A two-parameter measurement of the correlation of positron age with the momentum of the annihilating positron-electron pair

Abstract

The momentum spectrum of annihilating positron-electron pairs is normally analysed in terms of two or more momentum groups. If the medium is heterogeneous, or if it permits the formation of positronium, the annihilation time spectrum also becomes complex with as many as four resolvable exponential components. The interrelationship of these groups is difficult to establish because the usual measurements of time and momentum spectra are done with instruments of low absolute efficiency. We have made simultaneous measurements of time and energy spectra using a Ge(Li) spectrometer as a momentum analyser, obtaining an approximate hundredfold increase in data output at a cost of a factor of 3 in momentum resolution.

The momentum spectrum of the intermediate lifetime component in a plastic scintillator is shown to be very similar to that of the long-lived orthopositronium. Recent developments in fast timing and in high purity Ge detectors should improve both the quality and quantity of data from these techniques, making them feasible for routine studies of polymers, chemical solutions and metallic voids.