Papers by Author: Sergey V. Stepanov

Abstract: The thickness of the near-surface melting layer of ice is estimated by measuring the melting kinetics of the ice spherical balls, temperature readings of the ice surface, and the bulk ice temperature. The temperature monitoring system is programmed on an Arduino-based controller with thermocouple sensors and non-linear semiconducting thermistors. By the melting front, we consider the near-surface layer where multiple types of structural defects accumulate (owing to the latent heat absorption during melting). The melting front contains "point" defects (e.g. vacancies, interstitials) and macroscopic defects, such as dislocations, cracks, stress and strain fields, and vacancy pores. We also measured positron annihilation lifetime spectra in ice and water at temperatures above -8 °C and up to room temperature. We demonstrate that efficient accumulation of the structural defects in melting ice begins only when some temperature overheating above the melting temperature (+0.1...+0.4 °C) is applied and does not occur at negative temperatures.

Abstract: Here we discuss the e⁺ fate from its injection into a liquid till its annihilation, namely: 1) energy losses and track structure of fast positrons. Parameters of the terminal e⁺ blob; 2) electric interaction between blob constituents; 3) picosecond stage: hot (epithermal), quasi-free (dry), solvated (hydrated) e^- and e⁺, capture of hot particles by a scavenger; 4) positronium formation in condensed matter: the Ore model, the diffusion-recombination model, formation of quasi-free Ps and Ps in a bubble, a “non-point” positronium; 5) application of an external magnetic field, mixing of ortho- and para-Ps states, R(H)-parameter, Ps contactdensity; 6) diffusion-controlled reactions involving Ps (oxidation and ortho-para conversion) at nanosecond stage, temperature dependence of the reaction rate constants.

Abstract: Experiments on investigation of the radiation defects produced as a result of high energy proton irradiation of single crystal Si wafers are carried out. Parameters of the proton irradiation facility are presented. It is shown that the most efficient radiation defect formation correlates with the position of the Bragg peak of ionization losses. LT spectra were measured just after irradiation and then after keeping Si samples during 3 months of at room T. We did not observe any variation of the number density of the defects, except for the 7th wafer, where most part of protons was stopped. An efficient annealing of the vacancy-type defects starts at temperatures slightly lower than 100 °C (during 10 min). Annealing at about 700 °C leads to recovering of the monoexponrntial shape of the LT spectra.

Abstract: Taking liquid water as an example, we have considered energetics of the positronium formation. It is shown that quasi-free Ps as well as the Ps localized in a bubble cannot decay into hydrated electron and positron. The most probable value of the positron work function in water, V₀⁺, is 1.5…2 eV. Ps formation from the hydrated electron and positron is energetically possible. By the end of thermalization major fraction of positrons escapes the blob and hydrates outside. The low-mobile е⁺_aq has no time to diffuse back and form Ps with intrablob e-. These е⁺_aq mostly annihilate as “free positrons”. Positrons, which are thermalized within the blob, recombine with the quasi-free intrablob electrons and form Ps.

Abstract: The terminal part of the e₊ track (the positron blob) is formed during ionization slowing down and subsequent ion-electron recombinations produced by a positron. It releases up to 1 keV of energy, which is converted into heat within few picoseconds. If a bulk temperature of a medium is below, but close enough to its melting point, some region of a substance may melt, yielding a peculiar temperature dependence of the lifetime (LT) spectra. We have estimated properties of the molten region with a help of macroscopic heat con- duction equation and suggested a model describing temperature dependence of the ortho- positronium lifetime in frozen methanol, ethanol, butanol and water close to their melting points.

Abstract: In application to positron annihilation spectroscopy, Ps atom is considered not as a point particle, but as a finite size e⁺e^- pair localized in a bubble-like state in a medium. It is shown that during transition from quasifree to localized state, variation of the internal Coulombic e⁺-e^- attraction energy (several eV) plays an important role, which may govern Ps localization process.

Abstract: Ps and radiolytic hydrogen yields anticorrelate in saturated hydrocarbons when molecular structure changes from a normal to a cyclic form. This fact is explained by much higher mobility of primary radical-cations in cyclic hydrocarbons than in normal ones.

Abstract: A self consistent interpretation of the positron lifetime experiments in water at different temperatures (2-93 0C) and magnetic fields (H ≤ 2 T) is given. By using the blob model of Ps for-mation we have obtained the contact density in the positronium atom in water, which is in agree-ment with the previous measurements.

Abstract: A unified mechanism describing formation of Ps and Mu atoms in condensed matter, radiolytic hydrogen in water and products of Auger ionization in Moessbauer atoms is presented. All these processes are similar to the Ps formation mechanism in a condensed molecular milieu.

Abstract: A new strategy for the treatment of positron annihilation lifetime (PAL) spectra in viscous liquids is proposed, enabling to extract values of the Ps reaction rate constants with intratrack radiolytic products as well as parameters of the free volume distribution in viscous media.