N. V. Klassen

Nanoscintillators for Microscopic Diagnostics of Biological and Medical Objects and Medical Therapy

The main focus of this paper is the description of qualitatively new facilities for diagnostics of biological and medical objects and medical therapy obtained by applications of nanocrystalline scintillators. These facilities are based on abilities of nanoscintillators to selective conjugation with various biomolecular objects and noticeable variations of their atomic structures, X-ray diffraction (XRD) patterns, and light-emission characteristics induced by modifications of conditions on their external surfaces. Experimental results presented in this paper provide development of detection in vivo just inside a living organism of various viruses, cancer cells, and other pathological macromolecules by means of scanning X-ray diffractometry of nanoparticles introduced into the body. These data are produced by selective adsorption of pathological bioobjects by these nanoparticles and subsequent modifications of their XRD patterns. Application of narrow collimated X-ray beams and new types of X-ray detector matrices providing microscopic spatial resolution due to usage of nanoscintillators enables determination of the regions where these pathologies are localized with high accuracy. The procedure of detection of pathological organelles by this method improves possibilities for effective destruction of these pathologies by low-dose X-ray irradiation of the places of their localization. High effectiveness of this X-ray destruction is provided by concentrated absorption of X-ray quanta by the nanoscintillators and direct transfer of the absorbed energy to the pathological objects that are attached to the absorbing particles. Constructions of 3-D radiation detector matrices providing necessary microscopic spatial and angular resolutions of X-ray imaging are described on the basis of nanoscintillators, fiber light guides, and microcapillary matrices.

Advantages and Problems of Nanocrystalline Scintillators

Our experiments with nanocrystalline scintillating rare earth oxides and rare earth fluorides have shown that in some cases nanoscopic dimensions provide essential improvement of the most important scintillation parameters: light yield, kinetics of scintillations, radiation hardness, etc. We found that in the range from 20 to 100-nm of the oxide and fluoride particles there are 3 types of layered structures: with expanded exterior layer, with changed phase structure, and with changed chemical composition. These layered structures can strongly influence scintillation parameters: cause an increase or decrease in the light yield, vary scintillation kinetics, modify radiation hardness, etc. Control of dimensions and structures of nanoscintillators can be used for significant modifications of parameters of radiation detectors (radical acceleration of kinetics, enhancement of light yield, increase in radiation hardness, etc.). Radiation detectors based on nanoscintillators have promising prospects for applications in new generations of devices for medical diagnostics, security inspection, radiation monitoring of nuclear reactors.

Laser and Electric Arc Synthesis of Nanocrystalline Scintillators

Two new methods of preparation of nanocrystalline scintillators are described. Laser ablation of microscopic powders immersed in optically transparent liquid was used to produce spherical nanoparticles, which preserved the initial compositions. Electric arc discharge between electrodes of definite metals immersed in water solutions of different salts produces a vast variety of scintillating compounds with nanoscopic dimensions and morphologies having crystallographic symmetry of the corresponding equilibrium phases. A wide range of different compositions and structures of tungsten oxides are obtained during one synthesis process, which is due to variety of temperatures and other conditions around the arc channel. It was found that the light emission spectroscopy of the discharge is a rather informative method of diagnostics of the process of the nanoparticle synthesis inside the discharge chamber. The synthesis of nanoscintillators by arc discharge turned out to be rather efficient and capable to create nanocrystalline scintillators of easily regulated compositions. Hydrogen injection into nanoparticles of tungsten oxide is detected by light emission and infrared absorption spectroscopy. Hydrogenated nanoscintillators obtained by this method are interesting for registration of fast neutrons.

Spectroscopy of composite scintillators

The spectral and temporal characteristics of X-ray luminescence of composites consisting of microparticles of “heavy” components (oxides, fluorides, sulfates) and an organic polymer binder containing optically active impurities have been investigated. It has been found that, in the case of pulsed X-ray excitation of the composites with a photon energy of 130–150 keV, the fast component (τ < 10 ns) of the luminescence arises whether or not the “heavy” component of the composite is doped with an optically active impurity. A mechanism has been proposed for the formation of the fast component of the luminescence: electrons and low-energy X-ray photons generated during the interaction of high-energy X-ray photons with the “heavy” component of the composite are effectively absorbed by the polymer binder and, thus, induce its luminescence. It has been shown that, in order to produce a composite-based fast scintillator with a high light yield, it is necessary to use a binder prepared from an organic material with a short scintillation decay time and another component prepared from a compound whose composition includes an element of a large atomic number Z.

Modifications of Light Emission Spectra and Atomic Structure of Europium Molibdate Bulk Crystals Caused by High Pressure and Heat Treatment

Essential enhancement of light emission ability of europium molybdate - Eu2(MoO4)3 (EMO) crystals has been achieved by combined application of high pressure (9 GPa) and heat treatment (a procedure was repeated from 3 to 25 times for different samples). Multifold increase in light emission of EMO single crystals can be achieved, without deterioration of the optical transparency of the material. Simultaneously the light emission spectra change substantially. Nature of this phenomenon is explained by the modifications of the crystal field caused by structural transformations induced by the treatments. These modifications of the crystal fields bring to increase in probability of light emission optical transitions between 4f sublevels of europium ions.

“Isomorphous” phases in nanodispersed powders of rare-earth oxides

Using x-ray diffraction, a structural state consisting of two “isomorphous” phases was revealed in nanocrystalline powders of simple oxides Re2O3 (Re = Eu, Gd, La, Lu) and Y3Ga5O12 prepared by solvent thermolysis from simple-oxide solutions followed by annealing of obtained precursors at elevated temperatures. A model is proposed explaining such two-phase states in terms of the excess energy of nanograin surface layers which causes the formation of a surface phase isomorphous to the core phase having a smaller lattice parameter. Analogous two-phase states were also obtained in microcrystalline LuBO3 and Eu2(MoO4)3 powders subjected to long-term grinding.

Growth of α-PbF2 single crystals from aqueous solutions of inorganic acids

Abstract Growth of α-PbF 2 crystals in 0.28M perchloric acid is described. The choice was based on an analysis of equilibria with the participation of fluoride ions and investigations of the solubility of lead fluoride in aqueous solutions. The enthalpy of dissolution of α-PbF 2 in 0.28M perchloric acid is 1.6 kJ/mol, which is close to the value for pure water (2.4 kJ/mol). Orthorhombic lead fluoride single crystals were grown at spontaneous crystallization by a slow cooling method in the shape of plates (3 × 3 × 0.1 mm 3 ) or prisms (4 × 1.5 × 0.2 mm 3 ), depending on the cooling rate, α-PbF 2 was grown on a seed crystal. The grown layer of about 1 mm in thickness has no visual inclusions and cracks.

Pulsed X-ray luminescence of composites consisting of inorganic particles and organic phosphors

A fast luminescence component with a duration of ∼2 ns has been observed upon pulsed X-ray excitation of composites composed of microparticles of a heavy constituent (heavy-metal oxides and fluorides) and optically active polymer adhesive. The intensity and temporal parameters of this component depend on neither the structural state of the heavy constituent nor the presence of optically active impurity. A mechanism of the formation of the fast luminescence component of composites upon pulsed X-ray excitation is proposed; according to it, when high-energy X rays interact with the heavy constituent of the composite, electrons and low-energy X-ray photons, which are intensely absorbed by the polymer adhesive and thus cause its luminescence, are generated due to the photoelectric effect and X-ray scattering.

Peculiarities of nanostructured silicon carbide films and coatings obtained by novel technique

A new method has been developed for obtaining various versions of nanostructured SiC films and coatings, whose structure can be altered in a controlled way for different applications. The films and coatings obtained can be useful in metallurgy, nuclear power industry, microelectronics, and high-temperature furnaces.

New Shaped Ceramics Based on Silicon Carbide

New techniques have been developed for producing inexpensive shaped SiC ceramics with certain structure and porosity for a wide variety of applications. These techniques are based on the interaction of silicon melt with carbon from a previously pressed blank of definite composition (carbon, silicon carbide, organic bond) and porosity.