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Positron Annihilation

Positron work was motivated by interest in the momentum distributions of core electrons which can be determined from the Doppler shift of the annihilation radiation induced by the momentum of the electron participating in the process. This method permits to identify the local chemical environment of trapped positrons (cf. Fig.4 showing results of a study of the chemical composition of precipitates in an AlAg alloy). The accuracy in the determination of the Doppler shifts is presently limited by the energy resolution of Ge detectors which is slightly above 1 keV for annihilation radiation of 511 keV. On the other hand, perfect crystal spectrometers at the ILL permit measurements of gamma quanta with a resolution down to 1 eV thus offering a potential improvement of three orders of magnitude over conventional techniques. A first test experiment at the ILL showed that the employment of gamma spectrometers for analyzing annihilation radiation is indeed feasible, however, requires further work in order to optimize the set-up with respect to background level, sample geometry and other parameters.

Physics of Functional Materials
Fakultät für Physik
Universität Wien
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