Anomalous magnetic quenching of quasi-positronium in solid octadecane

Positron annihilation in liquid as well as in solid octadecane has been investigated by means of lifetime spectroscopy combined with magnetic quenching experiments. An anomalous magnetic quenching in solid octadecane was detected, which cannot be explained in terms of the usual Zeeman effect on positronium with a modified electron contact density at the positron. A technique to extract the value of the relative contact density without making use of magnetic fields is used. The measurements indicate that the longest lifetime component both in the liquid and solid phases must be ascribed to the decay of a positron-electron bound state.