SUMMARYMeasurements have been made of the activity collected by a stream of distilled-water droplets falling through an atmosphere saturated with tritiated water vapour. The results, combined with those of Friedman et al. (1962), confirm theoretical predictions of the rate of exchange between droplets of radius 40-1500 microns and the surrounding vapour.
A continuous flux of thermal neutrons was passed into a large cylindrical tank of heavy water that sat on a graphite column containing a source of photoneutrons. The neutron density in the tank was explored with small indium detectors at selected points along two diameters at right angles in each of three horizontal planes and at 10-cm. intervals along the axis of the tank. The transverse measurements were fitted to Fourier–Bessel series in which the amplitudes of the harmonics were relatively small. The effective radius derived from the transverse analysis was the same in the three planes. The relaxation length of the first component was found from the axial measurements. Corrections were applied for the absorption of neutrons in the axial detector tube, enclosed framework, and indium detectors, and in the light hydrogen in the heavy water. The final value of the diffusion length of thermal neutrons in heavy water (100% D2O) is 171 ± 20 cm. Combining this with the known transport mean free path, the capture cross-section is (0.92 ± 0.22) × 10−27 cm.2 per molecule D2O for neutrons of velocity 2200 m. per second.
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