A high-resolution thermometer for the range 1.6 to 5 K

This paper presents a detailed design, a theoretical analysis, and experimental tests of a high-resolution thermometer for use in the temperature range from 1.6 to 5 K. The device uses a dc-SQUID magnetometer to determine the change in magnetization with temperature of a paramagnetic salt in a magnetic field. The field is provided by a small permanent magnet attached to the thermometer. Measurements of the sensitivity of the device agree well with the theoretical analysis. Near 2.17 K (the superfluid transition of He-4 at saturated vapor pressure) the thermometer has a specific sensitivity of 4000 phi(o)/K Gauss. There it achieves a temperature resolution better than 10(-9) K when it is charged with a field of about 300 Gauss. At 4.2 K, the specific sensitivity is smaller by a factor of 50, but should still allow temperature measurements with a resolution better than 10(-7) K. Near 2.17 K, drifts of the device are below the level of 10(-13) K/s. The thermometer has a small mass of about 7 g (excluding the magnet), and thus the advantage of relatively small cosmic radiation heating during microgravity experiments in Earth orbit.