Positron surface detection

The cosmic radiation incident on the earth is generated in our galaxy. It is effectively absorbed in the atmosphere, and the flux density is reduced from about 20 cm s to about 1 cm" s at the surface of the earth. By interaction with the atoms and the molecules in the atmosphere showers of elementary particles are produced, making up the secondary cosmic radiation. Positrons, muons, several kinds of mesons and baryons were first detected in the secondary cosmic radiation. Furthermore, nuclear reactions induced by secondary cosmic radiation lead to the production of cosmogenic radionuchdes, such as T and (section 1.2). 

The limited application of positron beams is explained by the fact that Doppler-broadening spectroscopy is the only detection method that works with beams without any difficulty. Although angular correlation measurements are possible with positron beams, it requires a very high efficiency beamline. Only very few beams fulfill the requirements of ACAR measurements, but their results are very impressive. The combination of low-energy positron beams and 2D-ACAR measurements is the only possibility to gain direct information on the momentum distribution of electrons at the surface of metals and semiconductors (Chen et al. 1987). 

Positrons can be used as particle probes, suitable to detect low concentrations of defects in materials. Positron physicists generally are in need of intense positron beams for applying positron annihilation techniques such as two dimensional (2D) Angular Correlation of Annihilation Radiation (ACAR) for investigating surfaces and interfaces of materials. The 2D-ACAR technique allows high resolution measurements of the electron momentum distribution for depth, localized defects, thin layer systems, and interfaces. In addition, a submicrometer size positron beam can be created for defect depth profiling on a lateral scale smaller than a micrometer. Vacancy type defects can be mapped in a three dimensional fashion. 

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