Positron annihilation spectroscopy

For solids containing free electrons (such as metals or semiconductors), the implanted positrons annihilate rapidly unless voids such as vacancy defects are present.

Para-positronium, p-Ps, is a singlet state (the positron and electron spins are anti-parallel) with a characteristic self-annihilation lifetime of 125 ps in vacuum.

[4] Ortho-positronium, o-Ps, is a triplet state (the positron and electron spins are parallel) with a characteristic self-annihilation lifetime of 142 ns in vacuum.

Even approaches to obtain fractional free volume from the PALS data that claim to be independent on other experiments, such as PVT measurements, they still do employ theoretical considerations, such as iso-free-volume amount from Simha-Boyer theory.

Even the simplest case, that of a single positron immersed in a homogeneous gas of electrons, has proved to be a significant challenge for theory.