In Chapter I the existing literature on secondary emission is reviewed from the point of view of possible effects which might be observed with positrons. Secondary emission by electrons of energy ~ 2 KeV has been extensively studied, and work on positive ions has been reported. No work at all on positrons has been published, and in the energy region covered by the present research ( ~ 5 - 500 KeV) there has been no theoretical work and practically no experimental work on electrons. Some simple theoretical ideas concerning secondary emission by positrons are put forward in Chapter II. Secondary emission by ''Potential Ejection", which has been observed for positive ions but is impossible for electrons, is considered as a possible process for positrons; if such a process can occur for positrons it would only be predominent for particles with an energy ~ 1eV. Some known differences in the behaviour of positrons and electrons are then discussed, from the point of view of any effect these might have on the secondary emission of the particles. It is concluded that no large differences are to be expected in the energy range which can be investigated by experiments which are feasible at present. The second part of Chapter II outlines the basic principle of the experiment, which was to compare the secondary emission by positrons and electrons of the same energy under identical conditions of geometry and target surface, A ?-spectrometer and a copper 64 source, which emitted positrons and electrons, provided focused beams of particles. The secondary electrons were detected with an Allen type electron multiplier, and the number of primary particles was counted with a thin windowed Geiger counter. Chapter III describes the electron multiplier and the associated electronics, and discusses briefly some measurements on its performance. In Chapter IV a preliminary experiment on secondary emission without using the spectrometer is described, which confirmed that there were no large differences in secondary emission by electrons and positrons at high energies. Chapter V describes the ?-spectrometer and the rotating coil method used to measure the magnetic field. Chapter VI describes the main experiments to determine, the relative secondary emission of electrons to that of positrons. Some absolute measurements were also made. It was found that above ~ 50 KeV was about 1.04; as the energy was reduced began to rise rapidly, exceeding 2 below 10 KeV. As such large values of were not expected on any existing theory, a very thorough investigation was carried out to establish that the results were not due to any instrumental errors. The final results for platinum, after all the corrections, none of which was very large, had been applied, were as follows;- Energy KeV 6.5 10 20 50-500 m 3.25 + 1.15 1.7 + 0.3 1.2 + 0.1 1.04. + 0.025 A copper-beryllium target gave similar results. In the final Chapter some tentative explanations for the results are put forward, For energies greater than 100 KeV a semi- quantitative theory is given. It was assumed that the secondary yield was proportional to the energy loss of the particles, and that a primary could produce a secondary as it entered the target, or as it left the target, if it did so as a result of scattering within the target. Using recent data on the energy loss of positrons and electrons, and the results of Seliger, who found that electrons were backscattered by ~30% more than positrons, values of m of the right order of magnitude are predicted. Below ~100 KeV the simple theory breaks down, but other factors which become important at lower energies enable this theory to be extended, so that it can possibly account for the results down to 20 KeV. This extended theory does not seem adequate to explain the large values of m observed below 20 KeV. Some very tentative ideas are put forward concerning processes by which positrons and electrons might liberate secondary electrons, which suggest qualitatively that electrons may be favoured. It is concluded that more experimental and theoretical work is required before the results at low energies can be understood
