Compositional dependence of positron annihilation in HgTl alloys

Ratios of coincidence counting rates of annihilation gammas at 0 and 10 milliradians for 13 atomic concentrations from 0 to 50% of thallium in liquid mercury were measured. A sharp discontinuity at about 5% of thallium indicates an abrupt change of cluster size and concentration, which may be related to the peaking of electric thermopower of the alloy.