Thermal resistance of cryogenic thermometers at ultra-low temperatures

Self-heating of a resistive temperature detector (RTD) is a common problem in cryogenic thermometry, especially at millikelvin temperatures. Power dissipation in the sensor causes its temperature to rise above that of the surrounding environment adding to the uncertainty of the measurement. The amount of self-heating is directly proportional to the thermal resistance between the sensor and its environment. This paper presents thermal resistances data and analysis for temperatures ranging from 50 mK to 1 K in vacuum on germanium, ruthenium oxide, and Cernox resistance temperature sensors. This is an extension of previous studies that ranged from 1.4 K-300 K. The thermal resistance depends on the temperature and the details of the sensor type, packaging and mounting (including heat sinking). The experimental technique is discussed, and the data are compared to models of the thermal resistance. These data allow the optimum operating point that maximizes the sensor sensitivity while minimizing the measurement error to be calculated. (Author)