In-situ diffusivity measurement technique

We have developed a technique for the measurement of diffusivities in liquids over a wide temperature range. A radiotracer, initially located at one end of the cylindrical diffusion sample, is used as the diffusant. The sample is positioned in a concentric isothermal radiation shield with collimation bores located at defined positions along its axis. The intensity of the radiation emitted through the collimators is measured vs, time with solid state detectors and associated energy discrimination electronics. Diffusivities are calculated from the signal difference between pairs of collimation bores. Self-diffusivities obtained with In/In-114m in space and on Earth illustrate the high precision obtainable with this technique. The In/In-114m space data were close to the ground results, however, the data scatter was much less. By employing a tracer that emits photons of different energy, and thus, different self-absorption, transport in the bulk of the sample can be distinguished from that in the proximity of the wall. No difference was found. In support of this work 2-D numerical modelling of the effect of various blockages on the concentration profile and the resulting apparent diffusivity were conducted. For the methodology that we use very little effect was seen except in the case of extreme blockages. (C) 1999 COSPAR. Published by Elsevier Science Ltd.