Positron annihilation lifetime spectroscopy uses positrons to probe material defects/voids/free volume at the sub-nm scale. The general working principle is based on correlating the lifetime (dwelling time) of the injected positrons against the sample void size (longer lifetimes correspond to large void sizes). The table-top spectrometer has a time resolution of ~ 270 ps, and uses a 22Na positron source sandwiched between two thin Kapton foils. The source is sandwiched between the material of interest and the annihilation lifetimes and intensities are deconvoluted by fitting exponential decay functions using commercially available software. Here, the spectrometer is primarily used to understand polymeric materials and fiber reinforced composites. Thermalized positrons form a quasi-bound state with electrons, known as positronium (Ps) that localize within void-content of polymers. The Ps eventually pick-off an electron from neighboring atoms to annihilate within 1-10 ns which can then be correlated against the free volume nanohole radius using a semi-empirical Tao-Eldrup equation.
 Gidley DW, Peng H-G, Vallery RS. Positron Annihilation As a Method To Characterize Porous Materials. Annu Rev Mater Res 2006;36:49–79. doi:10.1146/annurev.matsci.36.111904.135144.