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At least 91 records · Page 5
All sourcesNASA NTRSDOE OSTI
DOE OSTI2016

Optimizing ZnS/6LiF scintillators for wavelength-shifting-fiber neutron detectors

In this paper we compare the performance of grooved and flat ZnS/6LiF scintillators in a wavelength shifting-fiber (WLSF) detector. Flat ZnS/6LiF scintillators with the thickness L=0.2-0.8 mm were characterized using photon counting and pulse-height analysis and compared to a grooved scintillator of approximately 0.8 mm thick. While a grooved scintillator considerably increases the apparent thickness of the scintillator to neutrons for a given coating thickness, we find that the flat scintillators perform better than the grooved scintillators in terms of both light yield and neutron detection efficiency. The flat 0.8-mm-thick scintillator has the highest light output, and it is 52% higher compared with a grooved scintillator of same thickness. The lower light output of the grooved scintillator as compared to the flat scintillator is consistent with the greater scintillator-WLSF separation and the much larger average emission angle of the grooved scintillator. We also find that the average light cone width, or photon travel-length as measured using time-of-flight powder diffraction of diamond and vanadium, decreases with increasing L in the range of L=0.6-0.8 mm. This result contrasts with the traditional Swank diffusion model for micro-composite scintillators, and could be explained by a decrease in photon diffusion-coefficient or an increase in micro-particle content in the flat scintillator matrix for the thicker scintillators.

Crow, Lowell↗
DOE OSTI2014

Study of Plastic Scintillator Properties for Radioactive Sources Dosimetry - Paper 108

Plastic scintillator is a simple material which has the density close to that of human soft tissue (1.06 g/cm{sup 3}). Its energy response is close to that of human soft tissue and it is relatively energy independent. This prospect provide a desired characteristics in measuring dose by plastic scintillator either for radiation monitoring or personal dosimeter. Generally the energy calibration of organic scintillator will be done by using gamma ray sources. The positions of Compton edge in pulse height spectra, which have known energy values will be identified. However the Compton edge normally is very broad and difficult to determine the point corresponding to that particular Compton electron energies. Many researchers used several fitting functions and Monte Carlo simulations to determine the position of Compton edge. For example, Proctor and Wellman (2012) used a straight line fit to the linear part of Compton edge and extrapolated to zero count. While L. Swiderski et al. used a method based on a back-scattered coincidence measurement to measure in a wide energy range the position of Compton edge in organic scintillators. For a simple and quick approach of calibration, we used a Gauss-error function to fit the Compton edge region of interest by assumed that the location of maximum energy of Compton electrons is half-height of the Compton maximum (CM). The reasons is because the CM position could be determined easily from the spectra throughout wide energy range. Interpreting gamma dose rates from accumulated plastic scintillation detector spectra is often a complex task because the probability of a full energy deposition event in the detector is often negligible. Despite no full energy peaks in plastic scintillator spectra, several attempts have been made by the other researchers to obtain the dose from the energy distribution spectra of gamma radiation. For example, Proctor and Wellman used the modified spectrum dose index (SDI) to calculate the exposure rates by multiplying the energy corresponding to its channel and all products are summed before multiply by a calibration constant to yield an exposure rate. While Lobdell and Hertel calculated dose using a response matrix based on spectral unfolding technique. From these points of view, we have made an attempt to correlate the counts obtained in scintillator with absorbed dose or ambient dose equivalent. We aimed to calibrate the plastic scintillator energy deposition for dose, by correlate the counts obtained with a particular value of dose. By designed the scintillator to measure at 1 cm tissue depth might be helpful to be used as radiation monitoring or personal dosimeter since the energy response of plastic scintillator is relatively energy independent compared to that of inorganic scintillators. The energy calibration proposed from simple fitting approach had been verified with calculated spectra. This might be considered as a practical approach for technician or radiation workers with sufficient accuracy. The linear behavior of counts per absorbed dose give promising to calibrate scintillator energy deposition to read dose (equivalent dose or absorbed dose) for photons. Thus the scintillator could provide both the energy and dose response simultaneously, which may important in some radiation field of measurements. Currently the scintillator is able to give dose rate value for gamma sources of Cs-137 and Mn-54. The number of counts per unit absorbed dose would likely to vary somewhat with photon energy, although the plastic scintillator should have a tissue dose response that is relatively energy independent. Further analysis will be done on conversion factor parameters so that it could be used in such situations where the radiation field is complex or unknown. (authors)

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2015

Characterizing energy dependence and count rate performance of a dual scintillator fiber-optic detector for computed tomography

Purpose: Kilovoltage (kV) x-rays pose a significant challenge for radiation dosimetry. In the kV energy range, even small differences in material composition can result in significant variations in the absorbed energy between soft tissue and the detector. In addition, the use of electronic systems in light detection has demonstrated measurement losses at high photon fluence rates incident to the detector. This study investigated the feasibility of using a novel dual scintillator detector and whether its response to changes in beam energy from scatter and hardening is readily quantified. The detector incorporates a tissue-equivalent plastic scintillator and a gadolinium oxysulfide scintillator, which has a higher sensitivity to scatter x-rays. Methods: The detector was constructed by coupling two scintillators: (1) small cylindrical plastic scintillator, 500 μm in diameter and 2 mm in length, and (2) 100 micron sheet of gadolinium oxysulfide 500 μm in diameter, each to a 2 m long optical fiber, which acts as a light guide to transmit scintillation photons from the sensitive element to a photomultiplier tube. Count rate linearity data were obtained from a wide range of exposure rates delivered from a radiological x-ray tube by adjusting the tube current. The data were fitted to a nonparalyzable dead time model to characterize the time response. The true counting rate was related to the reference free air dose air rate measured with a 0.6 cm{sup 3} Radcal{sup ®} thimble chamber as described in AAPM Report No. 111. Secondary electron and photon spectra were evaluated using Monte Carlo techniques to analyze ionization quenching and photon energy-absorption characteristics from free-in-air and in phantom measurements. The depth/energy dependence of the detector was characterized using a computed tomography dose index QA phantom consisting of nested adult head and body segments. The phantom provided up to 32 cm of acrylic with a compatible 0.6 cm{sup 3} calibrated ionization chamber to measure the reference air kerma. Results: Each detector exhibited counting losses of 5% when irradiated at a dose rate of 26.3 mGy/s (Gadolinium) and 324.3 mGy/s (plastic). The dead time of the gadolinium oxysulfide detector was determined to be 48 ns, while the dead time of the plastic scintillating detector was unable to accurately be calculated due to poor counting statistics from low detected count rates. Noticeable depth/energy dependence was observed for the plastic scintillator for depths greater than 16 cm of acrylic that was not present for measurements using the gadolinium oxysulfide scintillator, leading us to believe that quenching may play a larger role in the depth dependence of the plastic scintillator than the incident x-ray energy spectrum. When properly corrected for dead time effects, the energy response of the gadolinium oxysulfide scintillator is consistent with the plastic scintillator. Using the integrated dual detector method was superior to each detector individually as the depth-dependent measure of dose was correctable to less than 8% between 100 and 135 kV. Conclusions: The dual scintillator fiber-optic detector accommodates a methodology for energy dependent corrections of the plastic scintillator, improving the overall accuracy of the dosimeter across the range of diagnostic energies.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2016

SU-F-T-239: Evaluation of Plastic Scintillator Light Output for Various Lead Doping Concentrations: Towards LET Detection

Purpose: This work seeks to develop a beam quality meter using multiple differently doped plastic scintillators that are thus intrinsically beam-quality dependent. Plastic scintillators spontaneously emit visible light upon irradiation; the amount of light produced is dependent on stopping power (closely related to LET) according to Birks’ law. Doping plastic scintillators can be used to tune their sensitivity to specific LET ranges. Methods: GEANT4.10.1 Monte Carlo (MC) was used to evaluate the response of various scintillator dopant combinations. MC radiation transport and scintillator light response were validated against previously published literature. Current work involves evaluating detector response experimentally; to that end, a detector prototype with interchangeable scintillator housing was constructed. Measurement set-up guides light emitted by the scintillator to a photomultiplier tube via a glass taper junction coupled to an optical fiber. The resulting signal is measured by an electrometer, and normalized to dose readout from a diode. Measurements have been done using clinical electron and orthovoltage beams. MC response (simulated scintillator light normalized to dose scored inside the scintillating volume) was evaluated for four different LET radiations for an undoped and 1%Pb doped scintillator (σ=0.85%). Simulated incident electrons included: 0.05, 0.1, 0.2, 6, 12, and 18 MeV; these energies correspond to a range of stopping power (related to LET) values ranging from 1.824 to 11.09 MeVcm{sup 2}g{sup −1} (SCOL from NIST-ESTAR). Results: Initial MC results show a distinct divergence in scintillator response as LET increases. The response for undoped plastic scintillator indicated a 35.0% increase in signal when going from 18 MeV (low LET) to 0.05 MeV (high LET) while 1%-Pb doped scintillator indicated a 100.9% increase. Conclusion: After validating MC against measurement, simulations will be used to test various concentrations (2%, 4%, 6%) of different high-Z material dopants (W, Mo) to optimize the scintillator types for the beam quality meter. NSERC Discovery Grant RGPIN-435608.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2021

Fabrication of Low-Cost Large-Volume Ceramic A 2 HfX 6 (A= Cs or Tl, X = Cl, Br, or I) Scintillators for Gamma Ray Detection (SBIR Phase I Final Technical Report)

Scintillator crystals play an important role in the radiation detection field. Widespread use of scintillators as gamma-ray detectors is largely generated by their extensive availability and tunable properties, such high light output, high stopping power (Z eff ), fast decay time, and good proportionality. Additionally, the cost for manufacturing a scintillation detector like NaI:Tl is usually considerably lower than the cost for manufacturing a semiconductor detector like CdZnTe. Because there is no such thing as an ideal scintillation material, an application requiring certain detection characteristics may incorporate a scintillator tailored to its specific properties. The vast variety of applications and requirements necessitates more research into new scintillation materials and/or better methods of producing existing materials.The goal of this project was to grow low cost and environmentally stable inorganic transparent ceramic scintillators with excellent gamma ray resolution, excellent energy proportionality, excellent detection efficiency due to high density (>5 g/cm 3 ) and very high Z eff (55-80), and good light yields (>40,000 ph/MeV). In Phase I Xtallized Intelligence, Inc. (XI, Inc) developed a novel ceramic fabrication technique to produce low cost and environmentally stable highly efficient inorganic transparent ceramic scintillators of various dimensions. XI, Inc., collaborating with Fisk University (Fisk), investigated the scintillation properties of these new ceramic scintillators and compared them to in their single crystal counterparts. The results of this Phase I project show that successful production of high-quality inorganic halide ceramic scintillators Cs 2 HfCl 6 (CHC) and Tl 2 HfCl 6 (THC). Both ceramic CHC and THC scintillators have achieved good performance close to the performance of their single crystal counterparts. Fabricating these inorganic ceramic scintillators mitigate many issues encountered during conventional bulk crystal growth by melt methods. Additional benefits of the ceramic fabrication technique include high production yield, low production cost, fast production time, and no material waste Inorganic transparent ceramic scintillators produced in this project will enhance cost effectiveness at the instrument level based on low projected cost of the proposed compounds, as much smaller crystal sizes would be required to achieve similar efficiency as current radioisotope identification devices (RIID’s) used in homeland security applications as well as spectrometers in high energy physics applications.

36 MATERIALS SCIENCE↗
DOE OSTI2014

TH-C-19A-11: Toward An Optimized Multi-Point Scintillation Detector

Purpose: The purpose of this work is to characterize a 2-points mPSDs' optical chain using a spectral analysis to help selecting the optimal components for the detector. Methods: Twenty different 2-points mPSD combinations were built using 4 plastic scintillators (BCF10, BCF12, BCF60, BC430; St-Gobain) and quantum dots (QDs). The scintillator is said to be proximal when near the photodetector, and distal otherwise. A 15m optical fiber (ESKA GH-4001) was coupled to the scintillating component and connected to a spectrometer (Shamrock, Andor and QEPro, OceanOptics). These scintillation components were irradiated at 125kVp; a spectrum for each scintillator was obtained by irradiation of individual scintillator and shielding the second component, thus talking into account light propagation in all components and interfaces. The combined total spectrum was also acquired and involved simultaneous irradiation of the two scintillators for each possible combination. The shape and intensity were characterized. Results: QDs in proximal position absorb almost all the light signal from distal plastic scintillators and emit in its own emission wavelength, with 100% of the signal in the QD range (625–700nm) for the combination BCF12/QD. However, discrimination is possible when QD is in distal position in combination with blue scintillators, total signal being 73% in the blue range (400-550nm) and 27% in QD range. Similar results are obtained with the orange scintillator (BC430). For optimal signal intensity, BCF12 should always be in proximal position, e.g. having 50% more intensity when coupled with BCF60 in distal position (BCF12/BCF60) compared to the BCF60/BCF12 combination. Conclusion: Different combinations of plastic scintillators and QD were built and their emission spectra were studied. We established a preferential order for the scintillating components in the context of an optimized 2-points mPSD. In short, the components with higher wavelength emission spectrum should be distal and lower wavelength in the proximal position.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2011

Highly lead-loaded red plastic scintillators as an X-ray imaging system for the Laser Mega Joule

The scope of this project intends to record spatially resolved images of core shape and size of a DT micro-balloon during Inertial Confinement Fusion (ICF) experiments at Laser Mega Joule facility (LMJ). We need to develop an X-ray imaging system which can operate in the radiative background generated by an ignition shot of ICF. The scintillator is a part of the imaging system and has to gather a compromise of scintillating properties (scintillating efficiency, decay time, emission wavelength) so as to both operate in the hard radiative environment and to allow the acquisition of spatially resolved images. Inorganic scintillators cannot be used because no compromise can be found regarding the expected scintillating properties, most of them are not fast enough and emit blue light. Organic scintillators are generally fast, but present low X-ray absorption in the 10 to 40 keV range, that does not permit the acquisition of spatially resolved images. To this aim, we have developed highly lead-loaded and red-fluorescent fast plastic scintillators. Such a combination is not currently available via scintillator suppliers, since they propose only blue-fluorescent plastic scintillators doped with up to 12%w Pb. Thus, incorporation ratio up to 27%w Pb has been reached in our laboratory, which can afford a plastic scintillator with an outstanding Z{sub eff} close to 50. X-rays in the 10 to 40 keV range can thus be detected with a higher probability of photoelectric effect than for classic organic scintillators, such as NE102. The strong orange-red fluorescence can be filtered, so that we can eliminate residual Cerenkov light, generated by {gamma}-ray absorption in glass parts of the imaging system. Decay times of our scintillators evaluated under UV excitation were estimated to be in the range 10 to 13 ns. (authors)

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2015

Development of new Polysiloxane Based Liquid Scintillators

In the last decade, attention toward neutron detection has been growing in the scientific community, driven by new requirements in different fields of application ranging from homeland security to medical and material analysis, from research physics, to nuclear energy production. So far neutron detection, with particular attention to fast neutrons, has been mainly based on organic liquid scintillators, owing to their good efficiency and pulse shape discrimination (PSD) capability. Most of these liquids have however some main drawbacks given by toxicity, flammability, volatility and sensitivity to dissolved oxygen that limits the duration and the quality of their performances with worse handiness and increased costs. Phenyl-substituted polysiloxanes could address most of these issues, being characterized by low toxicity, low volatility and low flammability. Their optical properties can be tailored by changing the phenyl distribution and concentration thus allowing to increase the solubility of organic dyes, to modify the fluorescence spectra and to vary the refractive index of the medium. Furthermore, polysiloxanes have been recently exploited for the production of plastic scintillators with very good chemical and thermal stability and very good radiation hardness and the development of polysiloxane liquid scintillators could allow to combine these interesting properties with the supremacy of liquid scintillators as regarding PSD over plastics. For these reasons, the properties of several phenyl-substituted polysiloxane with different phenyl amounts and different viscosities have been investigated, with particular attention to the scintillation response and the pulse shape discrimination capability, and the results of the investigation are reported in this work. More in details, the scintillation light yield towards gamma rays ({sup 60}Co and {sup 137}Cs) of several polysiloxane liquids has been analyzed and compared with the light yield of a commercial non-toxic liquid scintillator (EJ309). The results have been related to the optical characterization of these materials, especially as regarding the fluorescence response, and the best performing material (1,1,5,5-Tetraphenyl 1,3,3,5-Tetramethyl Trisiloxane) showed a scintillation light-yield only slightly lower than EJ309, proving to be a promising candidate for the production of an efficient polysiloxane based liquid scintillator. The results as regarding the neutron-gamma pulse shape discrimination capability of the best performing materials are also reported in this work and the scintillation decay time of these materials are compared to the results of fluorescence lifetime analysis. PSD tests have been performed at CN accelerator in Legnaro National Laboratories with a 2.2 MeV pulsed neutron beam using TOF procedure and the pulses have been analyzed in order to evidence the PSD capability of every sample. The reported results pave the way to the development of a new promising class of non-toxic liquid scintillating materials for neutron detection, with good light output and interesting PSD characteristics. (authors)

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗