Positronium emission from MgO smoke nanocrystals

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 approximate to 100 nm or 30 mu m of the powder-vacuum interface. The transverse kinetic energy of Ps atoms emitted into vacuum was measured via the Doppler broadening of 1(3)S(1) -> 2(3)P(J) transitions, and found to be E-x approximate to 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.