Khromiuk I. The mechanism of delayed radioluminescence pulse formation in organic heterostructured scintillators and their ability to separately register ionizing radiation based on the scintillation pulse shape

The dissertation is devoted to determining the influence of the migration characteristics of triplet excitons in organic heterostructured scintillators, namely polycrystals and composite scintillators based on stilbene, p-terphenyl (both activated and unactivated) and anthracene, on their ability to separate by the form of a scintillation pulse (pulse shape discrimination, PSD) the signals from ionizing radiation with different specific energy losses dE/dx. In the work, the diffusion coefficients of triplet states in similar heterostructured scintillators were calculated for the first time, and it was also shown that the spatial dimensions of these coefficients are equal to 3, which indicates the isotropic nature of the propagation of these states. Also, proposed two simplified methods for fast numerical determination of the ability of an organic scintillator, both single crystal and heterostructured, to separate ionizing radiation by pulse shape, namely by determining the inflection point and by approximating the fast component. It is proved theoretically and experimentally that the migration of triplet excitons can be limited by the geometric dimensions of the grain of a heterostructures scintillator, and it is shown that such a limitation leads to an increase in the recombination effect of triplet excitons due to their reflection from the boundaries of the grains if their initial number in the grain is sufficient. Chapter 1 presents the basic physical principles and mechanisms that describe the appearance and recombination of excited states in organic scintillators. The factors affecting the shape of the radioluminescence pulse are shown, the main scintillation parameters of the investigated substances and types of scintillators are given. A literature review was conducted on the ability to separate ionizing radiation based on the form of the scintillation pulse of the studied samples. The need to study the physical mechanisms of the appearance and recombination of triplet excitons in heterostructured scintillators is substantiated in connection with the possibility of variation in the sizes of the grains on the basis of which they are made. Chapter 2 describes and substantiates the methods of obtaining the studied samples of organic scintillators, the methods of conducting experimental studies taking into account the features related to the fulfillment of the tasks of the dissertation work, the mathematical principles of using the new methods proposed by the author for calculating the ability to separate ionizing radiation by pulse shape are given. In order to study the properties of the studied samples, we used installations for measuring the spectra of scintillation amplitudes, the kinetics of radioluminescence pulses, determining the FOM of the scintillator, as well as measuring the spectra of optical transmission and luminescence. Single crystal, polycrystalline, and composite samples based on stilbene, pterphenyl (both activated and pure) and anthracene, of different geometries (cylindrical or rectangular) were chosen as the studied samples, depending on the specifics of the research. The size of stilbene and p-terphenyl grains was varied from40 μmto 2.5 mm. Single-layer composite samples with different grain fractions in the range from 40 μm to 1 mm were produced. In chapter 3, the types and sizes of the samples that were used in the dissertation work are given, experimental data of both optical and ionizing radiation studies are given for single-layer and bulk samples. A preliminary discussion of the features that can be seen in the drawings and evaluated qualitatively was carried out. The technical features of the installations are discussed, which could lead to the occurrence of certain changes in the experimental data and are not related to the nature of the response of the studied samples. Chapter 4 analyzes experimental data obtained using ionizing radiation, such as scintillation amplitude spectra, radioluminescence pulse kinetics, and determination of the FOM of the studied samples. For the first time, the presence of isotropic propagation of triplet excitons in heterostructured samples was shown, and it was proved by calculating diffusion coefficients that the process of recombination of triplet states in heterostructured scintillators with a grain size of 1.7-2.2 mm takes place within one grain. Numerical methods show that the methods of quantitative assessment of the ability to separate ionizing radiation by pulse shape (both by the inflection point and by the approximation of the fast component) proposed by the author of the dissertation correlate with the physical properties of the studied scintillators and can be used in the future.