Chapter 8
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Applications of Nanoparticles in Scintillation Detectors Suree S. Brown, Adam J. Rondinone, and Sheng Dai* Chemical Sciences Division, Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, T N 37831-6201
Applications of commercially available, highly efficient inorganic scintillators (particle size: μm) are limited by their low solubilities in both polymeric and sol-gel matrices. On the other hand, organic scintillators, though highly soluble in polystyrene-based matrices, are not compatible with an efficient neutron inorganic absorber, L i , and their applications with L i as neutron scintillators are strictly limited. Here, preparation and surface modification of organic nanoparticles and inorganic nanocrystals are demonstrated as a means to increase dispersion and compatibility of scintillators with neutron-absorbing materials and matrices. A survey of nanoparticles, including PPO, POPOP-doped polystyrene nanoparticles, CdSe/ZnS core/shell quantum dots, Y 0 : C e (5%), LaP0 :Ce (10%), and L i P 0 nanocrystals, in various matrices, along with their results in beta, alpha, or neutron detection is discussed. 6
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© 2008 American Chemical Society In Antiterrorism and Homeland Defense; Reynolds, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2007.
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Introduction Advanced radiation detectors, especially for neutron and gamma radiation, are important for detecting "dirty" bombs, monitoring U-235 and Pu-239 for nuclear arms control/verification, nuclear smuggling, special nuclear material (SNM) detection, and waste characterization, as well as for fundamental research in nuclear physics, solid-state physics, chemistry, biology, and neutron radiography. One of the oldest, and still the most useful, methods for detecting ionizing radiation (e.g., alpha particles, beta particles, gamma rays) is through the scintillation process. Scintillation occurs when the energy of ionizing radiation is absorbed by certain crystalline inorganic or organic materials, resulting in the emission of UV-vis light from the absorbing materials (7). The key to the development of advanced radiation detectors lies in the synthesis and characterization of efficient scintillation materials. In the case of neutron detection, the conversion of incident neutrons into detectable charged particles is normally required. O f particular importance in the detection of slow neutrons, ca. below 0.5 eV, are B(n,a) and Li(n,a) reactions. We report that by taking advantage of the high g-value (i.e., large excess energy imparted to charged particles) of the L i nuclear reaction ( L i + *n -> H + He, 0-value = 4.78 MeV), we can detect slow neutrons. The energy of alpha particle produced as a secondary radiation is deposited directly onto scintillation materials. The ideal scintillation material should meet the following criteria: high scintillation efficiency, wide range of linear energy conversion, transparency to the wavelength of its own emission, short emission decay time, good optical quality, index of refraction near that of glass (-1.5) (2), emission spectrum matched to the spectral sensitivity of the detector, and high durability (5). Commercially available inorganic scintillation materials possess many of the desired properties; however, their particle sizes in the range of microns lead to the opacity of scintillation detectors and, consequently, lowered scintillation efficiencies. During the past two decades, there have been extensive investigations on inorganic nanocrystals, especially semiconductor nanocrystals or quantum dots (QDs). With appropriate surface functional groups, nanocrystals can be dispersed in various solvents, sol-gels, or polymers, yielding transparent products. In the case of QDs, quantum confinement effect (4-7) offers the plausibility of a better matching between the emission spectrum and the sensitivity of a photomultiplier tube (PMT) or other light sensors used. Here, a survey of representative organic nanoparticle and inorganic nanocrystals embedded in various matrices, along with their results in beta, alpha, or neutron detection, is presented. The ultimate goal of this study is to be able to identify the most suitable nanoparticle-based scintillation materials for specific radiation detection purposes. 10
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In Antiterrorism and Homeland Defense; Reynolds, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2007.
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119 A better understanding of how certain nanoparticle-based scintillators behave under radiation interactions is also expected to be gained.
Experimental Section
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Reagents and Syntheses A l l reagents used were of highest grades commercially available. Most were used as received, except styrene and methylstyrene, which were distilled freshly prior to usage. A l l syntheses and handling were carried out under an inert atmosphere. 2,5-diphenyloxazole (PPO), l,4-bis-2-(5-phenyloxazolyl)-benzene (POPOP)-doped polystyrene (PS) and polyvinyltoluene (PVT) nanoparticles were synthesized by ultrasonication of micelle solutions of styrene and methylstyrene, respectively, under conditions modified from the procedures reported by Biggs and Grieser (8). CdSe/ZnS core/shell quantum dots were prepared by a selected procedure published previously (9). Cerium-doped yttrium oxide ( Y 0 : C e ) nanocrystal samples were prepared by three different methods. The detailed synthesis by the first method, Pechini-type in-situ polymerizable complex (IPC) method, has been reported (10). This method utilizes polyesterification between metal complexes of an a-hydroxycarboxylic acid (e.g., DL-malic acid) and a polyhydroxy alcohol (e.g., ethylene glycol), followed by calcination, yielding homogeneous Y 0 : C e . The second method employed the slow decomposition of urea and hexamethylenetetramine ( H M T A ) at elevated temperatures (up to 235 °C), providing an in-situ generated controlled amount of ammonia. The third method involved the formation of complexes between the polyethylene glycol, PEG-8000, as a chelating agent and the yttrium and cerium metals. Heat treatment of yttrium, cerium-PEG complexes led to the formation a mixed-metal oxide. Cerium-doped lanthanum phosphate (LaP0 :Ce) nanocrystals were prepared by a modified method from the detailed synthesis reported by Haase and co-workers (11-13). Surface modification of nanocrystals was done by injecting a large excess of mixed surfactants to the reaction mixture at 100 °C. A mixture of dodecylamine (DDA) and bis(2-ethylhexyl) hydrogenphosphate (BEHP) and a mixture of oleylamine (OA) and B E H P were compared in this study. The surface modification was allowed to proceed for 1 h at 100 °C before the reaction mixture was worked up by precipitation with methanol. Synthesis of lithium-6 phosphate ( L i P 0 ) from Li-6 oleate (95% L i ) for neutron detection was modified from the synthesis of LaP0 :Ce nanocrystals. 2
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In Antiterrorism and Homeland Defense; Reynolds, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2007.
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120 Commercially available, highly efficient liquid and solid scintillators were tested as standards for comparison with our samples. Ultima Gold™ (PerkinElmer) is a standard liquid scintillation counting cocktail, containing PPO (ca. 1 wt %) in a mixture of aromatic compounds, and phosphate and succinate surfactants. BC-400™ (Bicron) is a plastic scintillator composed of organic fluors, PPO and POPOP, at
