Homogeneous porous silica for positronium production in AEgIS

Positronium (Ps) formation measurements in homogeneous porous silica (Xerogel of 85 mg cm-3 and swollen MCM-41 of 390 mg cm-3) were performed at different temperatures (8K-293K) by means of a variable energy positron beam equipped with a Ge detector. The results indicate that Xerogel and swollen MCM-41 samples have a high Ps production, which is independent on the temperature. An estimation of the ortho-Ps mean diffusion length was obtained by measuring samples capped with an Al film (~110 nm). An efficient formation of cooled Ps atoms is a requisite for efficient production of antihydrogen, with the aim of a direct measurement of the Earth gravitational acceleration of antimatter, a primary scientific goal of AEgIS (Antimatter Experiment: gravity, Interferometry, Spectroscopy, CERN). Porous materials with open pores at the surface are necessary to produce a high yield of Ps atoms as well as to cool Ps through collisions with the inner walls of the pores before emerging in the free space outside the target. The results indicate that Xerogel and swollen MCM-41 are good candidates for an efficient formation of cold Ps atoms within the framework of the AEgIS project.