V. V. Kedrov

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.

Evolution of the spectral characteristics upon annealing of lithium borate glasses containing europium and aluminum

The spectral and structural characteristics of lithium borate glasses containing europium and aluminum have been investigated upon annealing at different temperatures. It has been found that the spectral characteristics of the studied system change nonmonotonically with an increase in the annealing temperature. After annealing at a temperature of 600°C, the luminescence spectra of the glasses exhibit broad structureless bands that are specific for the amorphous phase containing Eu3+ ions. Then, after annealing at T = 700°C, narrow lines appear in the wavelength ranges 585–595 and 610–620 nm, which correspond to the luminescence of the Eu(BO2)3 and EuAl3(BO3)4 borates. A further increase in the annealing temperature (T = 800–900°C) leads to the disappearance of europium aluminum borate. In the luminescence spectra of these samples, there are narrow bands in the wavelength range λ = 585–595 nm, which are typical of europium metaborate. Finally, at a temperature of 1050°C, these bands disappear and narrow lines appear again in the wavelength range 610–620 nm, which are characteristic of the EuAl3(BO3)4 borate. Thus, the temperature annealing makes it possible to purposely change the spectral characteristics of the studied system in the wavelength range 590–615 nm.

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.

Spectral Characteristics of Different Structural Modifications of Lu 1- x Eu x BO 3

The spectral and structural characteristics of polycrystals of Eu3+-doped lutetium borates Lu1 − xEuxBO3) annealed at different temperatures have been investigated over a wide range of europium concentrations. The conditions for the preparation of Lu1 − xEuxBO3 in the calcite and vaterite phases have been determined. It has been found that there is a radical difference between the excitation spectra of the main emission bands of the calcite and vaterite phases of the Lu1 − xEuxBO3 borates. The influence of the europium concentration on the structure of Lu1 − xEuxBO3 has been analyzed. It has been established that, at europium concentrations of higher than 15 at %, only the vaterite structure is formed independently of the annealing temperature. Thus, by varying the Eu3+ concentration and the annealing temperature of Lu1 − xEuxBO3, it is possible to directionally synthesize a specific structural modification and, consequently, to control the spectral characteristics of this compound.

Spectral and structural features of Lu1 − xRExBO3 compounds

The luminescence spectra, luminescence excitation spectra, IR absorption spectra, and crystal structure of orthoborates Lu1 − xRExBO3 (RE = Eu, Gd, Tb, Y, Dy) have been investigated. It has been found that the solid solution consisting of a LuBO3 orthoborate, which has two stable structural modifications (calcite and vaterite), and an REBO3 orthoborate, which has one structural modification (vaterite), crystallizes only in the vaterite structure when the concentration of a rare-earth ion substituting for lutetium exceeds 15–20 at %. The investigation of the photoluminescence spectra has demonstrated that, for rare-earth ions Lu3+, Eu3+, Y3+, and Gd3+ in the vaterite modification of Lu1 − xRExBO3 orthoborates, there are at least two positions that are not equivalent in the symmetry of the local environment. It has been established that the maximum intensity of the luminescence of the vaterite modification of Lu1 − xTbxBO3 compounds synthesized at 970°C, which is observed at a terbium concentration of 15 at %, is several times higher than the maximum intensity of the luminescence of the calcite modification.

Spectral characteristics and energy transfer from Ce3+ to Tb3+ in compounds Lu1 – x – yCexTbyBO3

The structure, IR absorption spectra, morphology, and spectral characteristics of compounds Lu1 – x – yCexTbyBO3 have been investigated. It has been shown that the Tb3+ luminescence excitation spectrum of the Lu1 – x – yCexTbyBO3 compounds is dominated by a broad band coinciding with the excitation band of Ce3+ ions, which clearly indicates energy transfer from the Ce3+ ions to the Tb3+ ions. The spectral position of this band depends on the structural state of the sample: in the structures of calcite and vaterite, the band has maxima at ~339 and ~367 nm, respectively. By varying the ratio between the calcite and vaterite phases in the sample, it is possible to purposefully change the Tb3+ luminescence excitation spectrum, which is important for the optimization of the spectral characteristics of Lu1 – x – yCexTbyBO3 when it is used in light-emitting diode sources. An estimate has been obtained for the maximum distance between Ce3+ and Tb3+ ions, which corresponds to electronic excitation energy transfer. It has been shown that the high intensity of Tb3+ luminescence in these compounds is due to the high efficiency of electronic excitation energy transfer from the Ce3+ ions to the Tb3+ ions as a result of the dipole–dipole interaction.

Influence of nanoparticles of cesium sulfate and deformation on structure, vibration spectra, and luminescence of polystyrene

Significant changes in infrared absorption spectra of polystyrene upon the addition of cesium sulfate nanoparticles to it are observed, which is due to two variants of attachment of organic molecules to nanoparticles (through hydrogen and pi bonds). The relation between the bonds changes significantly during deformation of polystyrene/cesium sulfate composite owing to differences in the mechanical strength thereof. In its turn, the differences in atomic and electronic bond structures result in a significant differentiation of their influence on light emission processes, making it possible to control the scintillation characteristics of polystyrene/nanoparticle composites via regulating their composition and morphology.