We report experiments in which positronium (Ps) atoms were created in a thick layer of MgO
smoke powder deposited on a thin silicon nitride substrate. The experimental arrangement was
such that a positron beam could be implanted directly into the top of the MgO layer or be
transmitted through the substrate, allowing Ps to be produced within ≈100 nm or 30 μm of the
powder-vacuum interface. The transverse kinetic energy of Ps atoms emitted into vacuum was
measured via the Doppler broadening of 13
S1 2 3PJ transitions, and found to be
Ex ≈ 350 meV, regardless of how far Ps atoms had traveled through the powder layer. Our data
are not consistent with the model in which energetic Ps atoms emitted into the internal free
volume of a porous material are cooled via multiple surface collisions, and instead indicate that
in nanocrystals lower energy Ps is generated, with negligible subsequent cooling in the large
open volumes of the powder. Our experiments also demonstrate that SiN substrates coated with
MgO smoke can provide a simple and inexpensive method for producing Ps transmission targets
