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.
Radiation Portal Monitors (RPMs) based on polyvinyl toluene (PVT) scintillators have been in use since the 1970s. Recently it has been discovered that under certain conditions PVT scintillators can fog at low temperatures. In collaboration with researchers at national laboratories, Eljen Technology has developed methods for the production of Intrinsic Anti-Fogging Scintillators. The production methods have been scaled up to allow for castings of rods and large sheets of scintillators that are deployable in RPMs and other extreme locations. The work is ongoing. The presentation will focus on the properties of the new scintillators and on the ongoing testing results from exposure to extreme environmental conditions.
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.
Plastic scintillators incorporating 8 weight percent elemental Bismuth offer enhanced sensitivity and distinct photopeak spectra in the <1000 keV range typically used in radiation portal monitors. The Bismuth-loaded plastic is based on polyvinyl toluene with standard singlet fluors. It produces ~6,000 photons/MeV with a maximum emission at 430 nm and a ~10 ns decay. Energy resolution of 49% at 59.5 keV and 16% at 662 keV are obtained for a 14 in3 Bismuth plastic scintillator plate. Count rates compared to standard plastic scintillator of the same size reveal a sensitivity improvement of >5x in the <200 keV range. Future spectroscopic radiation portal monitors based on the Bismuth plastic scintillator could provide moderate resolution spectroscopy for radioisotope identification. In addition, the Bismuth plastic offers outstanding environmental stability to weathering effects, in contrast with standard plastic scintillator formulations.
Plastic scintillators are widely deployed for ionizing radiation detection, as they can be fabricated in large sizes, for high detection efficiency. However, commercial plastics are limited in use for gamma spectroscopy, since their photopeak is very weak, due to low Z, and they are also limited in use for neutron detection, since proton recoils are indistinguishable from other ionizing radiation absorption events in standard plastics. We are working on scale up and production of transparent plastic scintillators based on polyvinyltoluene (PVT) loaded bismuth metallorganics for gamma spectroscopy. When activated with standard organic fluors, PVT scintillators containing 8 wt% bismuth provide energy resolution of 11% at 662 keV. When Iridium complex fluors are used, we can load plastics up to 20 wt% bismuth, while obtaining energy resolution of 10% at 662 keV. Another formulation, activated with Ir fluors for use as neutron radiography scintillator may be used for high energy neutron radiography.
Acknowledgements
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 National Nuclear Security Administration, Defense Nuclear Nonproliferation Research and Development under Contract No. DE-AC03-76SF00098
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