Hrytsak A. Radiation and thermally stimulated processes in scintillation and thermochromic materials for ionizing radiation detection.

The work is dedicated to the targeted search and study of the properties of effective scintillation and thermochromic materials that can be used for ionizing radiation detection, on the basis of investigations of the inherent to them thermally and radiation stimulated processes. Special attention was devoted to study of the scintillation characteristics of maximally pure and defect-free single crystals as sensitive materials for cryogenic detectors to ensure favorable conditions for the realization of intrinsic excitonic luminescence. Therefore, a main part of this dissertation work was devoted to study of the materials such as cesium iodide (CsI), zinc telluride (ZnTe) and calcium tungstate (CaWO4). At the same time, an alternative method of the targeted introduction of an activator impurity was used to optimize the luminescence parameters of zinc tungstate (ZnWO4). Significant attention was also devoted to study of the thermochromic materials, particularly those based on diethylammonium tetrachlorocuprate [NH2(C2H5)2]2CuCl4 (DEACC), which are characterized by the ability to change their optical properties under temperature variations, enabling their use in sensor technology, particularly for ionizing radiation detection. Although the single crystalline scintillators and thermochromic materials are employed for different purposes, they can be considered complementary to one another, opening up new opportunities for enhancing radiation safety systems and ionizing radiation detection technologies. As a result of study of X-ray luminescence spectra and their correlation with the thermally stimulated processes, presence of Vk and H centers responsible for the X-ray luminescence and thermoluminescence bands was confirmed in nominally pure CsI crystals. Additionally, the nature of the ionic conductivity in this material was established. The thermally stimulated luminescence in combination with the analysis of X-ray luminescence spectra and scintillation decay curves in nominally pure calcium tungstate crystals was investigated. CaWO4 crystals with maximal excitonic luminescence intensity were grown using the Czochralski method from the raw materials of the highest purity (99.99%), obtained using solid-phase high-temperature synthesis. These crystals are promising for the development of cryogenic scintillation detectors. The introduction of Li₂O impurities into ZnWO₄ scintillator leads to increase in the luminescence band intensity at 2.59 eV, that implies its potential use in the sensor devices. A patent for a utility model has been obtained concerning the scintillation material based on ZnWO₄ crystal, improved by doping with lithium. This modification allowed increasing of the luminescence light output and a reduction of the light sum accumulation at deep trapping levels during X-ray excitation. Among the studied materials ZnTe and CaWO₄ crystals possess the optimal scintillation characteristics. Undoped ZnTe exhibits a competitive light output value of 117 ± 20% in respect of that of the CaWO₄ scintillator, with a relatively short scintillation decay time. This implies the potential use of zinc telluride as a sensitive element in the traditional scintillation detectors operating at the liquid nitrogen temperature (T = 77 К). This material may be particularly attractive for confirming the possibility of observation of the neutrino-less double beta decay (0νDBD) in the 130Te nuclei, which are the part of the scintillator. The proposed explanation of the observed extremely high sensitivity of the thermochromic properties of the microcomposites based on diethylammonium tetrachlorocuprate (DEACC) crystals to relatively low doses of ionizing radiation involves a significant shift in the thermochromic phase transition temperature determined from the D(T) hysteresis loops, towards lower values. The key factor in the radiation effect is related to changes at the interface between the matrix and the microcrystal. The shift in phase transition temperature under relatively low radiation doses is attributed to the breaking of the chemical bonds between the polymer matrix and the microcrystals. There was developed the technology for producing of the thermochromic indicator based on a DEACC polycrystalline film coated with a polymer, which can be used for fabrication of the sensitive elements for sensors including those used for measurement of the doses of ionizing radiation.