VARIATIONS IN THE AMOUNTS OF POSITRONIUM FORMED IN LIQUIDS AND AMORPHOUS SOLIDS

Abstract

Measurements are reported of the intensity, I₂, of the long-lived (τ₂) component of positron annihilations in a number of liquids and amorphous solids. Large variations in I₂ have been found with different samples of similar substances. It is shown, for example, that I₂ for fused quartz of good quality is about 51% of all the positron decays, and that an earlier published value of 29% probably referred to a 96% silica glass rather than to pure fused quartz. The new value, taken with the results of angular correlation measurements, gives convincing evidence that the annihilation of triplet positronium in condensed materials proceeds mainly by the "pickoff" mechanism. It is shown that the addition of NO₃⁻ and NO₂⁻ ions to water in increasing concentration depresses I₂ from its normal value of (21 ± 5)% to a value at least as low as 2% without any marked change in τ₂. This behavior is explained in terms of the formation of the positron compound e⁺NO₃⁻ in the water. This hypothesis explains the shape of the curve of I₂ as a function of NO₃⁻ ion concentration and also gives a cross section for the capture of positrons by NO₃⁻ in rough agreement with expectation.