Temperature dependence of positron annihilation rates on atoms and molecules

The authors previously reported experimental data on positron annihilation rates, Zeff, measured in a Penning trap geometry at room temperature for a large variety of organic and inorganic substances. They have now been successful in heating the stored positrons to temperatures up to 1 eV in the presence of a test gas. Subsequently, positrons cool in a reproducible manner while simultaneously annihilating on the test gas. This has allowed the authors to measure the temperature dependence of positron annihilation and investigate annihilation on a variety of molecules in a regime not previously accessible to experiments. In this paper they present results on the temperature dependence of annihilation rates from noble gases in the range between 0.025 eV and 0.6 eV. They observe a decrease in Zeff with increasing temperature, which is more pronounced for the heavier noble gases. In the case of Xe and Kr, Zeff changes by a factor of 4 and 2, respectively, over the temperature range studied. He and Ne exhibit only minor changes in Zeff. A comparison of this temperature dependence with theoretical predictions reveals good agreement for all noble gases. Similar measurements using organic molecules are currently underway