Goddard began work on the first space-based Adiabatic Demagnetization Refrigerator (ADR) in 1979. The ADR developed by Goddard is a heat pump that operates between liquid helium temperature and very low temperatures. Goddard developed an ADR for the X-Ray Spectrometer ( XRS) to cool detectors to 0.065 Kelvin (65 mK). Ground-based ADRs have been constructed that have reached approximately 2 mK. An ADR was chosen for space instead of a dilution refrigerator because it does not require gravity and because it is exceedingly efficient. The ADR for the X-Ray Spectrometer has an efficiency of approximately 50% of Carnot. This efficiency is necessary for space-based systems to allow the liquid helium dewar to operate for extended periods of time on orbit.
The XRS flight salt pill(non-NASA site) was built by the Space Science and Engineering Center, University of Wisconsin, Madison. Visit their site and see how.
References, with some absracts.
The Advanced ADR project is developing a continuously running ADR. The heat sink for the advanced ADR will be a mechanical cooler, not a liquid helium bath.