A simple fix-point device for temperature scale definition below 1K.

It is clear that one of the main problem of the low temperature physics is the definition of the temperature scale. While the temperature scale from about 1300 K down to 1 K is internationally adopted as ITS-90 and it is defined by melting points of pure materials (like Cu, Au, Ag, etc.) and vapour pressures and triple points of pure substances ( like H2O, Hg, Ar, .. , He ), the definition of temperature scale below 1K is more difficult. The best way to define the temperature scale is a using of primary thermometers like, for example a nuclear orientation thermometer, a noise thermometer, a gas thermometer etc. and according to these calibrate secondary thermometers. Another way is the application of fix-point devices (FPD), the devices with samples of the pure superconductors with well defined temperatures of superconducting transitions. Here we present design and properties of a very simple fix-point device we have developed.

Figure 1: The 3-D sketch of the FPD crosssection.

A drawing of the fix-point device cross section is presented in Fig. 3. The holder of fix-point device is made from a copper and it is thermally anchored to the mixing chamber of a dilution refrigerator via thread. The rod of holder serves as a thermal link to cool the samples of superconductors. Five superconductors: AuIn2, Ti, Zr, Zn, Mo were put into a Stycast tube and they are "immersed" in a silver powder. The silver powder provides small overheating due to eddy currents and good thermal conductivity between samples and the rod. Changes in magnetic susceptibility due to superconducting transitions are measured by a transformer using a standard lock-in technique. A secondary coil of the transformer consists of two parts wound in opposite to each other. The primary coil is a one layer single coil wound directly on the top of secondary coil. In order to minimize external perturbations and to reduce residual magnetic field including the Earth's magnetic field the transformer is shielded by Nb and m-metal tubes thermally connected to the holder. The samples of superconductors are divided into two parts: AuIn2 are placed in one part of the secondary coil closer to the rod (in future we plan to add W, Be and Ir samples to define the temperature scale down to 15 mK) and the rest of samples (Ti, Zr, Zn, and Mo) are placed in the second part. This produces a "positive" and a "negative" jumps in measured signal.

Figure 2: Properties of FPD in temperature range from 200 mK up to 600 mK measured during warming up and cooling down regime of the dilution refrigerator. The inset shoes details of the Zr transition.

The properties of the fix point device in the temperature range from 0.7 K down to 200 mK are presented in Fig. 4. The induced signal was measured during warming and cooling cycle of the dilution refrigerator. The first sharp jump in the signal can be seen at temperature 200 mK as AuIn2 sample is coming from the superconducting to its normal state. After this the signal is constant for a short time. As temperature is rising up to about 0.4 K the transition of the Ti sample takes place. The opposite change in the signal is due to position of the Ti sample in second part of the secondary coil (as it was already mentioned above).

The fix-point device seems to be very simple instrument for temperature scale definition below 1K owing to their ease of detection, reproducibility, good accuracy and fast response time. The details can be found in:

E. Gažo, Ľ. Lokner, R. Scheibel, P. Skyba, N. Smolka:
Simple fix-point device for temperature scale definition below 1K,
Cryogenics 40/7 (2001) 441.