ANGULAR-CORRELATION STUDIES OF POSITRON-ANNIHILATION IN THE NOBLE-GASES

Two-dimensional angular correlation distributions have been measured f or the first time for positron and positronium annihilation in low-den sity helium, neon, argon, krypton and xenon, and in a range of helium- xenon mixtures. The full widths of the (cylindrically) averaged) free positron components are in reasonable agreement with theoretical value s. The intensity of the mixed-state positronium (Ps) component (mean v acuum lifetime 9.7 ns in a magnetic field of 0.8 T) in helium is consi stent with the Ps fraction previously determined in lifetime measureme nts; the widths of the Ps component in helium, neon and argon are cons istent with a Ps-atom scattering cross section below 6.8 Ev in all thr ee gases of approximately (9-0.5E) pia0(2), where E is the Ps energy i n eV.