Ortho- and Para-Positronium Formation Measurements for Positron Scattering by C$_2$H$_2$ and CH$_4^\ast$

We are investigating ortho- and para-positronium formation for positrons interacting with C$_2$H$_2$ and CH$_4$ in a gas scattering cell. These measurements involve the detection of two $\gamma$ rays in coincidence for energy windows (1) centered at 511~keV resulting from the decay of short-lived (0.1~ns) para-Ps and the destruction of longer-lived (0.1~ms) ortho-Ps at the scattering cell walls, and (2) from 300 to 460~keV resulting from the three gamma decay of ortho-Ps.$^1$ By taking the ratios of these signals versus positron impact energy we find that near the Ps formation threshold the $3\gamma/2\gamma$ ratios have their largest values of about 1.6, which is where Ps has its lowest kinetic energy and ortho-Ps decays without breakup at the cell walls. Comparing these ratios with ones obtained for Ar$^1$ reveal interesting differences, which include the formation of Ps with inner orbital electrons. The threshold we observe for forming Ps with CH$_4$ is consistent with this being an adiabatic ionization process, as opposed to a vertical ionization process.\\ $^\ast$Research supported by NSF Grant PHY 99-88093.\\ $^1$W.E. Kauppila et al. Phys. Rev. Lett. 93, 113401 (2004).