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The circular polarization of the 1.081 MeV y rays from "F has been measured to be (1.7+5.8)X10 . "F was produced by a 4.05 MeV He+ beam impinging on a recirculating water target with a thin entrance window. The circular polarization was measured with four magnetic transmission polarimeters, each backed by a 150 cm' n-type Cxe detector. A fast data acquisition system limited overall peak losses to 30% at a count rate of 60 kHz per detector. Supplementary experiments were carried out to investigate the sensitivity of the circular polarization measurement to systematic variations of beam position and intensity. These effects were found to be negligible relative to the statistical uncertainty. The experimental result is interpreted in terms of an isovector parity-nonconserving matrix element between the~0+, T = 1, 1.042 MeV ) and 0,T =0, 1.081 MeV) states in ' F that is dominated by the weak pion exchange contribution.The present result for the weak pion-nucleon coupling strength is~f'~=(0.4+O4)&& 10 ', which is significantly smaller than recent theoretical predictions based on the Weinberg-Salam model.'Present address:

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The passage of the alkali metals Li, Na, K, Rb, and Cs through saturated phosphatidylcholine membranes has been measured using particle induced gamma ray and x-ray emission to observe the ions. Simultaneous measurements of these five cations has not been possible with more traditional methods involving ion specific electrodes or radioactive tracers. To the authors’ knowledge this is the first time this technique has been used in lipid bilayer research. The ion leakage was determined for an incubation period of 30 min at several temperatures which spanned the bilayer melting temperature. The dependence of this permeability on temperature, acyl chain length, and ion size is described theoretically in terms of the density fluctuations in the bilayer. A leakage rate which depends on the mass of the ion was observed. This dependence shows that the heavier the ion the faster it diffuses out of the vesicle. The effect of this selectivity is shown to be most pronounced near the melting temperature of the lipid. Moreover, this size dependence suggests that the permeating entity is a hydrated ion, rather than a naked ion or a neutral, bound ion pair.

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