Day, David John; (1993) An electrostatic positron beam and its use in an experimental investigation of the first excited state of positronium. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

The purely leptonic character of positronium, the bound state of an electron and positron, presents the opportunity to test quantum electrodynamics in the absence of quantum chromodynamical effects. In recent years persistent discrepancies have arisen between experiment and theory for values of the decay rate of the l3S1 state, thus far unreconciled, and therefore investigations of other positronium states desirable. The main body of this work has involved a study of the first excited state of this system, known as Ps*. Its production by positron impact on solid surfaces was examined, and an experiment to measure the radiative lifetime of the 23P1-13S1 transition performed. In order to carry out this work it was necessary to design an electrostatic positron beam, operable at low transport energies, ∼ 100eV, with reasonable efficiency and beam quality. Measurements of the Ps* yield have supported recent disparate measurements by providing a link between, and possible re-interpretation of, some aspects of these studies. Peak yields of several percent were observed from a variety of targets, all without prior surface treatment. Studies of positron re-emission from the same untreated targets were also made in order to estimate the energy regime of the Ps* formed and therefore shed some additional light on the formation process. An attempt to make the first measurement of a positronium radiative lifetime was made, with many unforeseen difficulties arising. This work has identified the main problems that may be encountered in an experiment of this kind, and steps were taken to minimise their impact, to the point that the transition was isolated. Such steps however have precluded a meaningful analysis of the lifetime by virtue of their effects on the resolution of the measuring system, however as a result of the experience gained through this work the prospects for future successful experiments are much improved.

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