Spin conversion and pick-off annihilation of ortho-positronium in gaseous xenon at elevated temperatures

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We present experimental results for the annihilation rate of ortho-positronium in gaseous xenon at 250 kPa using a digital-oscilloscope-based positron annihilation lifetime spectrometer as a function of temperature (300--623 K). Owing to the Zeeman mixing of positronium in a magnetic field, the annihilation rate is divided into two components: one is due to ortho-para postronium spin conversion induced by spin-orbit interaction, while the other is due to pick-off annihilation. The spin-conversion annihilation rate is proportional to T2.1, where T is the temperature. We attributed this dependence to the fact that the spin conversion occurs only in p-wave scattering during positronium--xenon collisions and to the fact that, after thermalization of the ortho-positronium atoms, their velocity profile follows a Maxwell-Boltzmann distribution. The results also show that the pick-off annihilation rate is almost linear in temperature. This increase is a dependence exceptionally stronger than that observed in other gases.