Electric Field and Temperature Dependence of Positron Annihilation in Argon Gas

Abstract

The annihilation rate of positrons in argon gas has been measured over the temperature range of 135 to 573 K with applied d.c. electric fields per unit gas density up to 35 V cm−1 amagat−1. An analysis of these data was carried out assuming that the annihilation rate and the momentum-transfer rate can both be approximated by functions of the form AνB over this velocity range. Results of the analysis indicate a strong velocity dependence for both the annihilation rate and the momentum-transfer rate. For velocities in atomic units [Formula: see text], the annihilation rate could be represented by va(ν)/D = (1.195 × 106) ν−0.480 s−1 amagat−1 over the velocity range: 0.03 to [Formula: see text]. The momentum-transfer rate was similarly determined as ν(ν)/D = (2.11 × 1011) ν−0.49 s−1 amagat−1. The statistical errors in these results are velocity dependent, with the largest being 16%. A modified effective range parameterization of the momentum-transfer cross section gives a zeroth-order scattering length of A0 = (−4.4 ± 0.5)a0 for positron–argon collisions.No evidence of a velocity dependence in the orthopositronium quenching rate at room temperature was detected.