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Positron techniques

The positron was discovered by Anderson in 1932, shortly after its existence had been predicted by Dirac. However, condensed matter studies with positrons did not begin until 1949, when DeBenedetti et al. discovered that the two gamma-rays produced by the annihilation of a positron with an electron in a solid were not perfectly collinear. This deviation from collinearity is due to the effect of the momentum of the electron. By their nature, positrons are very sensitive to the presence of open-volume defects, and may be used to determine both their type and concentration. In addition, due to the diverse variety of processes through which they may interact with the surface region of a solid, positrons can be valuable tools in the field of surface physics.

However, it soon became apparent that the potential of positrons as a probe of condensed matter was severely limited by the broad energy distribution of positrons produced by beta-decay. It wasn't until the 1970's that technology had advanced sufficiently to allow the production of relatively intense, monoenergetic beams of slow positrons. The use of slow positron beams allows the study of surfaces, as well the investigation of defects in a depth-profiled mode. Slow positron beams are currently used in variations on standard condensed matter analysis techniques, as well as in techniques which are unique to the positron field. Some of the many probes currently in use which exploit the properties of positrons include: Low-Energy Positron Diffraction (LEPD), Positron annihilation-induced Auger-Electron Spectroscopy (PAES), REemitted-Positron Energy-Loss Spectroscopy (REPELS), Angular Correlation of Annihilation Radiation (ACAR), Doppler Broadening Spectroscopy (DBS), Positron Annihilation Lifetime Spectroscopy (PALS), and Reemitted-Positron or -Positronium Spectroscopy (RPS).