Norbert Mulders

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 of4He at saturated vapor pressure) the thermometer has a specific sensitivity of 4000φ0/K Gauss. There it achieves a temperature resolution better than 10−9K 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 10−7K. Near 2.17 K, drifts of the device are below the level of 10−13K/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.

Critical properties of4He in aerogel near the ?-line

We measured the superfluid fraction ρs/ρ and the isobaric thermal expansion coefficient βP of4He confined in an aerogel. Data were obtained near the λ-line along several isobars. Powerlaw fits to the results for ρs/ρ as a function of t≡T/Tc−1 (Tc is the transition temperature) give a pressure—independent exponent ς=0.755 when a confluent singular term is included. Fits to the βP data of power laws yield the specific-heat exponents α≅−0.6 and α′≅−1.0≠α above and below Tc respectively. When an analytic background term a×t is included in the fit, the pressure-independent value α=α′≅−0.59 is permitted, but the amplitude ratio A′/A is found to be near zero and the coefficient of the analytic term is large. The measured values for ς and α′ or α are inconsistent with hyperscaling in three dimensions.

A high-resolution susceptibility thermometer for use near 2 K

A relatively small and easily constructed high-resolution thermometer is described. It uses the magnetic salt copper ammonium bromide dihydrate (CAB). The temperature is read out using a dc-SQUID. We determined the sensitivity of the thermometer as a function of temperature from 1.5K to 4.3K and for magnetic fields from 40 to 800 Gauss. Subnano-Kelvin resolution is achievable near 2K. A mathematic model is used to understand the measured sensitivity and to improve the design.