Low-temperature formation of positronium in nonpolymer organic systems

The phenomenon of the low-temperature formation of positronium (bound electron-positron system) observed during positron irradiation of a test substance is discussed. The effect is additional relative to the formation of positronium under normal conditions and, like the principal phenomenon, is radiation-chemical in nature. This effect has been discovered and studied mainly for polymers, in which the role of weakly bound electrons and positron localization (trapping) in the intensity of manifestation of this phenomenon was revealed. This review focuses on studies concerning barely investigated nonpolymer (polar and nonpolar) organic systems, in which a relatively high mobility of molecules as compared to polymers can open an opportunity for a more pictorial manifestation of positron and electron trapping by polar centers, both belonging to separate molecules and resulting from collective orientation of the molecules.