Positronium emission from MgO smoke nanocrystals

Abstract

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 13S 1 → 2 3 P J transitions, and found to be E x ≈ 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.

Resonant shifts of positronium energy levels in MgO powder

“…Ps excitation in vacuum [ Fig. 2(a)] shows a typical Doppler-broadened spectrum corresponding to atoms with a mean transverse kinetic energy of ≈350 meV, as has been observed in previous experiments using MgO powder targets [48]. To measure confined Ps the lasers were directed into the sample [ Fig.…”

“…The presence of even a small number of micron-size cubes means that the open spaces in the MgO powder may also be similarly large. In separate measurements [48] we have found that Ps atoms are able to travel through tens of microns of MgO powder without significant energy loss or annihilation, indicating that the internal spaces between crystals are large and interconnected. The lack of observed cooling can be used to set a lower limit of ≈1 μm on the Ps mean free path between collisions, based on a collisional cooling model [49].…”

“…Using IMAGEJ analysis software [47], we determined that the mean crystal edge length was 120 nm, with a broad distribution ranging from approximately 10 nm to 1 μm. We estimate the MgO layer thickness to be on the order of 30 μm [48].…”

“…Ps atoms emitted from MgO nanocrystal surfaces have kinetic energies on the order of 350 meV [48], and the Pssurface scattering rate ω S (assuming a maximum 1 μm mean free path) will therefore be at least 250 GHz while the atoms remain in the powder. For the laser parameters used in the present experiments, the Rabi frequency for 1 3 S 1 → 2 3 P J transitions ω R is estimated to be on the order of 10 GHz (e.g., [50]), meaning that during the excitation process atoms will scatter from MgO surfaces multiple times.…”

Positronium density measurements using polaritonic effects

“…Open cavity systems have been largely employed to overcome the issue of growing the active material on top of the mirrors, allowing both noninvasive investigation and good control over the microcavity resonances and mode volumes [46,47]. These systems are particularly suitable to the present investigation as different kinds of microscopic Ps producing materials, such as porous silica layers [48] or MgO powders [49], can be placed upon a prefabricated DBR with different thickness and surface extension [see Fig. 1(a)].…”