Positronium in low temperature mesoporous films

We investigate the formation and annihilation of ortho-positronium atoms (o-Ps), the spins parallel electron (e(-))-positron (e(+)) bound state, in mesoporous films from 400 to 50 K. At room temperature up to 20% of the implanted e(+) end up as o-Ps which self-annihilates (the bound e(+) and e(-) annihilate); this is 50% of the formed o-Ps. One would expect self-annihilation to be suppressed at lower temperatures since, although o-Ps trapped in pores of diameter phi > 1 nm, found in these films, is more likely to self-annihilate, several effects could decrease o-Ps formation and/or o-Ps trapping in a pore. Instead we find that at 50 K the amount of e(+) ending up as self-annihilating o-Ps is up to 19% greater than predicted if no suppressing effects played a role. Copious amounts of o-Ps atoms self-annihilate at 50 K (up to 30% of implanted e(+), 75% of formed o-Ps). This amount was found to increase even further down to 10 K making these films ideal substrates in which to confine large amounts of collisionally cooled, self-annihilating o-Ps for the eventual realization of an o-Ps Bose Einstein condensate (BEC).