Low-energy X-ray detection in cryogenic detectors with tungsten thermometers

In the course of our development of calorimetric particle detectors with superconducting phase transition thermometers, we have succeeded in depositing epitaxial α-tungsten films on sapphire which have critical temperatures Tc near 15 mK. To our knowledge this is the first time that the Tc of bulk tungsten has been observed in thin films. Such films used as thermometers are very sensitive and provide good energy resolution: with 4 g and 32 g sapphire crystals energy resolutions of better than 100 eV (FWHM) for 1.5 keV X-rays have been achieved.