanostructures. The optimization tests vary the height, width, and spacing of the photonic crystals. Preliminary optimization tests show an improvement in the light transmission by more than 60%. The optimized geometry will be manufactured in the lab using various manufacturing techniques such as ion milling, electron beam lithography, or 3D printing. Various gamma sources will be used to experimentally characterize the LYSO scintillators with and without photonic crystals. These experiments will also be used to validate the simulations and demonstrate the effectiveness of the photonic crystals in improving the energy resolution. Once validated, the simulations will be used to determine optimized photonic crystals for other inorganic scintillators, such as bismuth germanate, sodium iodide, and lanthanum bromide.
Purpose
High‐dose rate (HDR) and pulsed‐dose rate (PDR) brachytherapy would benefit from an independent treatment verification system to monitor treatment delivery and to detect errors in real time. ...This paper characterizes and provides an uncertainty budget for a detector based on a fiber‐coupled high‐Z inorganic scintillator capable of performing time‐resolved in vivo dosimetry during HDR and PDR brachytherapy.
Method
The detector was composed of a detector probe and an optical reader. The detector probe consisted of either a 0.5 × 0.4 × 0.4 mm3 (HDR) or a 1.0 × 0.4 × 0.4 mm3 (PDR) cuboid ZnSe:O crystal glued onto an optical‐fiber cable. The outer diameter of the detector probes was 1 mm, and fit inside standard brachytherapy catheters. The signal from the detector probe was read out at 20 Hz by a photodiode and a data acquisition device inside the optical reader. In order to construct an uncertainty budget for the detector, six characteristics were determined: (1) temperature dependence of the detector probe, (2) energy dependence as a function of the probe‐to‐source position in 2D (determined with 2 mm resolution using a robotic arm), (3) the signal‐to‐noise ratio (SNR), (4) short‐term stability over 8 h, and (5) long‐term stability of three optical readers and four probes used for in vivo monitoring in HDR and PDR treatments over 21 months (196 treatments and 189 detector calibrations, and (6) dose‐rate dependence.
Results
The total uncertainty of the detector at a 20 mm probe‐to‐source distance was < 5.1% and < 5.8% for the HDR and PDR versions, respectively. Regarding the above characteristics, (1) the sensitivity of the detector decreased by an average of 1.4%/°C for detector probe temperatures varying from 22 to 37°C; (2) the energy dependence of the detector was nonlinear and depended on both probe‐to‐source distance and the angle between the probe and the brachytherapy source; (3) the median SNRs were 187 and 34 at a 20 mm probe‐to‐source distance for the HDR and PDR versions, respectively (corresponding median source activities of 4.8 and 0.56 Ci, respectively); (4) the detector response varied by 0.6% in 11 identical irradiations over 8 h; (5) the sensitivity of the four detector probes decreased systematically by 0–1.2%/100 Gy of dose delivered to the probes, and random fluctuations of 4.8% in the sensitivity were observed for the three probes used in PDR and 1.9% for the probe used in HDR; and (6) the detector response was linear with dose rate.
Conclusion
ZnSe:O detectors can be used effectively for in vivo dosimetry and with high accuracy for HDR and PDR brachytherapy applications.
Multiplexing of radiation detectors reduces the number of readout channels, which in turn reduces the number of digitizer input channels for data acquisition. We recently demonstrated frequency ...domain multiplexing (FDM) of pulse mode radiation detectors using a resonator that converts the detector signal into a damped sinusoid by convolution. The detectors were given unique ”tags” by the oscillation frequency of each resonator. The charge collected and the time-of-arrival of the detector pulse were estimated from the corresponding resonator output in the frequency domain.
In this paper, we demonstrate a new method to recover the detector pulse from the damped sinusoidal output by deconvolution. Deconvolution converts the frequency-encoded detector signal back to the original detector pulse. We have developed a new prototype FDM system to multiplex organic scintillators based on convolution and deconvolution. Using the new prototype, the charge collected under the anode pulse can be estimated from the recovered pulse with an uncertainty of about 4.4 keVee (keV electron equivalent). The time-of-arrival can be estimated from the recovered pulse with an uncertainty of about 102 ps. We also used a CeBr3 inorganic scintillator to measure the Cs-137 gamma spectrum using the recovered pulses and found a standard deviation of 13.8 keV at 662 keV compared to a standard deviation of 13.5 keV when the original pulses were used. Coincidence measurements with Na-22 using the deconvolved pulses resulted in a timing uncertainty of 617 ps compared to an uncertainty of 603 ps using the original pulses. Pulse shape discrimination was also performed using Cf-252 source and EJ-309 organic scintillator pulses recovered by deconvolution. A figure of merit value of 1.08 was observed when the recovered pulses were used compared to 1.2 for the original pulses.
This paper reports the characterization of two scintillating materials in powder form, Gadox and YVO embedded in a light-activated resin, used in a probe developed for oncological brachytherapy ...in-vivo dosimetry. The materials were characterized in terms of internal absorption, scintillation decay time, and light yield. The measurement of the optical characteristics highlighted a significant internal absorption at the scintillation light wavelength, with values of 6.5 dB/mm for Gadox and 14.1 dB/mm for YVO.
Measurements of the characteristics scintillation time and of the light yield were performed with a novel method based on single photon counting, profiting from the long decay time of the materials under study. Measurements have been complemented by a two-step simulation with Geant4 to study the energy deposition followed by a ZEMAX OpticStudio® ray tracing to estimate the light collection efficiency. The decay time for scintillating materials were measured to be τGadox = (458 ± 3 ± 3) μs and τYVO = (451 ± 8 ± 3) μs and the estimated values of the light yield are (7.1 ± 0.5) × 104 photon/MeV for Gadox and (4.8 ± 0.5) × 104 photon/MeV for YVO.
Frequency domain multiplexing of pulse mode radiation detectors Mishra, M.; Mattingly, J.; Mueller, J.M. ...
Nuclear instruments & methods in physics research. Section A, Accelerators, spectrometers, detectors and associated equipment,
09/2018, Volume:
902, Issue:
C
Journal Article
Peer reviewed
Open access
The capability to multiplex scintillation detectors or other pulse mode radiation detectors is necessary in some applications where a large number of detectors is required. Frequency domain ...multiplexing has been previously implemented for applications in astronomy using amplitude modulation on radiation detectors such as transition-edge sensors. We propose an alternative method for multiplexing pulse mode radiation detectors in the frequency domain using convolution. We pass the detector signal to a resonator circuit that converts a detector pulse to a damped sinusoid of a specific frequency which gives a unique tag to the detector. We have developed a prototype frequency-domain multiplexed system for four EJ-309 organic scintillator detectors using four resonators of unique frequencies. The resonator outputs are combined using a fan-in circuit which is then connected to a single digitizer input. Using this system, we demonstrate that the charge collected under the original anode pulse can be estimated from the power spectrum of the damped sinusoid with a relative uncertainty of about 2%. The time-of-arrival of the anode pulse can be estimated using constant fraction discrimination applied to the leading edge of the damped sinusoid with an uncertainty of about 450 ps. We also used a CeBr3 detector to test the performance of our system for spectroscopic applications and found only small degradation in the resolution for a multiplexed detector.
The temperature dependence of four inorganic scintillation detectors was examined spectrally using the HYPERSCINT Research Platform 200. The detectors were subject to 6 MV photon irradiations from a ...linear accelerator, varying only the temperature of the detectors. Optical fibre connectorisation was characterised and found to be essential to the repeatability of measurements. The magnitude of the temperature effect on each detector was evaluated by calculating the percentage change in total photon counts for a range of temperatures. Gd2O2S:Tb and Gd2O2S:Eu exhibited the lowest change in counts with temperature, 0.12%/°C and −0.10%/°C, respectively, with ZnS:Ag showing the highest change in counts of −0.55%/°C. Gd2O2S:Pr photon counts changed by −0.25%/°C. All the scintillators demonstrated linearity when the change in photon counts with temperature in the full-width at half maximum of their spectrum are integrated. Establishing the magnitude of the temperature dependence of the materials is critical to decide whether correction factors are required. This is especially true in applications such as brachytherapy, where detectors equilibrise to body temperature.
•Temperature effect on scintillators established in a clinical temperature range.•HYPERSCINT RP-200 used with inorganic scintillators using 6 MV photon beam.•Change in photon output up to 0.55%/°C demonstrating scintillator choice is key.•Linearity of effect demonstrated permitting a range correction methods.
Inorganic scintillators with high density and high light yield are of major interest for applications in medical imaging and high energy physics detectors. In this work, the optical and scintillation ...properties of Mg co-doped Ce:Gd3Al2Ga3O12 crystals, grown using Czochralski technique, have been investigated and compared with Ce:Gd3Al2Ga3O12 ones prepared with identical technology. Improvements in the timing performance of the Mg co-doped samples with respect to Ce:Gd3Al2Ga3O12 ones have been measured, namely a substantial shortening of the rise time and scintillation decay components and lower afterglow were achieved. In particular, a significantly better coincidence time resolution of 233ps FWHM, being a fundamental parameter for TOF-PET devices, has been observed in Mg co-doped crystals. The samples have also shown a good radiation tolerance under high doses of γ-rays, making them suitable candidates for applications in harsh radiation environments, such as detectors at future collider experiments.
Research in ceramic scintillators has steadily progressed alongside the research in bulk single crystal scintillator growth. As interest in faster scintillation material production with lower cost ...increases, more research on scintillating ceramics is needed. Research targeting optimization of optically transparent ceramics that can rival bulk-grown crystals grown may lower cost, increase yield, increase volume, and improve energy resolution in applications and systems currently using sodium iodide and alike. Ceramic scintillators that are dense (>5 g/cm3), have high effective Z (>60), are bright (>40,000 photons/MeV), and are not sensitive to moisture as well as those that can be handled without protection are desired. Ultra-fast ceramic materials are also of interest. This paper presents an equipment design and technique to produce inorganic halide ceramic scintillators Cs2HfCl6 (CHC) and Tl2HfCl6 (THC). Improvements and optimization of CHC and THC ceramic scintillator fabrication are gauged by monitoring the energy resolution and peak position of 137Cs full energy peak at 662 keV. With a 1-inch diameter CHC ceramic scintillator, energy resolution of 5.4% (FWHM) and light yield of 20,700 ph/MeV are achieved, while with a 16-mm diameter THC ceramic scintillator, energy resolution of 5.1% (FWHM) and light yield of 27,800 ph/MeV are achieved. Decay times of 0.6 μs (21%) and 3.0 μs (79%) are measured for CHC and 0.3 μs (13%) and 1.0 μs (87%) for THC. Here, both ceramic CHC and THC scintillators have similarly good proportionality data when compared to their single crystal counterparts.
There is a global trend to increase the light yield of CsI scintillators used in neutrino and dark matter detection by operating undoped crystals at cryogenic temperatures. However, high light yield ...alone is not sufficient to guarantee a low-energy threshold. The response of undoped crystals to nuclear recoils at cryogenic temperatures is equally important. A liquid nitrogen-based cryostat was developed to measure the nuclear quenching factor of a small undoped CsI crystal using monoenergetic neutron beams at the Triangle Universities Nuclear Laboratory (TUNL). To minimize neutron scattering, high-Z materials were reduced around the crystal. The structure and performance of the cryostat are described in detail. Using this cryostat, a system light yield of 33.4±2.0 photoelectrons per keV electron-equivalent (PE/keVee) was observed at 5.9 keVee, enabling the measurement of nuclear quenching factors at very low energies. The results of the quenching factor measurement will be reported in a subsequent paper.
Non-negligible negative overshoot was observed in the tails of the observed light pulses. The origin of this issue and the correction procedure are described in detail. This information may be useful for others who encounter similar technical challenges.
In this paper we report on the crystal growth, density functional theory (DFT) calculations and scintillation properties of TlMgX3 (X = Cl, Br, I). Crystals were grown by the Vertical Bridgman ...technique up to 16 mm in diameter and 25 mm long. Crystals of TlMgCl3 and TlMgBr3 belong to the family of the Perovskites and have the orthorhombic crystal structure. As a congruently melting composition, TlMgI3 does not exist but probably forms Tl2MgI4 instead.
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