Regularities in positronium formation for atoms and molecules

Abstract

In an effort to aid the modelling of positron and positronium (Ps) transport in biological media we have compiled recent experimental results for the total Ps formation in positron scattering from atoms and molecules. A simple function was found to adequately describe the total Ps formation cross section for both atoms and molecules. The parameters of this function describe the magnitude and shape of the Ps formation cross section and are compared to physical characteristics of the target atoms and molecules. A general trend in the magnitude of the total Ps formation cross section is observed as a function of the target atom/molecule dipole polarisability. The functional form may enable quick estimation of the Ps cross section for molecules for which experimental measurements or theoretical estimates do not exist.

Positron scattering from pyridine

“…This is seen to hold in the case of the present measurements for pyridine. In this case, we see very good agreement with the recent scaling model, 37 but only marginal agreement with the phenomenological model used in the IAM-SCAR+I calculation. The IAM-SCAR+I calculation does not closely reproduce the energy dependence but does provide a reasonable estimate of the peak value of the Ps formation cross section and to some extent the peaked nature of the cross section as a function of energy.…”

“…• , 35 using the suggested shape parameters. 37 The general trend from experimental results for the Ps formation cross section, for both atoms and molecules, is a rapid turn-on of the cross section with a peak in the cross section at approximately 10 eV above the threshold. This is seen to hold in the case of the present measurements for pyridine.…”

Theoretical study of positron scattering by group 14 tetra hydrides: A quantum mechanical approach

“…Investigation on Ps formation is still in the nascent stage. Machacek et al investigated a range of atoms and molecules and found a linear relationship between Q ps and the dipole polarizability of the respective targets. However, from the work of Surdutovich et al we can infer that such relationships are not general in nature.…”

Benchmark for the Determination of the Positronium Formation Fraction in Interstellar Media

“…The threshold for the Ps formation process for any particular target is open for energies that are the Ps-binding energy (6.8eV) below the ionization process for the target. However, the cross section for Ps formation is significant only within a few 100eV of this threshold, which has been observed for a number of atomic and molecular targets (Machacek et al 2016). Conversely, direct annihilation of the positron with a target is negligible at energies where the Ps channel is open (σ ann /σ Ps <10 −5 ), and remains small even relative to elastic cross sections at room temperature.…”