Positronium Formation Dynamics in Radiolytic Tracks:  A Computer Simulation Study

Abstract

Positron track structures were simulated by stochastic modeling of the collision-by-collision slowing down of positrons in n-hexane. The details of the inelastic scattering of positrons and of electrons produced in ionizing collisions were taken into account until the particle energies had degraded to less than 25 eV. Further slowing down to thermal energy was assumed to result in a spherically symmetric thermalization distribution around the position where the particle energy becomes less than 25 eV. The dynamics of positronium formation in competition with electron-cation recombination and positron annihilation was followed by a simulation of the diffusive motion of the charged species in each other's Coulomb field. Positronium formation was found to involve reaction of the thermalized positron with electrons produced along the last few hundred nm of its track-end, corresponding to several keV of energy attenuation. Positronium formation occurs mainly during the first few tens of picoseconds after positron implantation and extends to hundreds of picoseconds. Simulation reproduces the experimental positron lifetime spectra reported in the literature. The experimentally reported electric-field dependence of the ortho-positronium yield in liquid n-hexane could be reproduced with an exponentially decaying positron thermalization distribution with a mean distance of 50 nm, while a Gaussian positron thermalization distribution was found to be inappropriate.

Physical aging of polycarbonate far below the glass transition temperature: Evidence for the diffusion mechanism

“…In addition to the information on free volume hole size, a direct relation was proposed in the past between the intensity of o-Ps ͑I o-Ps ͒, namely, the probability of Ps formation and the free volume hole density. 21 Although the I o-Ps can be affected by a number of other factors, such as chemical composition of the polymer, 22 source strength and charging effects due to prolonged irradiation, [23][24][25] electric fields, 26,27 and visible light, 28 the proportionality between I o-Ps and the concentration of free volume holes can be still assumed, pro-vided that all the mentioned effects are eliminated. This is the case of this study, where the chemical structure of PC does not change with time and the other factors affecting I o-Ps are not present.…”

Amorphous-amorphous transition in glassy polymers subjected to cold rolling studied by means of positron annihilation lifetime spectroscopy

“…16 The intensity of o-Ps ͑I o-Ps ͒, which is proportional to the probability of positronium formation, has been related in the past to the density of free-volume holes in the polymer. 17 However, it was found that I o-Ps is also affected by the chemical composition of the polymer, 18 the source strength and electrostatic charging due to prolonged irradiation, [19][20][21] electric fields 22,23 and visible light. 24 Although the interpretation of I o-Ps as a measure for free volume has been questioned, 25 we believe that, provided that all the previously mentioned effects are carefully eliminated, or at least minimized, a direct relation between I o-Ps and the hole density exists.…”

Positron Chemistry in Polymers

“…Nevertheless, we are still far away from the complete understanding of positron chemistry in polymers; (1) The variation of the Ps yield can be explained in many cases in terms of the inhibition and anti-inhibition effects, but quantitative prediction of the Ps yield in a given system is not possible at present [86,87]. (2) Eq.…”