At very low temperatures below 0.1 K, it is possible to construct devices with high responsitivity, low thermal noise and an energy resolution of a few electonvolt. such that heat pulses from individual X-ray photons can be sensed with a signal-to-noise ratio of up to one part in several thousand.

The Japanese X-ray observatory Suzaku, launched in June 2005, is the first satellite carrying a cryogenic X-ray detector (X-ray Spectrometer, XRS). XRS utilizes a microcalorimeter array of 32 pixels operating at 60 mK [10]. The basis of the XRS array is micro-machined silicon with ion-implanted thermistors and semimetallic crystal mercury telluride (HgTe) as X-ray absorbers. With an energy resolution of 10 eV at 6keV, it provides a spectral resolving power E /AE ~ 600, a value that was reserved in the past to dispersive elements only. The effort required for cooling such an instrument is huge and comprises three stages consisting of solid neon, liquid helium, and an adiabatic demagnetization refrigerator. Unfortunately, the instrument failed shortly after launch because of a leakage in the helium dewar.

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