Bismuth-Loaded Polymer Scintillators for Gamma Ray Spectroscopy
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Bismuth-Loaded Polymer Scintillators for Gamma Ray Spectroscopy Benjmain L. Rupert, Nerine J. Cherepy, Benjamin W. Sturm, Robert D. Sanner, Zurong Dai and Stephen A. Payne Lawrence Livermore National Laboratory, Livermore, CA 94550, USA ABSTRACT We synthesize a series of polyvinylcarbazole (PVK) monoliths containing varying loadings of triphenyl bismuth as a high-Z dopant and varying fluors, either organic or organometallic, in order to study their use as scintillators capable of gamma ray spectroscopy. A trend of increasing bismuth loading resulting in a better resolved photopeak is observed. For PVK parts with no fluor or a standard organic fluor, diphenylanthracene (DPA), increasing bismuth loading results in decreasing light yield while with samples 1 or 3 % by weight of the triplet harvesting organometallic fluor bis(4,6-difluoropyridinato-N,C2)picolinatoiridium (FIrpic) show increasing light yield with increasing bismuth loading. Our best performing PVK/ BiPh3/FIrpic scintillator with 40 wt % BiPh3 and 3 wt % FIrpic has an emission maximum of 500 nm, a light yield of ~30,000 photons/MeV, and energy resolution better than 7% FWHM at 662 keV. Replacing the Ir complex with an equal weight of DPA produces a sample with a light yield of ~6,000 photons/MeV, with an emission maximum at 420 nm and energy resolution of 9% at 662 keV. Transmission electron microscopy studies show that the BiPh3 forms small clusters of approximately 5 nm diameter. INTRODUCTION Due to their low cost and ease of fabrication in large volumes plastic scintillators find many uses in radiation detection, such as portal scanners at shipping docks aimed at detecting movements of enriched uranium, plutonium and other potentially dangerous radioactive materials. Scintillator-based gamma ray spectroscopy requires scintillator materials with high light yield, excellent light yield proportionality, prompt emission decay and high effective atomic number for high photoelectric cross-section. Unfortunately plastic scintillators have low light yields, and more importantly, their low effective Z (Zeff ยง 4.5) results in poor photopeak efficiency, and they therefore have not been used for gamma-ray spectroscopy. This lack of spectroscopic ability leads to many innocuous shipments of materials, such as ceramics, to be selected for secondary screening, which is time consuming and expensive. Attempts to increase Zeff by doping with heavy metals to induce a photopeak have been made in the past.1-3 However, difficulties due to poor solubility of the high-Z component and quenching of luminescence were never resolved. While the low light yields of polymer scintillators impacts their use for event counting to some extent, it becomes critical for gamma ray spectroscopy. Separate from any consideration of spectroscopy, Campbell and Crone produced thin films of PVK doped with an iridium complex fluor which were found to offer improved scintillation light yields.4 This is due to the same mechanism now widely employed in organic light emitting diodes (OLEDs), wherein it
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