Dresden 2009 – wissenschaftliches Programm
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TT: Fachverband Tiefe Temperaturen
TT 7: Postersession Matter at Low Temperature: Materials
TT 7.1: Poster
Montag, 23. März 2009, 13:00–16:45, P1B
The effect of temperature and magnetic field on the diffusion of positrons in slightly deformed copper — •Vera Koos, Matz Haaks, Torsten E.M. Staab, and Peter Herzog — Helmholtz-Institut für Strahlen- und Kernphysik, Nußallee 14-16, D-53115 Bonn
The density of lattice defects like dislocations and vacancies can be measured by positron annihilation spectroscopy (PAS) with outstanding sensitivity. This is due to the fact that positrons diffuse through the lattice and can be trapped by open volumes. Diffusion and trapping are temperature dependent. Additionally the diffusion is influenced by magnetic fields forcing the positrons on curved trajectories perpendicular to the fields direction. In slightly deformed metals changes in the diffusion are reflected by changes of the positron annihilation parameters (S-parameter).
In this study we present PAS results on slightly deformed copper over a temperature range from room temperature (300 K) down to 20 mK, using a 3He/4He dilution refrigerator. By comparison with the predictions of an adapted trapping model it could be shown that dislocations change their behavior from a shallow to a deep positron trap at about 100 K. Below trapping in prismatic dislocation loops plays a significant role.
At several temperatures the effect of a magnetic field (0 to 2 Tesla) was studied. Increase of the field causes a decrease in the S-parameter. Within the scope of the trapping model this can be explained by a decrease of the volume scanned by the positron during its diffusive motion.