Victor Yavna

Cascade decays of single vacancies in K to O shells of the gold atom are simulated by straightforward construction and analysis of the cascade decay trees. Portions of energies (a) absorbed by initially ionized Au atom, (b) reemitted by cascade-produced electrons, and (c) reemitted by cascade-produced photons are calculated with taking into account the shake-off processes caused by the change of atomic core potential during cascade transitions. Au atom absorbs noticeable amount of energy acquired in photoionization act only when ionized in its N and O shells, 33%–45% and 41%–54%, respectively. Except for the case of K-ionization, most of the energy is reemitted by cascade electrons: 57%–64%, 77%–85%, 55%–67%, and 46%–59% during the decay of vacancies in L, M, N, and O shells. Energies carried away by the cascade photons are only important for initial vacancies in K (89%) and L (29%–39%) shells. Inclusion of the shake-off branches in the decay trees increase substantially their complexity, however it leads to comparatively small corrections to calculated portions of energy redistributed through channels a, b, and c. This supports earlier qualitative prediction of small possible effect of the SO processes on the cascades in heavy atoms.

Final ion yields and mean final ion charges upon the cascade decay of single vacancies in the K, L, M, and N shells of atomic silver are calculated using the method of construction and analysis of the decay trees with accounting for the shake-offs caused by the change of atomic core potential due to cascade transitions. The shake-off effect on the final ion yields is found to be comparatively small, the largest increase of the mean final ion charge being 0.20e in the case of initial 2s vacancy. Total semi-relativistic Pauli–Fock energies of cascade-produced ionic configurations were used to calculate cascade transition energies, which made it possible to accurately account for the closing of some non-radiative transitions in multivacancy cascade configurations. For the transitions with overlapping multiplets of initial and final ionic configurations, only energetically allowed transitions between the multiplet components were counted when calculating mean transition energies and transitions branching ratios. Comparison of calculated charge spectra with those reported earlier by other authors demonstrated crucial importance of accurate calculation of transition energies and accounting for multiplet splitting in simulation of cascades in heavy atoms.

Mechanisms of absorption and redistribution of energy in liquid nitrogen irradiated by 0.027–17.4 keV photons are studied by Monte Carlo simulation, accurately accounting for the cascade relaxation of inner-shell vacancies and tracking all secondary electrons and photons. Densities of energy absorbed near the sites of initial photoionization due to inelastic processes caused by secondary electrons and photons and their spectra are calculated. The absorption of energy by the sample’s atoms that underwent initial ionization by incident photons is only important on a rather narrow incident photon energy interval in the UV range. With growing incident photon energy, the energy absorbed in primary photoionization acts decreases and becomes negligible in comparison with energy absorbed via other mechanisms. The main mechanism of energy transfer to the sample (average of 56% over the whole incident photon energy interval) is through secondary electrons causing ionization and excitation of atoms of the medium. The second most important energy absorption mechanism (22%) is by low-energy photons that do not produce ionization or excitation of the sample’s atoms. Low-energy secondary electrons are responsible for the absorption of 14% of energy, and 7% of the absorbed energy is transferred to the medium through photoionization by secondary photons. Low-energy electrons are produced in great quantities in liquid nitrogen after each primary photoionization act, and their number grows linearly against incident photon energy. The results of this study are in line with earlier reported results on energy absorption in photon-irradiated disordered solid neon, amorphous carbon, and water.

Final ion charge spectra produced by the cascade de-excitations of 1s- to 5p _3/2-vacancies of the gold atom are calculated by direct construction and analysis of the cascade decay trees. The energies of multivacancy configurations arising in the course of the cascade development, and the partial widths of radiation and radiationless cascade transitions are calculated using the Pauli–Fock approximation. The energies of the cascade transitions are calculated as the differences of total Pauli–Fock energies of initial and final ionic configurations which allowed excluding energy-forbidden radiationless transitions in numerous multivacancy configurations. Partial widths of transitions are expressed in the form that allowed accounting for the effect of electron subshell populations on the transition widths. The partial widths of the transitions between the states of the overlapping initial and final state multiplets are corrected so as to exclude energy-forbidden transitions between the multiplet states. It is demonstrated that accurate accounting for possible forbiddance of transitions between cascade configurations and the exclusion of energy-forbidden term-to-term transitions between the multiplets’ states are crucial in deep-initial-vacancy cascade simulations.

A ground-penetrating radar (GPR) technology was developed to study the process of water drainage in sand layers with an insignificant concentration of dusty and clayey particles when moistened from above. The technology includes a method of calibration of the GPR equipment, algorithms for processing the GPR information, and their software implementation. The technology was used to process the results of laboratory GPR measurements obtained during draining of water through sand layers from different quarries for 100 h. The absolute values and the changes in the refractive index and specific conductivity near the sand layer upper boundary and on average over the layer depth were calculated. The results show that the developed technology makes it possible to determine electrophysical properties with an accuracy of up to 10%. The developed method for calculating relative reflectivity and its derivative with respect to the depth of the layer made it possible to visualize the information contained in the radargrams on the distribution of water near the surface and deep in the sand layers. The application of the method makes it possible to quantitatively estimate the moisture content near the upper boundary of the layer and the depth of the location of the most moistened areas of the layer depending on the duration of water drainage.

This study presents data on the IR spectra of kaolinite at a moisture of 26% and after drying. The preparation of moistened samples was made with distilled water, and the solutions of NaCl with limiting and 50% of limiting concentrations at a temperature of 21 °C. To experimentally evaluate the effect of the interaction between liquid water and sorbing basal surfaces of kaolinite on the profile of the IR spectrum bands, the spectra of distilled water and NaCl solutions were additionally studied. Comparison of the band profiles in the wavenumber range of 2750–4000 cm⁻¹ for kaolinite samples allows the conclusion that, when going from distilled water to the most saline water, the adsorption effect is inferior to the effect of solvation, and it decreases with increasing salinity. During drying of the samples, an increase in the peak intensities of the bands in the wavenumber range of 3550–3750 cm⁻¹ is observed. Observed spectral variations are qualitatively interpreted by the results of calculations performed by the DFT method with the XLYP exchange–correlation potential. The presented data can be used to test theoretical approximations and computer models for calculating the structure and properties of moist minerals and salt adsorption mechanisms.

The processes in 2–128 nm iron nanoparticles (NP) under 10–800 Ry photon irradiation are studied by detailed Monte Carlo simulations, accounting for the cascade decays of inner-shell vacancies. The cascade decay of vacancies is a crucial factor in the theoretical description of the effect of ionizing radiation on matter. The NPs re-emit most of the energy of an absorbed photon into the surrounding medium with secondary electrons and photons emitted by NP. At incident photon energies under the Fe1s-ionization threshold, most of the energy is carried away by electrons; when photon energy is above the Fe1s-threshold, electrons and photons transfer the energy to the surrounding medium. In the case of large nanoparticles, many low-energy electrons are emitted by NP, over a hundred per one initial photon. Although the energy carried away by them is small, they contribute to forming a high positive charge on the NP, which results in great electric fields near the NP surface. The significance of the NP electric field effect on the DNA repair cycle is discussed.