Pylypchynets I.V. Photofission differentiation of shielded actinides 232Th, 235U, 238U and 239Pu – Qualifying scientific work as a manuscript.
Dissertation for the degree of Doctor of Philosophy in the field of knowledge E Natural Sciences, Mathematics and Statistics in the speciality E5 Physics and Astronomy, – Institute of Electronic Physics of the NAS of Ukraine, Uzhhorod, 2025.
The dissertation is devoted to complex simulation and experimental studies of the process of isotopic differentiation of shielded actinides 232Th, 235U, 238U, and 239Pu by their delayed gamma quanta from photofission products at the electron accelerator of the Institute of Electron Physics of the National Academy of Sciences of Ukraine - the M-30 microtron.
The introduction substantiates the relevance of the chosen topic, defines the goal and main objectives of the research, and outlines its object and subject. The scientific novelty of the results obtained is presented, and their practical significance is determined. Information about the applicant's publications on the topic of the dissertation and the testing of the main provisions of the work is provided. Information about the structure and scope of the dissertation is provided, and the author's personal contribution to the research is highlighted.
The first section analyses the methods of isotopic differentiation of actinides (passive, active), in particular with the use of photon or neutron stimulation. An analysis of the physical processes that occur when using the procedure of photofission differentiation of shielded actinides 232Th, 235U, 238U, and 239Pu to determine their isotopic composition and methods for describing them by theoretical models (Brosa, Seltzer-Berger, Beer-Lambert), which are the basis of modern nuclear Monte Carlo codes GEF, GEANT4 and computer programs NIST XCom, Phy-X/PSD. The tasks for ensuring photofission differentiation are formulated: selection of optimal pairs of fission products, determination of photon energy, optimisation of the detection scheme, and irradiation parameters.
The second section presents the results of modelling photofission processes using the GEF code. Mass distributions, energy characteristics of fragments, half-lives, and other parameters for selecting optimal pairs of fission products were calculated. It was established that the pair of 89Rb and 138Cs is the most suitable for differentiating both shielded and unshielded samples. The optimal energy of bremsstrahlung photons for stimulating photofission on the M-30 microtron was determined.
The third section describes the results of modelling and experiments on the stimulation of photofission of shielded samples. Using GEANT4, the characteristics of bremsstrahlung beams, the influence of structural elements of the activation scheme (Ta converter, B₄C filter, screen made of steel "12X18N10T") were simulated. The methods of conducting experimental studies to determine the characteristics of the components of bremsstrahlung beams and the influence of structural elements of the optimal irradiation scheme of shielded actinide samples on their characteristics are described in detail. Experimental data confirmed the reliability of the calculations. The own program “NPMA Reaction Yield” for estimating the yields of photonuclear reactions was made. It has been shown that background gamma rays from screen activation do not interfere with differentiation.
The fourth section discusses gamma-ray spectrometry. Using the Xcom and Phy-X/PSD programs, the optimal energy range for recording gamma quanta of 89Rb and 138Cs is established. The experimental setup based on the HPGe detector and the method for expanding the recording range to 3000 keV are described. A semi-empirical model of energy efficiency and the program “NPMU Detector efficiency” were developed, which were verified on another HPGe detector.
The fifth section describes in detail the method for determining the time dependences of the delayed gamma radiation from the fission products of actinides to ensure the process of their isotopic differentiation. The technical characteristics of the shielded actinide samples 232Th, 235U, 238U, and 239Pu are presented. As a result of the quantitative spectral analysis carried out on the basis of the SL-13 styloscope, the percentage content of the chemical constituent elements of the screen sample made of stainless steel “12X18H10T” was determined. The processes of stimulating the photofission reaction of shielded actinide samples on the M-30 microtron and detecting gamma rays from irradiated shielded actinide samples are described in detail. As a result of the conducted experimental studies, the time dependences of the intensity of gamma lines of light 89Rb and heavy 138Cs photofission products were established, which can be used for isotopic differentiation of shielded actinides 232Th, 235U, 238U, 239Pu.
