Superfluid 4He Velocities in Narrow Channels between 1.8 and 0.3°K
We have made an accurate determination between 1.8 and 0.3°K of superfluid velocities through submicron pores in irradiated and etched mica sheets of the type first used by Notarys.l The advantages of these narrow channels over previous types (Millipore filters, Vycor glass, etc.) are considerable. Each irradiating particle leaves a straight train of dislocations behind it in the mica, which provides an etching path for the HF used, normal to the crystal planes. Once the radiation damage has been removed the activity of the acid in the normal direction is greatly reduced, and a large number of parallel-sided, singly connected channels can be “grown” to a reproducible diameter. The channels were trapezoidal in shape, and had a 10 taper along their length. The spread in size was about 20% and each 5-μm-thick mica sample contained between 104 and 107 pores in a 1-cm2 area in the middle of a 1-in.2 sheet, depending on irradiation time. During an experiment the number of open pores was less precisely known, but all the data were extracted from each sample without letting it warm above the λ-point, so that the results were consistent for each channel size and only the absolute values of the superfluid velocity were subject to error. In calculating the velocities, it was assumed that all the pores were open, so that the experimental values provided a lower limit.
KeywordsNarrow Channel Synchronous Motor Channel Size Chemical Potential Difference Mica Sheet
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