We report orthopositronium $\left(o\text{−}\mathrm{Ps}\right)$ thermalization observed in cryogenic silica aerogel cooled from room temperature to 25 K. It was observed that the $o\text{−}\mathrm{Ps}$ thermalization rate gets slower as $o\text{−}\mathrm{Ps}$ itself is colder than room temperature. This energy dependence can be explained based on a model where the efficiency of kinetic energy exchange between $o\text{−}\mathrm{Ps}$ and silica is determined by the de Broglie wavelength of $o\text{−}\mathrm{Ps}$. The present measurement confirmed that a cooling method which utilizes the thermalization process at cryogenic temperatures followed by laser cooling is applicable for the efficient production of an $o\text{−}\mathrm{Ps}$ Bose-Einstein condensate.

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• Atom & ion cooling
• Atom or molecule scattering from surfaces
• Bose-Einstein condensates
• Electron & positron scattering
• Aerogel
• Positronium
• Positron annihilation spectroscopy