Positron impact ionisation phenomena

A magnetically guided beam of nearly-monoenergetic slow positrons has been used to study positron impact ionisation phenomena in gases. A novel hemispherical scattering cell incorporating an efficient ion extraction and detection system has been developed and has been utilised throughout this work. The energy spectra for the electrons ejected around 0° relative to the incident beam, following positron impact ionisation of Ar, have been measured by a time-of-flight method and a retarding electric field analyzer. The angular acceptance of the electron detection system has been estimated and used to compare the measured spectra with the double differential cross-sections calculated by Mandal et al (1986), Sil et al (1991) and Schultz and Reinhold (1990). The importance of the electron-capture-to-the-continuum process is discussed in this context and found to be minor at small forward angles, in contrast to the case of heavy positively charged projectiles. The apparatus was modified to produce a pulsed beam of slow positrons and utilised to measure in detail the total ionisation cross-section (Qt+) for a variety of atomic and molecular targets. For Ar, He and H2, Qt+ which includes contributions from Ps formation, has been subtracted from corresponding total cross-sections, in order to deduce the behaviour of the elastic scattering cross-section (Qel) in the vicinity of the Ps formation threshold (Eps). Here a small change in the gradient of Qel, has been found. The energy dependencies of the Qt+ for He, Ne and Ar, close to Eps have been interpreted in terms of threshold theory. In the case of Ar the outgoing Ps appears to be predominantly s-wave in character. For He and Ne the analysis suggests that the Ps contains significant contributions from a number of partial waves. In the case of O2, structure in Qt+ has been found, which is attributed to coupling between two inelastic channels, namely Ps formation and excitation to the Schuman-Runge continuum