The thesis is submitted to acquire the scientific degree of Doctor of Philosophy in the specialty 104 “Physics and Astronomy” – Institute of Electron Physics NAS Ukraine, Uzhhorod, 2023.
The dissertation is devoted to comprehensive experimental and theoretical studies of the structure of mass distributions of the photofission products of actinides 232 Th, 238 U, and 239 Pu stimulated by the bremsstrahlung radiation of the electron accelerator IEP of the National Academy of Sciences of Ukraine - microtron M-30, near the threshold of the second chance. The introduction substantiates the relevance of the dissertation's chosen topic and indicates the research's purpose, tasks, subject, and object. The scientific novelty of the obtained results and their practical significance are outlined. Publications on the dissertation topic are indicated, and data on the approbation of the work by the applicant are given. Information about the structure and scope of the dissertation work and the personal contribution of the dissertation worker is presented.
The first chapter, “Photofission of actinides: analysis of experimental data and research methods,” is devoted to the analysis of available literature on the yields of products (fragments), which are one of the main parameters characterizing the fission process of actinide nuclei and reflect the structure in their mass distributions. It has been established that one of the promising areas of study of this structure is photofission. It was established that the microtron, is the optimal for stimulating actinide photofission reactions. It has been established that when using computer modeling to obtain information about the characteristics of bremsstrahlung beams that stimulate photofission reactions of actinides, program code GEANT4 is optimal. The analysis of existing experimental data on the mass distributions of photofission products of individual actinides within the framework of the multi-Gaussian model is necessary to study their structure, especially around the second chance threshold.
In the second chapter, “Stimulation of the photodissociation reaction of actinides near the threshold of the second chance at the M-30 microtron,” an analysis of factors affecting the characteristics of bremsstrahlung beams formed at electron accelerators was carried out. The optimal parameters of the stimulation scheme for the photofission reaction of actinide samples on the M-30 microtron electron accelerator at the maximum energy of bremsstrahlung photons of 17.5 MeV were determined as a result of theoretical calculations using the GEANT4 toolkit. The contribution of accompanying nuclear reactions to the photofission of actinides 232 Th, 238 U, and 239 Pu was investigated in the result of the simulations. The second-chance threshold values of photofission of actinides 232Th, 238U, and 239Pu were established as a result of theoretical calculations using the TALYS and GEF software packages.
The third chapter, “Experimental studies of photofission product yields of actinides on the M-30 microtron,” describes in detail the process of conducting experimental studies of the photofission products of actinides 232 Th, 235 U and 238 U on the electron accelerator - the M-30 microtron; the computer programs “NPMA Prompt neutron yield version 1.0.2301” and “NPMU Prompt neutron parametrization version 1.0.2302” were created for routine calculations of neutron multiplicity - the total yield of instantaneous neutrons and their dependence on the fragment mass.
The fourth chapter, “Investigation of the structure of mass distributions of actinide photofission products,” presents the obtained experimental data of relative cumulative product yields and total yields of isobaric mass chains formed as a result of photofission of actinides 232Th, 238U, and 239Pu, at the maximum energy of bremsstrahlung radiation 17.5 MeV: for 232Th ‒ values of 23 products (17 obtained for the first time), from which total yields of 22 isobaric mass chains were found; for 238U - values for 29 products (14 obtained for the first time), from which total yields of 26 isobaric mass chains were found, and for 239Pu - values for 13 products (all obtained for the first time), from which total yields of 13 isobaric mass chains were found. The multi-Gaussian description model of the mass spectra of the photofission product yields of actinides was modified by introducing an additional Gaussian and was used to describe the mass spectra of photofission yield products of the actinides 232Th, 238U, and 23 Pu.
Keywords: photonuclear reactions, photofission, 232Th, 238U, 239Pu, yield of fission products, reaction cross section, activation method, microtron, production of bremsstrahlung, radiation, gamma spectrum, neutrons, GEANT4, GEF, TALYS
