MAGNETIC QUENCHING OF POSITRONIUM

The positron-electron bound system in condensed matter (Ps) is a probe of primary importance for the investigation of the microscopic struct ure of liquids and non-metallic solids. The physical properties of Ps in matter are generally different with respect to Ps in vacuo, owing t o the interactions with the surrounding electrons. Information on the structure of Ps can be obtained through the Zeeman effect, that induce s a mixing of the m = 0 Ps ground state sublevels with a shortening of the triplet lifetime (magnetic quenching). This method, which can be coupled with any positron annihilation experimental technique, showed its effectiveness to discriminate among competitive reactions between Ps and the surrounding medium, as well as to clarify the origin of lif etime components of uncertain attribution. The discovery of anomalous magnetic effects in different organic liquids and solids has opened ne w perspectives in the studies of positron-multielectrons bound systems . Magnetic quenching experiments carried out with polarized positron b eams display a complex and fascinating phenomenology, whose explanatio n could shed light on the role of the positron-medium interactions whi ch take place within the early instants after the emission of the posi tron. In the present paper various aspects of the magnetic quenching m ethod will be examined, with emphasis on recent experimental results.