[Visualization of convection in {sup 3}He-superfluid-{sup 4}He]

Low energy neutrons provide a unique opportunity for the visualization of interesting dynamical systems. Particularly for liquid helium mixtures, there are a number of very basic issues of current interest which can only be addressed by means of very novel techniques. The technique developed under this proposal is one of these and is designed to address specific issues for {sup 3}He-superfluid-{sup 4}He mixtures. These are: convection in these mixtures, and heat flow when the {sup 3}He concentration is very low. The neutron imaging device which the authors developed is potentially very useful for investigating the convection patterns when these mixtures are subjected to an adverse density gradient. There are a number of intriguing results obtained without the benefit of imaging. A full understanding of these results will depend crucially on obtaining images of the convection pattern. The second issue of interest, heat flow in very dilute superfluid mixtures, poses a particularly puzzling question. The expected divergence of the effective conductivity with vanishing {sup 3}He concentration, X, is not seen experimentally. This effect is intimately tied to the distribution of {sup 3}He in the direction of heat flow. However, no information on the distribution is available. The device which the authors constructed is potentially capable of addressing this need. The technique can also be used with any stem in which there is a spatially varying distribution of neutron absorbers. Hence, it has broad applicability