The Interaction of Monoenergetic Positrons with Solid Surfaces

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Abstract
Positrons, the antiparticles of electrons, were discovered by Anderson [1] in 1932. Their existence was experimentally established from observations of cloud chamber cosmic ray tracks that showed the presence of a positively charged particle with the same mass and numerical charge as that of the electron. About one year later positron sources became available as a result of the discovery of artificial radioactivity [2]. Positrons from such sources were found to have continuous energy distributions, ranging from hundreds to thousands of keV. No methods were then known for generating usable intensities of positrons with well-defined energies and, partly because of this, applied spectroscopies were limited. A relatively large area of study developed, however, which provided valuable information about the properties of matter through the measurement of positron annihilation characteristics in different media. Positrons may annihilate with electrons from a free state with the production of two coincident gamma rays. To conserve momentum the two photons must depart with approximate anticolinearity. Positronium, a quasi-atom bound state betweén an electron and a positron, may form prior to an annihilation event. Parapositronium, a singlist state in which the positron and electron spins are antiparallel, also undergoes annihilation with the creation of two gamma rays. Orthopositronium, the parallel spin triplet state, annihilates with the formation of three gamma rays. The orthopositronium state has a mean lifetime in vacuum of 142 ns; positrons in general, under most other conditions, annihilate at a faster rate. Measurements of annihilation rates, gamma ray energy variations, and deviations from anticolinearity for two-gamma annihilations provide information about variations in density, energy, and momentum for electrons in materials undergoing interaction with positrons. Positron annihilation spectroscopies are commonly used to map electronic band structure [3] and to study defects and voids in solids [4].