MEASUREMENTS OF ABSOLUTE TEMPERATURE BELOW 0.75-K USING A JOSEPHSON-JUNCTION NOISE THERMOMETER

In order to extend the international temperature scale of 1990, ITS-90, below its lower limit of 0.65 K, we have developed a temperature scale ranging from 6 to 750 mK Values of absolute temperature are defined on this scale by an R-SQUID noise thermometer. A review is given here of the decade of our experience in the operation of this thermometer and in modeling its systematic errors. The reproducibility of noise temperature values was assessed using superconductive fixed points and the melting curve of He-3. To assess the accuracy of the noise thermometer, it was compared with another absolute thermometer (based on nuclear orientation) at the lowest temperatures and with an internationally recognized scale above 0.5 K The noise thermometer temperatures were used to calibrate two paramagnetic salt thermometers, the result being the construction of a temperature scale that is smooth to approximately 0.01 %. On the basis of these comparisons, temperature values defined by the R-SQUID are deemed to be reproducible to within 0.1 % and accurate to within 0.3 %.