Position-sensitive detection of positronium for studying transfer and transfer ionization in e+–Ar collisions

Abstract

For detecting the positronium (Ps) formed in a positron–atom collision and emitted into a small solid angle, we utilize the impact-induced break-up into a positron–electron pair of which the electron subsequently starts an avalanche of secondary electrons. After exploratory measurements with a channel-electron multiplier (CEM) pivoted around the collision region, we employed a position-sensitive detector consisting of a stack of multi-channel plates (MCPs) and a two-dimensional delay-line anode. This experimental arrangement was used to study differential Ps formation in collision of 75 eV positrons with argon atoms. The two Ps-yielding reaction channels – Ar(e+, Ps)Ar+ (transfer) and Ar(e+, e−Ps)Ar++ (transfer ionization) – were distinguished by the flight time of the argon ion.