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At the MLZ, a positron beam is produced by thermal neutron capture in cadmium releasing high-energy gamma-radiation, which is subsequently converted into positron-electron pairs within a structure of platinum foils. By applying electrical fields, the positrons are separated from the electrons.
The positron technique is based on the fact that, after implantation in matter, positrons thermalize within picoseconds and diffuse over hundreds of lattice spacings until they annihilate with an electron either in the crystal lattice or after being trapped in crystal defects or at the surface. The annihilation process releases element-specific gamma rays that can be detected. This non-destructive spectroscopy technique is called Positron Annihilation Spectroscopy (PAS).
Fig. 1: Angular correlation and Doppler shift of the two annihilation γ-quanta with total energy of E = E1 + E2 = (511 keV - ΔE)1 + (511 keV + ΔE)2 = 1022 keV.
In the experiment, one observes a deviation from the 180° angular correlation and an energy shift of the emitted 511 keV quanta due to the Doppler effect caused by the electron momentum (figure, right) – the momentum of the thermalized positron is negligible. The positron lifetime in matter is strongly correlated with the local electron density.
Features of PAS at NEPOMUC:
In solid-state physics and materials science the positron is applied as a highly mobile nano-probe for the non-destructive investigation of lattice defects on an atomic scale – e.g. vacancies, grain boundaries and nano-voids, clusters or surfaces. The high sensitivity of PAS is related to the large positron diffusion length until annihilation and the attractive trapping potential of open volume defects.