A SEALED CELL FOR THE ACCURATE REALISATION OF THE λ-POINT OF ~(4)He

摘要

The superfluid transition in pure ~(4)He under its saturated vapour pressure (λ-point) provides an ideal reference fixed point for thermometry near 2 K. The λ-point is by far superior to the currently used thermometric fixed-point based on superconductive transitions because it is sharp and highly reproducible (typically within 10 μK). Moreover the superfluid transition is not affected by sample inhomogeneity, strain or magnetic field. At the Cryogenic Laboratory (CL) sealed-cell realisations of the λ-point have been developed. The principle of the design is based upon the dramatic change in the thermal conductivity and the large specific heat of liquid helium at the transition temperature. The λ-point is realised in sealed cells with a stainless steel capillary containing the HeI/HeII interface within it. The column of HeI within the capillary provides a self-adjusting effect that keeps the top chamber temperature constant at T_(λ), with fluctuations within 0,01 mK. A new version of the cell has now been developed in a collaboration between CL and IMGC. The paper describes the device and reports on the testing and measurements in both laboratories.