Positron Annihilation Lifetime Spectroscopy
Positron Annihilation Lifetime Spectroscopy (PALS) relys on detection of 511 keV gamma rays as a fuction of time elapsed from the entry of the positrons into the sample. The lifetime of the positrons in the sample changes as a function of various properties of the material, specifically on the presence of voids and defects.
On entering the sample, positrons thermalise and either annihilate or form positronium with one of the target electrons. Positronium can be formed in two configurations; para- and ortho-positronium (the singlet and triplet form of the positronium ground state, respectively). Para-positronium decays via 2-gamma emission (with both gamma rays having an energy of 511 keV) with a lifetime of 120 ps, while ortho-positronium decays via 3-gamma emission with a lifetime of 142 ns. However, inside a solid, the lifetime of ortho-positronium depends on how easily it can annihilate via pickoff annihilation, where the positron annihilates with one of the other electrons in the target (not the one it is bound to). The lifetime of direct annihilation is typically 100's of picoseconds.
If annihilation gamma-rays are detected as a function of time after entering the sample (usually the two 511 keV gammas are detected in coincidence to improve timing resolution), then a series of lifetimes can be seen in the spectrum. These lifetimes give information about the structural makeup of the sample, and the ortho-positronium lifetime is particularly sensitive to the size and distribution of voids and defects.
