Positronium formation dynamics in radiolytic tracks: A computer simulation study

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.