Plastic scintillators are widely deployed in radiation detection applications due to their low-cost, high scalability and generally favorable mechanical properties. However, due to their relatively low atomic number, plastic scintillators generally have low absolute efficiency and offer limited energy information for detected gamma-rays. To overcome these challenges, metal-organics can be included in these scintillators, offering higher efficiencies and gamma spectroscopy in plastics. LLNL is pursuing bismuth loading in plastics utilizing both conventional fluors (Bi-PVT) and triplet harvesting fluors (Bismuth-Loaded Iridium-fluor Plastics, BLIP). Bi-PVT is being produced by Eljen in sizes up to 1.5” x 6” x 30” as a drop-in replacement for portal monitors and LLNL is developing a modular spectrometer utilizing BLIP offering high sensitivity per unit mass of detector material.
Plastic scintillators incorporating up to 8 weight percent element bismuth are being developed as drop-in replacements for current portal monitor plastics. They use the same fluors with fast decay times (<10 ns) while offering enhanced sensitivity with more than 8x increased counts from Am-241 for the same detector volume. In this work, we report on the largest samples produced to date with volumes over 135 in3, and compare their performance to currently fielded plastic scintillators.
Transparent ceramic Cerium-doped Gadolinium Yttrium Gallium Aluminum Garnet, GYGAG(Ce), offers a combination of environmental stability, high light yield, good gamma spectroscopy and formability into large plates that is attractive for implementation into Radiation Portal Monitors. GYGAG(Ce) plates at 4” x 4” x 0.5” scale achieve energy resolution of R(662 keV) <7%. Production of high transparency 8 in3 GYGAG(Ce) plates is underway, as well as their integration into detector modules and portal systems.
Plastic scintillators are widely deployed radiation detectors due to their low cost and environmental ruggedness. Their effectiveness, however, is limited by their low atomic number resulting in low stopping power and poor photopeak efficiency. Here, we compare two different Bi-loaded plastic scintillator formulations to conventional plastic, demonstrating improved spectroscopy and stopping power at the ~18 in3 scale. One approach, Bi-pivalate plastics, uses conventional fluors and may be used as a drop-in replacement for currently deployed plastics such as EJ200. The other approach, Bismuth Loaded Iridium-complex Plastics (BLIP), uses an Iridium-based fluor for higher light yield and higher Bi loading.
Plastic-based scintillator detectors have many advantages over inorganic scintillators, including mechanical ruggedness and cost. However, their range of application has generally been limited by their lack of gamma spectroscopic performance. We have been developing metal-organic doped plastic scintillators which allow for spectroscopy while maintaining the advantages of plastics. These scintillators allow for the use of plastics in many new application spaces. Using iridium based fluors, bismuth loaded plastics have demonstrated high light yields of >20,000 photons/MeV and good energy resolution (<12% FWHM at 662keV) in modest sizes. We are working on scaling up these scintillators to larger sizes for use in radio-isotope identification (RIID) type application.
This work was supported by the US DOE Office of NNSA NA-22 DNN Program and was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.
We report on a small gamma spectrometer with directional detection, based on two modules. Each module contains an array of 128 scintillator pixels coupled to reverse-biased photodiodes, an ASIC and a microcontroller. Modules communicate via USB and are operated via an Android GUI on a WiFi-connected device. Transparent ceramic GYGAG ((Gd,Y,Ce)3(Ga,Al)5O12) garnet scintillator pixels offer energy resolution as good as 3.1% at 662 keV for individual pixels. We previously reported on an eight-module planar spectrometer, based on the same technology1. The current compact system uses two modules in a back-to-back configuration to improve its speed and accuracy in locating a gamma-emitting source in a 4-pi field of view. This dual module system (~2.5 x 2.5 x 3.5 inches) is suitable as a pager-sized RIID or spectroscopic personal radiation detector (SPRD). Multiple dual module units can be combined for improved efficiency. Spectroscopic and directional performance will be described.
High energy X-rays and neutrons can provide 3-D volumetric views of large objects made of multiple materials. Lenscoupled computed tomography using a scintillator imaged on a CCD camera obtains high spatial resolution, while a surface-mounted segmented scintillator on an amorphous silicon (A-Si) array can provide high throughput. For MeV Xray CT, a new polycrystalline transparent ceramic scintillator referred to as “GLO” offers excellent stopping power and light yield for improved contrast in sizes up to a 12” field-of-view. For MeV neutron CT, we have fabricated both contiguous and segmented plates of “Hi-LY” plastic scintillator, offering light yields 3x higher than standard plastic.
Inexpensive spectroscopic personal radiation detectors (SPRDs) are needed to monitor environmental radioactivity and search for sources. For gamma spectroscopy, excellent light yield, material uniformity, light yield proportionality, mechanical and environmental ruggedness can be achieved in polycrystalline ceramic oxide garnets. We are building a compact detector based on 14 cm3 of transparent ceramic garnet, formed into 256 pixels (3mm x 3mm x 6mm each) and mounted on two stacked silicon photodiode arrays. GYGAG(Ce) garnet transparent ceramics offer density = 5.8g/cm3, Zeff = 48, principal decay of <100 ns, and light yield of 50,000 Ph/MeV. We obtain R(662 keV) <4% for the full device, including Compton summing of coincident events in multiple pixels. In addition to excellent gamma spectroscopy, this device provides directional detection, via Compton imaging and active masking, for search applications.
This work was performed under the auspices of the U.S. DOE by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. We are grateful to Digirad for supporting our implementation of their photodiode array. Thanks to the US Department of Homeland Security, DNDO and CWMD offices, for funding under competitively awarded IAAs HSHQDC-12-X-00149 and HSHQDN-17-X-00016.
Plastic scintillators are in wide use in radiation portal monitors because of their low cost and availability in large sizes. However, due to their low density and atomic number (Z), they offer low intrinsic efficiency and little to no spectroscopic information. The addition of high-Z constituents to these plastics can greatly increase both their total stopping power and the amount of photo-electric absorption, leading to full-energy deposition and thus spectroscopic information in plastics. In this work, we present the performance of the largest bismuth-loaded plastics to date, showing useful spectroscopic information up to relatively high energy (~1 MeV) and their high stopping power compared the current commercially available plastics. These Bi-loaded plastics are based on 20 wt% Bi-pivalate (8 wt% elemental Bi) dissolved in a polyvinytoluene (PVT) matrix and conventional fast fluors (<10 ns decay time). A comparison of performance between slab and cylindrical plastics of similar volumes is presented and large performance improvements (greater than 9 times the sensitivity to 241Am) are shown when used as a drop-in replacement to conventional PVT based portal monitors.
This work was performed under the auspices of the U.S. DOE by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344, and has been supported by the US DOE, Office of NNSA, NA-22.LLNL-ABS-767130.
We have previously made improvements to the longevity of TlBr semiconductor gamma ray detectors by applying electrodes having the mixed semiconductor composition Tl(Cl,Br) via surface treatments in HCl, leading to a significant enhancement to the lifetime of the detectors. In order to examine the electron transport properties more closely, we have monitored the first-derivative of the cathode waveform (being proportional to velocity and number of carriers) as a function of time and the point of the gamma-interaction. The observed decay in this signal, especially at lower voltage, would naturally be interpreted as the usual trapping phenomenon. However, this phenomenon alone is not able to account for the observed waveforms, most dramatically for the case of increasing signal as the electrons approach the anode, for waveforms originating at the cathode. After detailed consideration of alternative explanations, the cathode waveform data has been interpreted in terms of a non-uniform field owing to variation in the resistivity as a function of position. We have interpreted the shape of the decay as a “built-in” resistivity profile and have further verified this interpretation by reversing the sense of the field (which as expected reverses the “sense” of the waveform). We modeled this effect in order to quantitatively deduce the resistivity profile and are currently working to relate the waveform observations to the relative orientation of the crystal growth direction and the applied electrodes.
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