Observation of orthopositronium thermalization in silica aerogel at cryogenic temperatures

We report orthopositronium (o-Ps) thermalization observed in cryogenic silica aerogel cooled from room temperature to 25 K. It was observed that the o-Ps thermalization rate gets slower as o-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-Ps and silica is determined by the de Broglie wavelength of o-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-Ps Bose-Einstein condensate.