The Adiabatic Demagnetization Refrigerator (ADR) is what keeps the XRS detectors cold enough to work. It has been developed at GSFC by the Cryogenics and Fluids Branch and X-Ray Astrophysics Laboratory.
An ADR works by first using a large magnet to align the magnetic poles (spins) of all the molecules in a block of salt (called the salt pill). The salt pill is then connected to a liquid helium bath via a "heat switch", allowing it to cool to the temperature of the liquid helium (about 1.5 Kelvin). Once it has reached equilibrium with the helium, the heat switch is opened, so that heat can no longer flow between the salt pill and the helium.
Once the heat switch is open, the magnetic field is slowly reduced nearly to zero, allowing the spins of the salt molecules to flop around in random directions. This absorbs heat from the salt pill, cooling it.
By carefully adjusting the strength of the magnetic field, the temperature of the salt pill can be kept constant for many hours. In the case of the ADR used in XRS, the temperature can be maintained for over 30 hours. Eventually the spins are all completely random and no more heat can be absorbed. Then the magnetic field is increased, heating up the salt pill, and the cycle is repeated.
Here's a picture of the ADR connected to the detector front end.
For more information, read the article about ADRs at the Cryogenics branch.