Thermal production of positronium in porous alumina

A porous anodic alumina sample with ordered channels was studied in order to characterise its efficiency as a positron-positronium converter at cryogenic temperatures. The effects of gas adsorption were found to be vacuum dependent and considerable up to 200 nm depth inside the sample. At higher depths it was possible to identify a temperature dependent positronium formation mechanism and the related activation energy was determined to be in an interval from 8.7 to 16 meV. Because of this mechanism the Ps yield decreases to a few percent with decreasing temperature and it can be considered unsuitable for an efficient cryogenic positronium production.