Vladimir D. Ryzhikov

Photochromic effect and its influence on scintillation properties of CdWO4 and PbWO4 crystals

Abstract Absorption induced in PbWO 4 and CdWO 4 scintillation crystals by UV and γ-ray irradiation was investigated by observing the dynamics of the absorption spectra after the irradiation of the samples with quanta of different energies and time evolution of the darkening effect at different rates of the irradiation. Dependence of the irradiation-induced absorption on growth conditions of the crystals, as well as bleaching of the samples by heating and illumination with visible light were studied. Trapping of free carriers at structural defects related with oxygen content in the lattice was shown to result in the creation of color centers in both tungstates under investigation.

Theoretical analysis of physical limits of energy resolution for detectors of scintillator-photodiode type and ways to improve their spectrometric characteristics

The development prospects of a scintillator-photodiode type detector with an improved energy resolution attaining few per cent, about 1.5 to 2.5%, are considered. The main resolution components have been analyzed theoretically, their theoretical and physical limits have been established. Empirical data on the properties of novel scintillators have been considered confirming the possibility of the energy resolution improvement. Ways have been proposed to optimize the detector statistical fluctuations and the scintillator intrinsic resolution. A specific importance of the intrinsic resolution is shown as a limiting threshold factor at the ionizing radiation energy values from 662 keV to 10 MeV and over.

Effects of structure microdefects on scintillation and photostimulated properties of CdWO4 crystals

CdWO4(CWO) scintillator single crystals find broad applications as detectors of ionizing radiation in tomographic and introscopic devices. Scintillation characteristics of the crystals are strongly dependent upon growth conditions. In this paper the nature of the defects is studied, related to deviations from stoichiometry and presence of admixtures, for the crystals grow by the Chochralski method. Negative effects of admixtures (Fe, Cu, Bi, Ga ions) on the light yield and afterglow in CWO crystals were established. Photosensitivity of CWO crystals under UV-radiation was noted in the region of the fundamental absorption edge, causing the induced absorption ((alpha) up to 1.5 cm-1) and a decrease of the light yield (to 30%). The studies have shown that the induced absorption depends upon both admixtures and non- stoichiometry. Basing on the results, preparation conditions were optimized for large-sized (55 X 250 mm) CWO crystals, allowing to obtain scintillation characteristics meeting the requirements for their use in radiation detectors for tomography and introscopy.

Radiographic spectroscopy of atomic composition of materials: a multi-energy approach

Theoretical model of multi-energy radiography (MER) for reconstruction of the atomic structure is proposed. It is shown that, using multi-channel absorption and detection of radiation, effective atomic number and quantitative chemical composition of the materials can be readily reconstructed. This approach opens prospects for improvement of efficiency of X-ray techniques in non-destructive testing, nuclear and safety monitoring, security customs control, and others.

Looking for new possibilities to improve properties of two-energy detectors of scintillator-photodiode type for inspection systems of international security: Multi-energy radiography against terrorism. Theory and experiments

Multi-energy radiography is a new direction in nondestructive testing. Its specific feature is separate detection of penetrating radiation in several energy channels. Multi-energy radiography allows quantitative determination of atomic composition of the objects. This is its principal advantage over conventional radiography. In particular, dual-energy radiography allows determination of the effective atomic number of a material with accuracy up to 80-90%. Development of three-energy radiography and radiography of higher multiplicity makes it possible to reconstruct the exact chemical composition also. This means, for example, detection of explosives and other illegal objects in luggage with reliability close to 100%. Thus, these developments can find application in anti-terrorist activities, in industrial testing and nuclear medicine

Semiconductor detectors for the erytheme region of UV radiation

Main parameters and characteristics of surface-barrier diodes sensitive for UV irradiation on the base of ZnSe are presented in the given paper. Possibilities of application of developed semiconductor photoreceivers for detectors of erytheme region are being discussed.

Multi-energy ZnSe-based radiography against terrorism: theory and experiments

Multi-energy radiography is a new direction in non-destructive testing. Its specific feature is separate detection of penetrating radiation in several energy channels. Multi-energy radiography allows quantitative determination of the atomic composition of objects. This is its principal advantage over conventional radiography. In particular, dual-energy radiography allows determination of the effective atomic number of a material with an accuracy of up to 80-90%. Development of three-energy radiography and radiography of higher multiplicity makes it possible to further improve the reconstruction of an objects chemical composition. This presents the possibility, for example, of detection of explosives and other illegal objects in luggage with a reliability approaching 95-98%. These developments can find application not only in anti-terrorist activities, but also in industrial testing and nuclear medicine.

Spectrometric universality and reduction of nonstatistical noises in detectors with regular light collection

Intrinsic resolution of the scintillator is one of the most important constituents of the full energy resolution of a detector. Intrinsic resolution contains components preserved at any ionizing radiation energy. A major role is played by the resolution of light collection. This is a component determined by geometric-optical non-uniformities of scintillation energy propagation and collection. In this work, theoretical studies of general light collection features have been carried out. A universal law has been predicted for light collection dispersion in detectors of regular beam dynamics. Such systems include scintillation blocks with mirror reflecting surface and regular geometry in the shape of cylinder, parallelepiped or sphere. An important regular collection feature is weak dependence of its dispersion on the scintillator material or shape. This allows to relate spectrometric efficiency and detection efficiency for any detector of the said type. The theoretically obtained law is confirmed by the available experimental data. The developed theory allows finding new ways to eliminate internal noises affecting the radiation measurements data. Such optimization is a necessary condition for creation of new detectors with improved characteristics.

Studies of interaction between components in system ZnSe-ZnTe-Se-H2-C

A thermodynamic analysis has been carried out for interaction of the components in the ZnSe-ZnTe-Se-H2-C system. Studies of thermodesorption, thermodynamics and kinetics of interaction reactions in the ZnSe-ZnTe-Se-H2-C system indicate high probability of formation of fine-dispersed fullerene-like particles of hydrated carbon black in this system at the stage of ZnSe(Te) charge synthesis. It has been established that carbon black particles preserve their stability at the stage of ZnSe(Te) crystal growth and can substantially affect crystallophysical, optical and physico-chemical properties of these crystals.

Use of GSO scintillators in x-range radiometers

In radiometers for X-range, the use of scintillators based on gadolinium orthosilicate (GSO) has many advantages over alkali halide scintillators, e.g., NaI(Tl). The shape of spectra obtained with GSO and cadmiug tangstate (CWO) scintillators has certain features that substantially increase the sensitivity of radiometers which use the windows method algorithm and are designed for simultaneous detection of low-energy gamma-radiation of 241Am (59.6 keV) and 137Cs (~33 keV). These features of oxide scintillators have allowed a substantial increase in sensitivity of the radiometer RK-AG-02M, which is designed for detection of 241Am on the background of 137Cs. The results obtained in this work show that the use of GSO and (in several cases) CWO scintillators in the instruments involving X-ray range of radiation (e.g., meters of thickness, radiometers) is very promising field of their application. We consider also the effects of different factors related to thermal treatment upon dopant-activated complex oxide crystal to improve its optic-luminescent, scintillation and other functional characteristics. In the selective radiometer RK-AG-02M are using a scintillation block based on GSO scintillator. To increase the measurement accuracy in the bulk density range of 0.6-2.5 g/cm3 correcting function has been obtained for the radiometer sensitivity.