Positron annihilation spectroscopy

Positron annihilation spectroscopy (PAS)[1] or sometimes specifically referred to as Positron annihilation lifetime spectroscopy (PALS) is a non-destructive spectroscopy technique to study voids and defects in solids.

The technique relies on the fact that a positron that comes in the immediate vicinity of an electron will cease to exist by annihilation. In the annihilation process of a positron and an electron, gamma photons are set free that can be detected. If positrons are injected into a solid body, their lifetime will strongly depend on whether they end up in a region with high electron density or in a void where electrons are scarce or absent. In the latter case, the lifetime can be much longer because the probability to run into an electron is much lower.

By comparing the fraction of positrons that have a longer lifetime to those that annihilate quickly, insight can be gained into the voids or the defects of the structure. In the case of materials with a lattice structure like semiconductors, this can be a dislocation or some other lattice defect. In the case of amorphous polymers, this might be the free volume between the chains of the polymer.

The technique requires a source of positrons, and a radioactive isotope of sodium is often used for this purpose.

References

1. ^ Positrons in Solids, P. Hautojaervi, Topics in Current Physics 12 Springer Heidelberg 1979