Positrons as a probe

Two years after its postulation by P. A. M. Dirac in 1930, C. D. Anderson discovered the positron, the anti-particle of the electron, by investigating particle traces using a cloud chamber in a magnetic field. Electrons and positrons as matter and antimatter particles have the same mass and spin, but opposite charge and magnetic moment.

After their implantation in matter, positrons thermalize within picoseconds and diffuse over hundreds of lattice spacings until they are annihilated either directly with an electron or after being trapped in crystal defects or at the surface. The annihilation process releases element-specific gamma rays that can then be detected. This non-destructive technique is called positron annihilation spectroscopy (PAS).

Features of PAS techniques using a mono-energetic positron beam can be summed up as follows:

• They are non-destructive,
• cause no activation of the sample,
• have a minimum atomic defect density of 10^-7 vacancies/atom,
• provide information about the chemical environment of defects,
• allow the investigation of surfaces, interfaces and thin layers,
• permit the study of electronic structures.

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 the atomic scale, e.g. vacancies, grain boundaries and nano-voids, clusters or surfaces.