Possible Experiments with High Density Positronium

Although it can probably only exist within a confining potential, a positronium (Ps) Bose-Einstein condensate (BEC) would be interesting because it could be viewed as the ultimate member of Wheeler's polyelectron series, e(+), Ps, Ps(-), Ps(+), Ps(2), ... , Ps-BEC [1], not counting high density neutral e(+)-e(-) plasmas. A Ps-BEC could be a source of very cold Ps atoms for precision spectroscopy, for gravitation experiments, and as the gain medium for an annihilation gamma ray laser [2]. It would also he interesting to look for other effects that are non-linear in the positron density, such as the production of Ps ions and highly polarized positrons at a metal surface. Specifically, a process in which one thermalized bulk positron passes into the vacuum accompanied by an electron and a surface positron (surface pseudo-Ps) to form a Ps(+) ion in vacuum, e(solid)(+) + PSsurf -> Ps(vac)(+) + Delta E, is energetically allowed for a Ni(100) remoderator surface, for which Delta E = 1.00 eV. The positive ions could he used in an experiment in which one positron undergoes a process in a material and the other positron reports on the process via its subsequent Ps emission, etc. Since the two positrons involved in the formation of Ps('+) and Ps(2) must have opposite spins, at sufficiently high implanted positron densities the minority positron spins will be significantly depleted by these processes, resulting in the enhancement of the spin polarization of the remitted positrons which might be useful for spin polarized angle -resolved positronium emission spectroscopy of the electronic structure of solids and surfaces.