Abstract
The mean square Johnson noise currents flowing in a low-temperature seriesL–R circuit are used to measure absolute temperatures in a new method using an rf-biased superconducting magnetometer. In one set of experiments a quantitative study was made of the noise currents in a beryllium copper resistor located in the mixing chamber of a dilution refrigerator in the temperature range 5.4 mK to 4.2 K as a function of the magnetic temperature determined from measurements of the static magnetization of powdered CMN. In this temperature range the measured mean square noise current is found to be linearly proportional toT* with a coefficient which agrees within the experimental accuracy of±3 % with the theoretically predicted value based on the N yquist relation and independent measurements of all necessary calibration factors. A model for a SQUID operated in the flux-locked loop configuration with a partly resistive input circuit is presented and is used in the determination of the “device noise” temperature, which for the above experiment is found to be 0.16±0.02 mK. A calculation is presented of the length of time necessary to average the output of our thermometer in order to achieve any desired precision in the estimation of the mean square Johnson noise currents, and a comparison with the observed precision is made. In a second set of experiments the relationship between the Johnson noise temperature of a copper resistor and the 16-Hz magnetic temperature of two powdered paramagnetic salts, CMN and CDP, each in the shape of a right-circular cylinder with diameter equal to height, located in an adiabatic demagnetization cell, was determined from 2 to 20 mK. The device noise temperature characterizing the measuring system for these experiments was measured to be ∼0.05 mK. A discussion of the possible sources of inaccuracy associated with Johnson noise thermometry is presented. Particular attention is paid to the possibility of a heat leak directly to the noise resistor.
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Work supported by the U.S. Atomic Energy Commission under Contract No. AT(04-3)-34, P.A. 143.
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Webb, R.A., Giffard, R.P. & Wheatley, J.C. Noise thermometry at ultralow temperatures. J Low Temp Phys 13, 383–429 (1973). https://doi.org/10.1007/BF00654076
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DOI: https://doi.org/10.1007/BF00654076