Positron trapping in solid and liquid metals

Abstract

Measurements of the peak counting rate in the angular correlation curve of positron annihilation radiation have been performed in the solid and liquid phase of indium, lead, and aluminum, and in the solid phase of magnesium, as a function of temperature. In the solid phase In, Pb, and Al show at low temperatures the thermal-expansion effect and at higher temperatures in addition the positron-trapping effect. At the melting point the peak counting rate increases abruptly and stays then constant with temperature. This behavior in the liquid phase is interpreted as "saturation trapping," i.e., all positrons annihilate from a trapping site and the thermal expansion of the lattice is not effective for these positrons. Trapped positrons are effectively "shielded" from the thermal expansion of the lattice. Therefore, it is possible to separate the thermal-expansion effect from the vacancy-trapping effect and very accurate values for the monovacancy formation energy can be obtained. A difference was found in the nature of trapping sites in solid and liquid aluminum. Magnesium shows only the thermal-expansion effect and no vacancy-trapping effect.