Positron annihilation in gaseous hydrogen and hydrogen-neon mixtures. II. Positronium

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Abstract
The annihilation rate of positrons bound in o-Ps atoms and the fraction of positrons which form Ps have been measured for H2 gas at 293, 250, and 77 K at densities in the range 5.9-429 amagat, and for H2-Ne gas mixtures at 77 K having H2 partial densities in the range 1.9-286 amagat and with Ne concentrations in the range 0.02-0.98. Large deviations from the annihilation rates expected for free o-Ps atoms are observed and are attributed to the formation of localized cavity states. The results obtained with H2-Ne mixtures strongly support o-Ps-induced cavity formation. Complex density and temperature dependencies are observed for the Ps fraction in H2, and the addition of partial densities of Ne enhances positronium formation. The annihilation of slow positrons in H2 is discussed in paper I of this series.