Positronium emission and cooling in reflection and transmission from thin meso-structured silica films

Measurements of the positronium (Ps) energy and formation fraction in reflection and transmission from a thin meso-structured silica target have been conducted using single-shot positron annihilation lifetime spectroscopy and Doppler spectroscopy. The silica sample is made using glancing angle deposition of vaporized SiO2 on a suspended thin carbon foil. Optical access through the silica sample facilitates measurement of the longitudinal Ps energy, and the Ps energy in the reflection geometry is found to decrease with positron energy as expected, with a minimum achievable Ps energy of 0.203(12) and 0.26(3) eV for the transverse and longitudinal directions, respectively. In the transmission geometry cooling of Ps becomes evident at the minimum positron impact energy required for the positrons to penetrate the carbon foil and enter the meso-structured silica. The minimum energies for this geometry are 0.210(12) and 0.287(14) eV in the transverse and longitudinal directions, respectively, and the minimum achievable Ps energy is found to be limited by the thickness of the structured silica target, since the same energy was found in both geometries