title: Measuring the three-photon self-annihilation fraction of positronium in and above thin films: a tool for determining film morphology.
abstract: A technique is described for evaluating the fraction of positrons F incident on thin film samples which form ortho-positronium and subsequently decay into three gamma photons. The method involves the measurement of two linked phenomena: the decrease in the number of annihilation events involving the emission of two gamma photons with approximately 511 keV in the germanium detector photopeak, and the increase in the number of decays into three gamma photons with energies in the range 395-505 keV. After the application of a number of systematic corrections to the raw data, these measurements allow the determination of the absolute value of F without the need for calibration on a sample with known F values, thereby avoiding problems with changing samples of different geometries measured under different conditions.
authors.affiliation: Department of Physics, University of Bath, Bath BA2 7AY, United Kingdom.
authors.forename: S
authors.initials: S
authors.lastname: Townrow
authors.forename: P G
authors.initials: PG
authors.lastname: Coleman