A Closed-cycle Refrigerator for Realizing Low-Temperature Fixed Points

In order to accommodate the MULTICELLS Project output devices, a cryogen-free refrigerator has been designed and built at the Istituto di Metrologia “G.Colonnetti” (IMGC). The cryostat is a Gifford-McMahon refrigerator with two cooling stages. With the wiring and radiation loading on the shields, the lowest temperature of the second stage is about 10 K. The refrigerator has been designed as an adiabatic calorimeter: it can be used to reproduce the cryogenic ITS-90 defining fixed points (triple points) or to perform accurate thermometer comparisons at low temperatures. In order to prevent serious damage to capsule-type resistance thermometers, the mechanical vibrations due to the moving piston have been measured at the cold tip and in the experimental chamber. To reduce the vibration level at the thermometer wells, the experimental apparatus has been appended by means of carbon reinforced fibres: a reduction of the vibrations of about one order of magnitude has been observed. The cryostat is interfaced with a system for process control and data acquisition. Therefore, triple point temperature measurements have been carried out automatically, with values for the heat capacity measured at temperatures close to the phase transition points. The main measurement steps include the approach to the phase transition and temperature measurement at the triple point. During the approach, a MATLAB routine using a statistical hypothesis test is used for the phase transition detection. The temperature control system, implemented in a LabVIEW™ program running on a PC, monitors and controls the temperature of the whole system during the measuring process. This paper describes firstly the design and the construction of the cryostat. The operational characteristics of the refrigerator and its performance during the triple point measurements are also presented. Finally, as an example of the thermometer calibration at a cryogenic gas fixed-point of the ITS-90, the realisation, during development of the system, of the neon triple point with an expanded uncertainty of the order of 0,1 mK is reported.