Robert D. Gilbertson

Large-scale synthesis of CexLa1−xF3 nanocomposite scintillator materials

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.

LaF3:Ce nanocomposite scintillator for gamma-ray detection

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.

The Couette configuration of the Los Alamos Neutron Science Center Neutron Rheometer for the investigation of polymers in the bulk via small-angle neutron scattering

A neutron rheometer in the Couette geometry has been built at the Los Alamos Neutron Science Center to examine the molecular steady-state and dynamic responses of entangled polymeric materials in the bulk under the application of shear stress via small-angle neutron scattering. Although similar neutron rheometers have been fabricated elsewhere, this new design operates under the extreme conditions required for measuring the structure and behavior of high molecular weight polymer melts. Specifically, the rheometer achieves high torques (200 N m) and shear rates (865 s(-1)) simultaneously, never before attainable with other neutron rheometers at temperatures up to 240 degrees C under an inert gas environment. The design of the instrument is such that relatively small sample sizes are required. The testing of the Los Alamos Neutron Science Center Neutron Rheometer in the Couette design both as a rheometer and in the small-angle neutron optical configuration on highly viscous polystyrene is presented. The observed anisotropic neutron scattering pattern of the polystyrene melt at a molecular weight above entanglement provides evidence that the conformation of the polymer chains are elongated in the direction of the melt flow, in agreement with the current theories concerning linear polymers in the bulk.

Ionic Liquid Polyoxometalates as Light Emitting Materials

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.

The Los Alamos Neutron Science Center neutron rheometer in the cone and plate geometry to examine tethered polymers/polymer melt interfaces via neutron reflectivity

Although several other neutron rheometers have been built to study soft matter under nonequilibrium conditions, none of them have the ability to measure the structure and behavior of the polymeric interfacial regions in highly viscous polymer melts which require high torques/high strain rates and high temperatures. A neutron rheometer in the cone and plate geometry has been constructed at the Los Alamos Neutron Science Center to rectify this lack of experimental instrumentation. It is also the first-of-its-kind to perform neutron reflectivity studies concurrently with rheological measurements. The details of both the development and testing of the Los Alamos Neutron Science Center neutron rheometer in the cone and plate configuration are described. Proof of principle neutron reflectivity results of end-grafted polystyrene against an identical melt under shear are presented, showing qualitatively that the structural attributes of the end-grafted polymer change when exposed to shear.

Computational screening of organic polymer dielectrics for novel accelerator technologies

The use of infrared lasers to power accelerating dielectric structures is a developing area of research. Within this technology, the choice of the dielectric material forming the accelerating structures, such as the photonic band gap (PBG) structures, is dictated by a range of interrelated factors including their dielectric and optical properties, amenability to photo-polymerization, thermochemical stability and other target performance metrics of the particle accelerator. In this direction, electronic structure theory aided computational screening and design of dielectric materials can play a key role in identifying potential candidate materials with the targeted functionalities to guide experimental synthetic efforts. In an attempt to systematically understand the role of chemistry in controlling the electronic structure and dielectric properties of organic polymeric materials, here we employ empirical screening and density functional theory (DFT) computations, as a part of our multi-step hierarchal screening strategy. Our DFT based analysis focused on the bandgap, dielectric permittivity, and frequency-dependent dielectric losses due to lattice absorption as key properties to down-select promising polymer motifs. In addition to the specific application of dielectric laser acceleration, the general methodology presented here is deemed to be valuable in the design of new insulators with an attractive combination of dielectric properties.

Accelerated Aging of BKC 44306-10 Rigid Polyurethane Foam: FT-IR Spectroscopy, Dimensional Analysis, and Micro Computed Tomography

An accelerated aging study of BKC 44306-10 rigid polyurethane foam was carried out. Foam samples were aged in a nitrogen atmosphere at three different temperatures: 50 °C, 65 °C, and 80 °C. Foam samples were periodically removed from the aging canisters at 1, 3, 6, 9, 12, and 15 month intervals when FT-IR spectroscopy, dimensional analysis, and mechanical testing experiments were performed. Micro Computed Tomography imaging was also employed to study the morphology of the foams. Over the course of the aging study the foams the decreased in size by a magnitude of 0.001 inches per inch of foam. Micro CT showed the heterogeneous nature of the foam structure likely resulting from flow effects during the molding process. The effect of aging on the compression and tensile strength of the foam was minor and no cause for concern. FT-IR spectroscopy was used to follow the foam chemistry. However, it was difficult to draw definitive conclusions about the changes in chemical nature of the materials due to large variability throughout the samples.

Development and characterization of nanocomposite scintillators for gamma-ray detection

Nanocomposite scintillators consist of nanoparticles of an inorganic scintillator dispersed in a matrix material. This technique is particularly well suited for applications requiring large or unusually shaped detectors, since these can be fabricated without requiring the growth of large crystals and using an inexpensive matrix. Nanocomposite scintillators may additionally make it possible to use scintillating materials not currently available in large sizes and quantities. Los Alamos National Laboratory is currently conducting a large nanocomposite scintillator development program. Several potential scintillator materials have been identified. We have successfully fabricated nanoparticles, for example CeF3, with sizes ≪ 10 nm and dispersed them in an optimized liquid scintillator solution and in several polymers. We have measured their size, structure, and radiation detection characteristics and have found good agreement between their scattering behavior and a Rayleigh scattering model. Current research results will be presented.