Tuesday, January 26, 2010

Inexpensive Nanophosphors for Wide Area Radiation Detection Created by Los Alamos National Security Scientists

Los Alamos National Security, LLC (Los Alamos, NM) scientists have created nanophosphor compositions that can be used for wide area radiation detection as well as an inexpensive nanofabrication method for the nanophosphors. 

FIG. 1a shows a photographic image of two pieces of the cerium doped transparent nanocomposite scintillator Ce:LaF3(oleic acid). FIG. 1b shows a transmission electron microscope (TEM) image of the nanocomposite of FIG. 1a.

Phosphors are currently used in many important devices such as fluorescent lamps, RGB (red, green, blue) screens, lasers, and crystal scintillators for radiation detectors, radiographic imaging, tagging and other security applications, lighting applications, and nuclear spectroscopy. Perhaps the most important property of any phosphor is its brightness, i.e. its efficiency, which is the ratio of the number of optical photons emitted by the phosphor to the energy absorbed.

Other important properties include the spectral region of maximum emission (which should match commonly-used photodetectors), optical absorption (minimum self-absorption is desired), decay time of the emission (for some applications fast is desired), and the density. In general, superior scintillators exhibit high quantum efficiency, good linearity of the spectral emission with respect to incident energy, high density, fast decay time, minimal self-absorption, and high effective Z-number. Specific scintillator applications determine the choice of phosphor. Scintillators used for active and passive radiation detection, for example, require high density, and brightness, whereas scintillators used for radiographic imaging also require fast decay time.

The compositions are nanophosphor particles capped with a ligand. The nanophosphor particles have a size of about 20 nanometers. The composition has at least one lanthanide and at least one halide. The weight percent of the lanthanide phosphor is about 5 percent. The light transmission of the composition is about 50 percent, according to inventors Anthony K. Burrell,  Kevin C. Ott, John C. Gordon,  Rico E. Del Sesto and Mark McCleskey.  The method for manufacturing the nanophosphors is detailed in U.S. Patent 7,651,633, granted on January 26th, 2010. 

 The Los Alamos nanophosphors are fast, bright, dense scintillators. Large area detectors (e.g. detectors useful for medical imaging or monitoring large objects such as shipping containers, boats, planes, etc.) may be prepared more easily using these fast, bright, dense nanophosphors than using single crystal scintillators. The brightness provides a detector with optimal light output, and the high density provides the detector with stopping power for the x-rays, gamma rays, neutrons, protons, or the like. Also, the new nanophosphors are inexpensive compared to more conventional spectroscopic detector materials.

Nanophosphors include monodisperse, or nearly monodisperse, doped or undoped lanthanide halides (halide=fluoride, chloride, bromide or iodide). Nanophosphors also include lanthanide chalcogens (chalcogen=oxygen, sulfur, selenium, tellurium). 

In one embodiment, nearly monodisperse nanophosphors were prepared from lanthanide triflate precursors. Lanthanide triflate was subjected to certain reaction conditions in the presence of a capping ligand and a source of acidic halide. The source of acidic halide participates in the removal of triflate from the lanthanide triflate precursor, and also with transfer of halide(s) to the lanthanide. A typical capping ligand is a relatively high boiling material that can chemically coordinate to the lanthanide and aid in controlling the nucleation and growth of the nanophosphor. The capping agent may also electrostatically interact with surfaces of the nanoparticles.   

Control over the nucleation and growth (and hence particle size), an appropriate surface capping with either ligands or additional inert lanthanum halide is used to optimize the light output of the phosphor. This lanthanide halide is suitable for pressing into a compact form, or dispersing in a plastic or glass composite having suitable properties for light transmission to prepare a large area scintillator body.

Los Alamos National Security, LLC (LANS) is made up of four top U.S. organizations that have extensive experience in nuclear defense programs, large-scale facilities management, applying science and technology to homeland security challenges, and safety and security—Bechtel National, University of California, The Babcock and Wilcox Company, and the Washington Division of URS.

FIG. 5b shows a TEM image of Ce doped LaBr3 nanocomposite scintillator made by Los Alamos National Laboratory Scientists  

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