Microcalorimeters operating at very low temperatures comprise a new class of x-ray detectors which cover a wide energy range with high resolution. The detectors are calorimeters cooled to 70 mK which respond to single photons with a temperature rise detected by a metal film whose conductivity transitions from superconducting to normal.

The transition edge sensor (TES) microcalorimeter x-ray detector developed at NIST offers abundant opportunities to conduct new experiments in chemistry and physics. Its high energy resolution should allow us to observe changes in oxidation state as a result of chemical reactions. For transitions that do not involve the valence band, the high resolution and broad energy range of individual detectors makes it possible to compare energies in atomic x-ray transitions. Many x-ray lines and satellites remain to be experimentally verified, in comparison with theory. We have a program to carry out these investigations using TES microcalorimeter detectors with 5 eV energy resolution. We are applying these detectors to electron microscopes, conventional x-ray sources, and synchrotron radiation.