Edward A. McKigney

Science and Application of Oxyorthosilicate Nanophosphors

Nanophosphor Y2SiO5:Ce (n-YSO), Lu2SiO5:Ce (n-LSO), and Gd2SiO5:Ce (n-GSO) were prepared by solution-combustion synthesis yielding nanophosphor crystallite sizes between 20 nm - 80 nm. Ce dopant concentrations were varied between 0.1%-10% for each the nanophosphors and concentration quenching curves were measured by radioluminescence (RL) and photoluminescence (PL). n-YSO exhibits concentration quenching at 1 at% and 4 at% under UV and X-ray excitation, respectively. Red shifted emission with a larger Stokes shift is observed for nanophosphors as compared to bulk crystals. The measured PL lifetime depended on the refractive index of the media, indicating that the PL originates from the surface. Measurements of the RL/PL intensity indicate that the light output of these materials is comparable to the bulk crystal.

EPR and Luminescence of

The main thermally stimulated luminescence glow peak in irradiated oxyorthosilicates occurs near 360-400 K and has been postulated to comprise an electron trapped at an oxygen vacancy (F+ center). We have used electron paramagnetic resonance spectroscopy to identify this defect in Ln2SiO5:Ce (Ln = Lu and Y) and show that it consists of a single electron trapped at a non-silicon-bonded oxygen vacancy in both bulk and nanophosphor oxyorthosilicates. Both Lu- and Y-based nanophosphors form seven- and nine-oxygen coordinated structures (P21/c) whereas the bulk phosphors form six- and seven-oxygen coordinated structures (C2/c). In each case the F+ center predominately forms at the larger oxygen site. A typical resonance comprises a single Gaussian line broadened by hyperflne interaction with g-values near the free electron value and hyperflne coupling ~0.4 mT. The F+ center can be annealed and radiation-induced, consistent with the thermally stimulated luminescence glow peak behavior.

{\hbox {F}}^{+}

Transparent nanocomposites have been developed which consist of nanocrystals embedded in an organic matrix. The materials are comprised of up to 60% by volume of 7–13 nm crystals of the phosphor CexLa1−xF3, and are greater than 70% transparent in the visible region at a thickness of 1 cm. Consistencies of the nanocomposites range from a solid polymer to a wax to a liquid, depending on the workup conditions of the nanoparticle synthesis. These transparent nanophosphor composite materials have potential applications in radiation detection as scintillators, as well as in other areas such as imaging and lighting, and can be produced on large scales up to near-kilogram quantities at near ambient conditions, much lower in temperature than typical nanoparticle syntheses.

Centers in Bulk and Nanophosphor Oxyorthosilicates

Nanophosphor LaF3:Ce has been synthesized and incorporated into a matrix to form a nanocomposite scintillator suitable for application to γ-ray detection. Owing to the small nanocrystallite size (sub-10 nm), optical emission from the γ / nanophosphor interaction is only weakly Rayleigh scattered (optical attenuation length exceeds 1 cm for 5-nm crystallites), thus yielding a transparent scintillator. The measured energy resolution is ca. 16% for 137Cs γ rays, which may be improved by utilizing brighter nanophosphors. Synthesis of the nanophosphor is achieved via a solution-precipitation method that is inexpensive, amenable to routine processing, and readily scalable to large volumes. These results demonstrate nanocomposite scintillator proof-of- principle and provide a framework for further research in this nascent field of scintillator research.

Large-scale synthesis of CexLa1−xF3 nanocomposite scintillator materials

A novel synthetic route to a series of cerium bromide solvates is reported. The combination of bulk cerium bromide and the ionic liquid (IL) 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide results in a precursor paste that enhances the solubility of the cerium(III)bromide moiety in a number of donor solvents. Crystallization from these solvents has resulted in the isolation and characterization of CeBr(3)(THF)(4) (2), CeBr(3)(2-Me-THF)(4) (3), and CeBr(3)(MeCN)(5)·MeCN (4). Additionally, 2 is shown to be an efficient precursor for the new species CeBr(3)(py)(4) (5) and CeBr(3)(bipy)(py)(3) (6).

LaF3:Ce nanocomposite scintillator for gamma-ray detection

The low melting point, negligible vapor pressure, good solubility, and thermal and chemical stability make ionic liquids useful materials for a wide variety of applications. Polyoxometalates are early transition metal oxygen clusters that can be synthesized in many different sizes and with a variety of heterometals. The most attractive feature of POMs is that their physical properties, in particular electrical, magnetic, and optical properties, can be easily modified following known procedures. It has been shown that POMs can exhibit cooperative properties, as superconductivity and energy transfer. POM ionic liquids can be obtained by selecting the appropliate cation. Different alkyl ammonium and alkyl phosphonium salts are being used to produce new POM ionic liquids together with organic or inorganic luminescent centers to design light emitting materials. Ammonium and phosphonium cations with activated, polymerizable groups are being used to further polymerize the ionic liquid into transparent, solid materials with high metal density.

An ionic liquid-mediated route to cerium(III) bromide solvates.

The Los Alamos Neutron Science Center (LANSCE) is a linear accelerator in Los Alamos, New Mexico that accelerates a proton beam to 800 MeV, which then produces spallation neutron beams. Flight path FP15R uses a tungsten target to generate neutrons of energy ranging from several hundred keV to ~600 MeV. The beam structure has micropulses of sub-ns width and period of 1.784 ns, and macropulses of 625 μs width and frequency of either 50 Hz or 100 Hz. This corresponds to 347 micropulses per macropulse, or 1.74 x 104 micropulses per second when operating at 50 Hz. Using a very fast, cooled ICCD camera (Princeton Instruments PI-Max 4), gated images of various objects were obtained on FP15R in January 2015. Objects imaged included blocks of lead and borated polyethylene; a tungsten sphere; and a tungsten, polyethylene, and steel cylinder. Images were obtained in 36 min or less, with some in as little as 6 min. This is novel because the gate widths (some as narrow as 10 ns) were selected to reject scatter and other signal not of interest (e.g. the gamma flash that precedes the neutron pulse), which has not been demonstrated at energies above 14 MeV. This proof-of-principle experiment shows that time gating is possible above 14MeV and is useful for selecting neutron energy and reducing scatter, thus forming clearer images. Future work (simulation and experimental) is being undertaken to improve camera shielding and system design and to precisely determine optical properties of the imaging system.

Ionic Liquid Polyoxometalates as Light Emitting Materials

Composite scintillators consisting of nanosize inorganic crystals embedded in an organic matrix have been actively pursued in recent years. One method of producing nanosize crystals is through wet milling; however, since milling is known to introduce defects, the light yield of the milled crystals must be characterized. In this work, a new method of characterizing the light yield of milled inorganic crystals will be explored and discussed; this method will take into account explicitly the concentration of the inorganic crystals and the difference in stopping power between the crystals and the solvent.

Time gating for energy selection and scatter rejection: High-energy pulsed neutron imaging at LANSCE

In recent years, composite scintillators consisting of nanosize inorganic crystals in an organic matrix have been actively developed. Ideally these scintillators would have efficiency and resolution similar to inorganic crystals, but at the same time would be inexpensive and easy to manufacture. In order to make composite scintillators optically transparent, McKigney et al. finds that nanosize inorganic crystals should be used in order to reduce optical scattering. One way to produce these nanosize inorganic crystals is through wet milling, where inorganic crystals are ground with microsize beads in an organic solvent to achieve size reduction. Milling is relatively simple in terms of preparation and equipment; however, milling is also known to introduce defects into the ground material. Therefore, a new light yield measurement technique is developed to evaluate the degree to which milling alters the light yield of the milled inorganic crystals. In this work, the light yield measurement technique is applied to samples containing BaFCl:Eu inorganic crystals milled in a tributyl phosphate (TBP) and cyclohexane mixture.

Light yield measurement method for milled nanosize inorganic crystals

A new neutron multiplicity counter is being developed that utilizes the fast response of liquid scintillator detectors. The ability to detect fast (vs. moderated) fission neutrons makes possible a coincidence gate on the order of tens of nanoseconds (vs. tens of microseconds). A neutron counter with such a narrow gate will be much less sensitive to accidental coincidences making it possible to measure items with a high single neutron background to greater accuracy in less time. This includes impure Pu items with high (alpha,n) rates as well as items of low mass HEU where a strong active interrogation source is needed. Liquid scintillator detectors also allow for energy discrimination between interrogation source neutrons and fission neutrons, allowing for even greater assay sensitivity. Designing and building a liquid scintillator multiplicity counter (LSMC) requires a symbiotic effort of simulation and experiment to optimize performance and mitigate hardware costs in the final product. We present preliminary Monte Carlo studies using the GEANT toolkit along with analysis of experimental data used to benchmark and tune the simulation.