Two-dimensional angular correlation distributions have been measured for the first time for positron and positronium annihilation in low-density 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 values. The intensity of the mixed-state positronium (Ps) component (mean vacuum lifetime 9.7 ns in a magnetic field of 0.8 T) in helium is consistent with the Ps fraction previously determined in lifetime measurements; the widths of the Ps component in helium, neon and argon are consistent with a Ps-atom scattering cross section below 6.8 eV in all three gases of approximately (9-0.5 E) pi a02, where E is the Ps energy in eV.

http://iopscience.iop.org/article/10.1088/0953-4075/27/5/017/pdf

Angular correlation studies of positron annihilation in the noble gases

Positronium annihilation spectroscopy has been used to study capillary condensation of nitrogen adsorbed in mesoporous VYCOR glass. The temperature and pressure dependences of the relative change in the 3γ:2γ annihilation ratio are reported. Both isobars and isotherms exhibit features which are associated with a shifted gas-liquid transition. The isotherm data suggest that pore filling occurs via progressive layer formation at the pore surface followed by an abrupt gas-liquid phase transition at some pressure below the bulk saturated vapor pressure. Such a picture is consistent with the theoretical treatment of adsorption in individual pores

Positron annihilation study of capillary condensation of nitrogen gas in a mesoporous solid.

Electrons injected from the photocathode of a diode measurement cell into liquid neon produce two types of current signals. A fast rising and decaying signal in the time domain of several tens of nanoseconds which is attributed to the movement of delocalized electrons, and a current signal in the time domain of milliseconds which is attributed to the movement of localized electrons. The electron drift velocities in the delocalized and localized states were measured as a function of temperature and/or electric field strength. The results are discussed taking into account the energetics and dynamics of electron transport in simple condensed systems. A theoretical model of the kinetics of electron autolocalization is presented and compared to experiment.

High- and low-mobility electrons in liquid neon

Helium and hydrogendashdecorated cavities in silicon

Elaborate wavefunctions representing low-energy positron - helium elastic scattering, which were obtained in the course of calculating accurate values of the scattering phase shifts, are used to determine the electron - positron annihilation rate and the Doppler-broadened annihilation -ray spectrum. This spectrum is also measured using room-temperature positrons in a Penning trap and a non-Gaussian lineshape is observed for the first time. Excellent agreement is obtained between the theoretical spectrum and the present results.

Annihilation in low-energy positron - helium scattering

The angular-correlation technique of positron annihilation has been used to detect and measure the localized bubble state of positronium (Ps) in liquid Ne, Ar, Kr, H2, and N2 and in liquid and solid He at various pressures and temperatures. No bubble state was seen in liquid O2 or in solid Ne and Ar. The dynamics of bubble formation is not yet understood. In the cases where theoretical calculations, and adequate data, exist, viz. He, Ar, and H2, there is reasonable agreement for the momentum of the photons from the annihilation of positrons with the outer electrons of these atoms. The Ps annihilations from the self-trapped bubble state are reasonably well described in terms of a simple finite potential-well model.

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J.J. Steinbacher

Papers

Positron annihilation in simple condensed gases

Magnetic Quenching Experiments in Liquid Rare Gases