Radiation Synthesis of High-Temperature Wide-Bandgap Ceramics.

This paper presents the results of ceramic synthesis in the field of a powerful flux of high-energy electrons on powder mixtures. The synthesis is carried out via the direct exposure of the radiation flux to a mixture with high speed (up to 10 g/s) and efficiency without the use of any methods or means for stimulation. These synthesis qualities provide the opportunity to optimize compositions and conditions in a short time while maintaining the purity of the ceramics. The possibility of synthesizing ceramics from powders of metal oxides and fluorides (MgF2, BaF2, WO3, Ga2O3, Al2O3, Y2O3, ZrO2, MgO) and complex compounds from their stoichiometric mixtures (Y3Al3O12, Y3AlxGa(5-x) O12, MgAl2O4, ZnAl2O4, MgWO4, ZnWO4, BaxMg(2-x) F4), including activators, is demonstrated. The ceramics synthesized in the field of high-energy electron flux have a structure and luminescence properties similar to those obtained by other methods, such as thermal methods. The results of studying the processes of energy transfer of the electron beam mixture, quantitative assessments of the distribution of absorbed energy, and the dissipation of this energy are presented. The optimal conditions for beam treatment of the mixture during synthesis are determined. It is shown that the efficiency of radiation synthesis of ceramics depends on the particle dispersion of the initial powders. Powders with particle sizes of 1-10 µm, uniform for the synthesis of ceramics of complex compositions, are optimal. A hypothesis is put forward that ionization processes, resulting in the radiolysis of particles and the exchange of elements in the ion-electron plasma, dominate in the formation of new structural phases during radiation synthesis.

Quality Control of Brachytherapy Equipment in Kazakhstan: Current Stage and Minimum Requirements.

OBJECTIVE: Brachytherapy is used in 17 radiotherapy facilities In Kazakhstan. Each institution has an individual quality control (QC) program in place to ensure the safe and accurate delivery of the treatment dose to the patient. The main objective of this paper is to explore current approaches to quality control of brachytherapy in Kazakhstan and reduce potential discrepancies in testing frequency and tolerance limits by identifying a set of basic quality control requirements. MATERIALS AND METHODS: A detailed brachytherapy quality control questionnaire was provided to 17 radiotherapy institutions for completion. A separate questionnaire was sent to two institutions associated with brachytherapy. Questions addressed safety aspects, radiation parameters, total time spent on quality control, and available imaging systems for dose determination. The results of the survey were compared with the recommendations set found in international brachytherapy quality control documents. RESULTS: The results of the questionnaires revealed significant differences in the frequency and methods of testing. For example, only two of the 17 centers have at least some kind of quality assurance program for brachytherapy treatment. Only five centers have equipment with the help of which dosimetric control can be performed, and only two centers have local medical physicists performing this control. One of the centers is checked quarterly, while the other is checked only once a year. In the remaining 15 centers, dosimetric control is performed by specialists who recharge the source without providing any document or protocol. There were also significant differences in the amount of time spent on quality control, mostly related to the variety of approaches to quality control and differences in the availability of resources. Almost all centers (15 of 17) rely only on inspections from the radionuclide source supplier and do not monitor the dosimetric and mechanical parameters of the facility at all. CONCLUSION: Based on the results of the survey and comparison with international recommendations, a set of basic requirements for brachytherapy quality control is needed.

Electron Beam-Assisted Synthesis of YAG:Ce Ceramics.

In this work, we present the results of the structure and luminescence properties of YAG:Ce (Y3Al5O12 doped with Ce3+ ions) ceramic samples. Their synthesis was carried out by sintering samples from the initial oxide powders under the powerful action of a high-energy electron beam with an energy of 1.4 MeV and a power density of 22-25 kW/cm2. The measured diffraction patterns of the synthesized ceramics are in good agreement with the standard for YAG. Luminescence characteristics at stationary/time-resolved regimes were studied. It is shown that under the influence of a high-power electron beam on a mixture of powders, it is possible to synthesize YAG:Ce luminescent ceramics with characteristics close to the well-known YAG:Ce phosphor ceramics obtained by traditional methods of solid-state synthesis. Thus, it has been demonstrated that the technology of radiation synthesis of luminescent ceramics is very promising.

The Optimization of Radiation Synthesis Modes for YAG:Ce Ceramics.

Synthesis in the radiation field is a promising direction for the development of materials transformation processes, especially those differing in melting temperature. It has been established that the synthesis of yttrium-aluminum ceramics from yttrium oxides and aluminum metals in the region of a powerful high-energy electron flux is realized in 1 s, without any manifestations that facilitate synthesis, with high productivity. It is assumed that the high rate and efficiency of synthesis are due to processes that are realized with the formation of radicals, short-lived defects formed during the decay of electronic excitations. This article presents descriptions of the energy-transferring processes of an electron stream with energies of 1.4, 2.0, and 2.5 MeV to the initial radiation (mixture) for the production of YAG:Ce ceramics. YAG:Ce (Y3Al5O12:Ce) ceramics samples in the field of electron flux of different energies and power densities were synthesized. The results of a study of the dependence of the morphology, crystal structure, and luminescence properties of the resulting ceramics on the synthesis modes, electron energy, and electron flux power are presented.

Express Synthesis of YAG:Ce Ceramics in the High-Energy Electrons Flow Field.

YAG:Ce ceramics by the direct action of an electron beam with 1.4 MeV energy were synthesized on a mixture of a stoichiometric composition of Y, Al, and Ce oxides without adding any substances to facilitate the process. The synthesis is realized in a time less than 1 s. The main structural phase of the obtained ceramics is YAG and YAP can be additional. The luminescence characteristics of the synthesized samples, the excitation, luminescence, decay time, and pulsed cathodoluminescence spectra, are similar to those known for YAG:Ce phosphors. The conversion efficiency of the excitation energy into the luminescence of the samples reaches 60-70% of those used for the manufacture of LED phosphors. The set of processes that determine the rate and efficiency of radiation synthesis differs from those occurring during thermal methods by the existence of a high degree of the initial compositions' ionization under the influence of a radiation flux and a high probability of the decay of electronic excitations into short-lived radiolysis products.