A Magnetic Thermometer for High-Resolution 10 mK Scale Thermometry

An innovative thin-film magnetic thermometer with integrated superconducting quantum interference device (SQUID) readout is described for fast, precision temperature measurements in the 10 mK range. The compact thermometer consists of a miniature DC SQUID susceptometer with a dilute paramagnetic alloy deposited in one of the two series-configured gradiometric SQUID pickup loops that form the SQUID inductance. Directly sensing the magnetic signal with the SQUID eliminates losses that would otherwise occur by transformer-coupling the signal to a remotely located SQUID, usually operating at a higher temperature and consequently with a higher noise floor. In addition, a novel superconducting flux concentrator deposited on top of the paramagnetic alloy homogenizes and concentrates the magnetic flux density in the paramagnet and reduces the inductance of the pickup coil, both of which improve the sensitivity and signal to noise ratio as compared with previous miniature susceptometer designs. The innovative magnetic thermometer design is expected to achive a temperature resolution well below 1 µK/Hz½.

Cryogenic detectors based on transition edge sensor (TES) microcalorimeters, magnetic microcalorimeters (MMCs), and superconducting tunnel junctions (STJs) are becoming increasingly attractive for several commercial applications including x-ray, gamma-ray and alpha particle spectroscopy for x-ray microanalysis, x-ray fluorescence, and nuclear forensics and non-proliferation. All of these detector applications require precision temperature control for proper operation of the detector arrays. Monitoring the temperature at the end of a cold finger or snout where the detector arrays typically are located is very difficult using a conventional resistive thermometer, which also can produce erroneously high readings due to self-heating effects. Noise thermometers are superior in this regard, but their slow response time (~10 s) and limited resolution (~1%) make these thermometers less attractive for precision temperature measurements. The precision thin-film magnetic thermometer described in this proposal will offer an attractive solution for precision temperature control for all of these commercial detector applications.

Several potential NASA missions stand to benefit from the proposed magnetic thermometer development effort, in particular missions that include instrumentation with arrays of cryogenic detectors requiring precision temperature control for a variety of spectroscopy applications. Potential missions include the measurement of the polarization of the Cosmic Microwave Background (CMBPol), Single Aperture Far-Infrared Observatory (SAFIR), International X-ray Observatory (IXO), and the space-based interferometer Submillimeter Probe of the Evolution of Cosmic Structure (SPECS). The innovative magnetic thermometer with integrated SQUID readout described in this proposal is easy to use, features a compact design to simplify integration, and will meet targeted needs for fast, high-resolution (better than 1 µK/Hz½) temperature control for these missions.

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