Ortho- and Para-Positronium Formation Measurements for Positron Scattering by N$_2$ and CO$^\ast$

We are investigating ortho- and para-positronium formation for positrons interacting with N$_2$ and CO 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 comparing our $3\gamma/2\gamma$ ratios to those for other target gases$^1$ we find a strikingly anomalous behavior at the Ps formation threshold for N$_2$ where there is an unexpected enhancement of the $2\gamma $ signal compared to the $3\gamma$ signal, while for CO (isoelectronic with N$2_$) the behavior is consistent with the other gases. The anomalous behavior for N$_2$ is consistent with an enhancement of annihilation at the threshold, which is as if Ps is forming and remaining bound to the N$_2^+$ ion until annihilation occurs.\\ $^\ast$Research supported by NSF Grant PHY 99-88093.\\ $^1$W.E. Kauppila et al. Phys. Rev. Lett. 93, 113401 (2004).