Annihilation of positrons in hydrocarbons and halogenated hydrocarbons adsorbed on silica gel

Positron lifetime spectra of a series of hydrocarbons and halogenated aromatic compounds adsorbed on silica gel have been studied as a function of adsorbate concentration. The fact that the anomolous long lifetime component found in pure silica gel is not affected by adsorbate concentration up to 1.5–1.7 mmole/g SiO2 indicates that thermalized positronium cannot react chemically with these compounds. The decrease of the intensity of the long lifetime component with the adsorbate concentration can be interpreted in terms of positron capture by these compounds or by chemical reactions of highly energetic Ps atoms with the adsorbate molecules.