Dielectric Low Temperature Thermometer for Use in High Magnetic Fields. I. KCl : Li

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Abstract
The temperature dependence of the dielectric constant of KCl crystals doped with Li+ ions has been shown to provide a useful measure of temperature over the range of 1–30 K, that is (at least between 1–10 K), independent of magnetic fields up to at least 100 kOe. The magnetic field dependence, measured by calibrating the thermometer at zero, then at high field, is immeasurably small and certainly less than the absolute temperature uncertainty (about 0.2%) at 4.2 K. The capacitance C varies with temperature T approximately as C=A+B/T, where A and B are positive constants. For a thermometer with A∼30 pF and B∼15 pF·deg the sensitivity δ lnC/δ lnT=−B/(AT+B) is about 0.3 at 1 K. Using a GR type 1615 A bridge a sensitivity of 0.3 mK at 1 K, and of 3.6 mK at 4.2 K was obtained. The thermometer calibration is reproducible to better than 0.2% on cycling between room temperature and helium temperature. A long range stability problem, possibilities for higher sensitivities, other geometric arrangements, and similar further development questions are briefly mentioned.

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