Small Low-Temperature Thermometer With Nanokelvin Resolution

The magnetic field is generated by permanent magnets instead of a solenoid.

An improved high-resolution thermometer (HRT) for use in scientific experiments at temperatures <4 K has been developed. Like other, previously developed low-temperature HRTs, this device is based on the strong temperature dependence of the magnetization of a paramagnetic salt exposed to a magnetic field. However, in comparison with other paramagnetic-salt HRTs, this one is smaller and less massive; hence, it is denoted "sHRT" — short for "small HRT."


The temperature-sensitive part of an HRT of this type is a pill-like piece of the paramagnetic salt GdCl3. The magnetization of the salt pill is measured by use of a superconducting quantum interference device (SQUID). In an older device of this type, the magnetic field needed to magnetize the pill is trapped in a long superconducting tube (flux tube) that must be charged by use of a superconducting solenoid; typically, the overall length and mass of such an HRT are ≈0.3 m and ≈10 kg, respectively. In contrast, the length and mass of the sHRT are ≈3 cm and ≈7 g, respectively.

The reductions in size and mass are made possible by using permanent magnets instead of a charging solenoid and flux tube to impose the magnetic field. In the sHRT (see figure), two small samarium cobalt permanent magnets are placed near opposite ends of a beryllium copper cylinder filled with GdCl3. To enhance the thermal link between the GdCl3 and the immediate surroundings, the ends of the Be-Cu cylinder are capped with oxygen-free high-conductivity (OFHC) copper blocks, into which numerous chimney-shaped fins have been machined. A SQUID pickup coil made of Nb-Ti wire is wound on the Be-Cu cylinder. The sHRT housing is made of Nb, which is a superconductor and thus effective in shielding the pickup coil against any ambient magnetic field.

In tests, the sHRT was found to yield measurements with a temperature resolution of ≈10–9 K at a temperature near the liquid-gas critical point of 3He (≈3.31 K). The drift rate of the sHRT was found to be <2 × 10-13 K/s.

This work was done by Inseob Hahn of Caltech for NASA's Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line at www.nasatech.com/tsp under the Test and Measurement category.


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