ИСТИНА

Application of the positrons and excess electrons as probes for experimental investigation of radiolysis of molecular media inevitably needs development of theoretical description of the processes involved during all the time of experiment starting from an implantation of the particle in matter. I. Ionization slowing down of the energized projectile is well described by known formulae of Bethe while at low energies (including thermalization stage) many questions remain. A rather general attempt to calculate vibrational and dielectric energy losses for straight-line and random motions of a charged particle was made by Tachiya and Sano (1978). Despite the coincidence between their results and those of Froehlich and Platzman (1953), their formula disagrees with the expression for the dielectric friction force for the case of a slow moving particle obtained by Zwanzig (1963). So we have reexamined calculation of the energy loss rate and obtained a correct formula [1] which reproduces in both limiting cases of fast and slow straight-line motion of the charged particle previous results of Froehlich and Platzman (1953) and Zwanzig (1963). Further progress in analytical calculation of the energy loss rate is possible for the diffusion motion of the particle with gaussian charge distribution [1]. Positron spectroscopy helps in verification of the theoretical predictions by means of studies of Ps formation in chemically equivalent, but isotopically different systems (for example, light and heavy water) [2]. II. Until now a puzzling difference of the yields of the solvated electrons, Ge , radiolytic hydrogen, GH2 , and positronium atom, PPs vs. concentration cS of the electron scavenger in aqueous solutions is debating. All these species have same common precursor, intratrack electron, so one may expect that all these quantities should behave in the same way vs. cS, but e--yield decreases exponentially: , obeys so-called “cube-root-low”: and Ps yield decreases as (and sometimes even increases!). All these phenomena get natural explanation in the frameworks of the diffusion-recombination model [3] taking following aspects into account: 1) both quasi-free and hydrated electrons are precursors of the radiolytic hydrogen and Ps atom; 2) there is a difference in onsets of the track electron capture reaction by scavenger (hot or epithermal electrons can be scavenged by S) and of the ion-electron recombination, solvation and Ps formation reactions (only thermalized electrons can participate there); 3) if e- is weekly bound in S-, it may serve as a Ps precursor (it is so called the anti-inhibition effect). Three comments are in order: a) There are at least two models, which utilize the recombination mechanism of Ps formation (namely, that Ps formation proceeds via combination of the thermalized positron with one of the knocked out electrons in the terminal part of the e+ track): the Onsager model (or the spur model) and the diffusion-recombination model (DRM or the blob model). The first one (in contrast to the second) neglects multiparticle track effects, but includes screened Culombic interaction within the e+-e- pair. However, both models predict the decrease of the Ps yield in electric field (just because it moves the positron and track electrons in different directions). Interesting that at low field limit the models give same expression for the Ps yield, but the meaning of parameters involved (the Onsager radius and initial e+-e- separation, r0) in case of DRM are completely different [4]. The Onsager expression has only one fitting parameter, r0 , which is not enough for fitting absolute values of the Ps yield and their variation vs. field; b) The “cube-root-low” for may be obtained analytically in the frameworks of 1-radical-1-scaveger diffusion model taking into account the fluctuation kinetics at small (it means that recombination initially proceeds within the spatial spots, which occasionally contain no S inside); c) Anti-inhibition effect on Ps formation may be observed in water-acetone solutions. Acetone molecule weekly bounds track electrons. So it efficiently suppresses ion-electron recombination but serves as a Ps precursor. Small addition of acetone in hexane even slightly increases the Ps yield. III. Following Ferrel (1957) it is commonly accepted that the final Ps state in a liquid is the Ps-in-the-bubble. Growth of the Ps bubble is due to an exchange interaction between the electron composing Ps and nearest molecular electrons. In contrast to the solvated electron, no polarization interaction with environment is involved (Ps is electrically neutral). For description of the Ps bubble growth and calculation of the dissipated energy we used the Navier-Stockes equation [5]. To avoid uncertainties related with application of the macroscopic hydrodynamic approach to the nanoscale system we tried molecular dynamics (NPT-ensemble) for simulation the growth of the bubble (Ps atom was considered as a point quantum particle in a spherically symmetric potential profile, proportional to the local molecular density). However straightforward application of the standard algorithms are not possible in this essentially nonequilibrium case because all of them are developed for calculations of equilibrium parameters of the system and by special means decrease V and T fluctuations (actually they treat growth of the Ps bubble as a fluctuation in the system and artificially suppress it). [1] Stepanov S.V. “Energy losses of subexcitation charged particles in polar media”. Radiat. Phys. Chem., 46(1), 29-37 (1995); Stepanov S.V., Byakov V.M. “Energy loss rate and thermalization of subionizing positrons and electrons”, NIM B 221, 235-238 (2004) [2] Stepanov S.V., Byakov V.M. “Slowing down and track structure of positrons and muons in ordinary and heavy water”, Nukleonika, 42(1), 245-252 (1997) [3] Stepanov S.V., Byakov V.M. “On the Mechanism of Formation of Intratrack Yields of Water Radiolysis Products upon Irradiation with Fast Electrons and Positrons” High Energy Chem. 39(3), 131-136, 2005; High Energy Chem. 39(5), 282-290, 2005 (Translated from Khimiya Vysokikh Energii). [4] Stepanov S.V., Byakov V.M. “Electric field effect on Ps formation in liquids”, JCP 116(4), 6178-6195 (2002); Stepanov S.V., Byakov V.M., Kobayashi Y. “Ps Formation in Molecular Media: Effect of the External Electric Field”, Phys. Rev. B 72(5), 054205 (2005) [5] Stepanov S.V., Mikhin K.V., Zvezhinskii D.S., Byakov V.M. “Energy Dissipation and Ps Bubble Growth in Liquids”, Radiat. Phys. Chem., 76(2), 275-279 (2007)