Reinterpretation of oxygen on Al samples by variable-energy positrons.

When low-energy positrons are implanted into crystals and diffuse back to the crystal surface, they can form positronium by escaping from the crystal. This positronium formation fraction is a function of temperature for an aluminum sample coated with oxygen. Lynn observed large differences in this positronium fraction in the heating and cooling curves and attributed the hysteretic behavior to the oxygen on the aluminum surface undergoing a phase transition from an amorphous to a crystalline state. This Brief Report offers an alternative explanation for the hysteretic behavior of the positronium formation fraction, namely, the oxygen leaves the surface region either by desorption or by migration into the bulk Al crystal.