Evaluation of Thermal Sensor Calibrations Below 1 K For a Horizontal Dilution Refrigerator in a Cryomagnet

Sammanfattning

Accurate thermometry is essential for experiments in cryogenic environments, particularly those using dilution refrigeration to reach milli-Kelvin temperatures. In this thesis, the calibration range of a Cernox CX1050 sensor was extended below its nominal lower limit of 1.4 K. In addition, the response of RuOx sensors at different positions within the dilution refrigerator was examined to assess their stability and accuracy under operational conditions. Controlled temperature sweeps between 250 mK and 700 mK were performed in the mixing chamber of a 17 T horizontal cryomagnet cryostat, with a pre-calibrated RuOx sensor serving as the reference. Resistance values from the Cernox sensor were recorded and fitted using Chebyshev polynomials to generate a reliable calibration curve in this ultra-low regime. Preliminary measurements between 0.7-1.4 K, above the tri-critical point of the 3He–4He mixture, are needed to broaden the effective operating range of the Cernox sensor and validate the findings of this thesis. Together, the investigations on thermometry enable more precise temperature control for future investigations of superconductivity and quantum materials.