Angular correlation of annihilation radiation (ACAR) from silica-powder pellets and silica aerogel has been measured in order to investigate the slowing down of free positronium (Ps) atoms by collisions with silica grains and gas molecules. The data for the pellets and the aerogel in vacuum show that the slowing down of parapositronium (p-Ps) in the free space between the silica grains depends on the number of collisions and hence on the mean distance between the grains. The momentum distribution of orthopositronium (o-Ps) shows further slowing down because of its long lifetime. From the ACAR data obtained from specimens of aerogel 611ed with gases (He, Ne, Ar, Kr, Xe, H2, CH4, CO2, and iso-C4Hqo), the momentum-transfer cross sections between Ps and the gas molecules are estimated. It is concluded that the Ps kinetic energy is transferred only to the translational motion of the gas molecules, i.e. , the excitations of vibration and rotation of the molecules are negligible. PACS number(s): 36.10.Dr, 34.50. -s, 78.70.Bj I. INTR. ODU CTIONIt was observed in the mid 1960s that the angular correlation of annihilation radiation (ACAR) and positron lifetime spectra in metal oxide and metal Huoride powders [1,2] showed formation of positronium (Ps). In 1968, Paulin and Ambrosino [3] reported that the Ps component in the positron lifetime spectra for silica powders depends on the grain diameter. It was postulated that the Ps atoms form inside the grains and then disuse out of them [4]. Paulin and Ambrosino also observed the eA'ect of air on the o-Ps annihilation. Following this, silica powders were used for investigating the interactions between Ps and paramagnetic gases [5 -8].

Abstract. Positron emission tomography (PET) has been utilized to obtain dynamic images of long distance nutrient translocation in plants. Positron emitting 18F, produced by a Van de Graaff accelerator using the reaction 18O(p,n)18F, was fed in solution to excised stems of Glycine max positioned vertically in a large‐aperture PET detector system. Images of tracer activity were recorded with a time resolution of 0.5 min and a spatial resolution of 4 mm. Maximum tracer activities at stem sites were obtained within 3 min of the pulse feed. A model is presented enabling evaluation of regional values for tracer flow, tracer binding, flow speed and flow volume. Analysis of data for one stem position yielded a flow volume of 2.1mm3 min−1 and a flow speed of 36cm min−1. Comparison with the distribution of 14C‐inulin, which was simultaneously fed to the cut stems, indicates the 18F is suitable for use as an apoplastic tracer; 92% of the tracer activity accumulated in the leaves. The fraction of 18F that remained bound was most concentrated at stem nodal regions, an observation consistent with the existence of transfer cells at these sites. Advantages and limitations of PET applied to plant physiological investigations are discussed.

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Previous experiments with positron annihilation in single crystals of quartz have revealed an interesting series of narrow peaks on the momentum distribution of the annihilation photons. Our experiments repeat these observations and also confirm that no very long lifetime exists. We outline a simple theoretical model based upon the necessity of treating the positronium electron and the remaining electrons in the crystal as indistinguishable. The model yields results compatible with both momentum and lifetime data.

Measurements of positron lifetimes in cold worked copper samples show clearly positron trapping by dislocations. The copper crystals were deformed under conditions that would lead to known densities of dislocations in near random arrays and were annealed to remove other defects associated with the deformation. Using a trapping model, the data yields the value of μ = 2.9 × 1015 s−1 for the trapping rate per unit defect concentration, somewhat higher than the rate for trapping by vacancies in copper. The increase in lifetime of positrons trapped at dislocations is 34 ps (26%), about half the change for positrons trapped at vacancies in copper. A discussion of positron trapping at dislocations contrasted to vacancies emphasizes the differences arising from the extended nature of the dislocation. The theoretical picture leads to a trapping rate at dislocations roughly proportional to the binding energy.