Vsevolod M. Byakov

Electric field effect on positronium formation in liquids

The comparison of different models (the Ore, spur, and blob models) of positronium (Ps) formation is presented. Because in molecular media Ps is formed in the terminal positron blob and not in an ordinary spur, the application of the blob model seems to be the most adequate. We extend this model for consideration of the Ps formation in the presence of an external electric field (<100 kV/cm). In the simplified limiting case, this approach provides a formula similar to the Onsager one for the geminate recombination probability. The influence of ion–electron recombination and other intrablob processes on Ps formation is taken into account. The role of quasifree positronium in the Ps formation process is discussed.

The nature of the precursors of radiolytic molecular hydrogen in water, and the mechanism of positronium formation in liquids

Abstract The dependence of H2 yields on the concentration of certain solutes indicates that molecular hydrogen is formed via recombination of its precursors in spurs. The order of efficiency of solutes in decreasing the formation of H2 does not correlate with the rate constants of their reactions with eaq− or H. Due to their diffusive motion at energies ⩽102eV and to their comparatively long lifetimes in the subvibrational energy range at the end of their tracks, positrons may form positronium by a reaction with (epi) thermal electrons. The order of the reactivity of solutes towards the precursors of positronium is the same as that towards precursors of the molecular hydrogen yield and is in agreement with the order of their relative rate constants towards the non-solvated electron. It seems that the non-solvated electron takes part in the formation of positronium and of radiolytic H2.

Investigation of nonsolvated electron reactions by means of the positron annihilation method

Abstract The validity of a model developed by us in which the positronium precursor is mainly the nonsolvated electron makes it possible to study the reactivity of nonsolvated electrons using data on positronium formation probabilities in solutions of electron acceptors. The rate constants thus obtained agree in most cases with those determined with picosecond pulse radiolysis techniques.

COMMON FEATURES IN THE FORMATION OF PS, MU, RADIOLYTIC HYDROGEN AND SOLVATED ELECTRONS IN AQUEOUS SOLUTIONS

A unified quantitative model of the formation of positronium, muonium and radiolytic hydrogen is proposed. It includes a description of the slowing down and thermalization stages, which are responsible for the initial spatial distribution of primary intratrack species. Possible reactions of “hot” electrons and positrons are discussed. Formation of Ps (Mu) and radiolytic hydrogen is described in terms of diffusion-recombination reactions between thermalized, but yet not solvated positron (muon), radical-cations and track electrons. It is suggested that this intratrack quasi-free electron released from a molecule by the primary ionizing particle is a common precursor of all radiolytic products.

What do we learn from the electric field effect on Ps formation in liquids

Abstract The diffusion–recombination model has been extended for consideration of positronium formation in the presence of an external electric field. The important basic ideas and main issues are discussed. It is shown that because of electrical neutrality of the terminal positron blob, e + , easily escapes from the blob during thermalization while intratrack electrons are kept in the blob by an electric field of the primary ions. Ion–electron recombination does not appear to significantly compete with Ps formation.

Microscopic surface tension of liquids with curved free boundary studied by positron annihilation

Abstract A method to determine the microscopic surface tension of nanobubbles is presented, based on the combination of positron lifetime and ACAR spectroscopies.

MODELING OF PRIMARY CHEMICAL PROCESSES OF WATER RADIOLYSIS AND SIMULATION BY SPUR DIFFUSION MODEL

Abstract Chlorite ion is selected as a solute reactive toward e aq − and OH. Experimentally observed G (ClO 2 ) is reproduced by the conventional spur diffusion model in even concentrated region without considering any reactions within 10 −12 s. For the solutes reactive toward the quasi-free electron, an extended spur diffusion model including in sub-picosecond time range is proposed by taking account of the experimental data of picosecond pulse radiolysis, femtosecond laser photolysis and positronium results are compared with corresponding experimental data.

Positronium formation in aqueous solutions of surfactant substances

Abstract Investigating the behaviour of positronium atoms in the solutions of surface actant (SA) substances Jean and Ache [Jean Y.-C. and Ache H. J. (1978) J. Am. Chem. Soc. 100 , 984] have observed a drastic decrease in the intensity I ortho of long-lived ortho-positronium component of Ps atoms when the concentration of SA molecules attains the critical value at which the micelle formation begins. This decrease is up to 1/3 from initial I ortho value and takes place irrespectively to the sign of electric charge of arising micelles. The aim of this communication is to give a physical interpretation and quantitative description of the discussed effect based on the model of Ps formation in liquids by combination of positron with one of the quasifree but yet nonsolvated electrons knocked out by positron during its slowing down process. It is shown that the decrease of I ortho and positronium formation probability P Ps is a result of the space segregation of quasifree electrons (e - ) and positron at the end part of its track containing micelles. According to the sign of its electric charge the micelles are able to accept electrons or positron, making their interaction weaker and thus preventing the Ps formation. The calculations show that the decrease of P Ps (and I ortho ) may be about several tens of per cent in accordance with available experimental data.

Antihydrogen Physics: gravitation and spectroscopy in AEgIS

AEgIS (Antimatter experiment: gravity, interferometry, spectroscopy) is an experiment approved by CERN with the goal of studying antihydrogen physics. In AEgIS, antihydrogen will be produced by charge exchange reactions of cold antiprotons with positronium atoms excited in a Rydberg state (n > 20). In the first phase of the experiment, controlled acceleration by an electric field gradient (Stark effect) and subsequent measurement of free fall in a Moire deflectometer will allow a test of the weak equivalence principle. In a second phase, the antihydrogen will be slowed, confined, and laser-cooled to perform CPT studies and detailed spectroscopy. In the present work, after a general description of the experiment, the present status of advancement will be reviewed, with special attention to the production and excitation of positronium atoms.

The nanoheterogeneous structure of aqueous solutions of n-propanol

The results of positron spectroscopy and molecular light scattering studies, the data on radiationchemical yields and optical absorption spectra of solvated electrons formed under the action of ionizing radiation, and the data on the concentration dependences of viscosity, adiabatic compressibility, the velocity of sound, and partial molar volume were used to determine the structure of water-n-propanol mixtures. The conclusion was drawn that the insertion of alcohol molecules into water network voids over the range of alcohol mole fractions 0 < x2 < 0.05 strengthened the structure of water. A further increase in the concentration of the alcohol caused the destruction of the aqueous component and solution homogenization. At 0.01 < x2 < 0.3, mixtures resembled an emulsion of alcohol “nanodrops” suspended in water. At 0.3 < x2 < 0.9, the system again became homogeneous. Lastly, when water was added to pure n-propanol (1 > x2 > 0.9), its molecules combined into nanodrops.