Evidence for trapping of positrons in thallium

For the first time the positron lifetimes have been measured in thallium in the temperature range 80 to 570 K. At 507 K the transition from the α- to the β-phase has been detected. By use of a three-state trapping model, including detrapping out of shallow traps a vacancy formation enthalpy of 0.39 eV and a binding energy in the shallow trap of 0.06 eV was found in the α-phase.