X‐ray detection using a superconducting transition‐edge sensor microcalorimeter with electrothermal feedback

We have developed a new type of x‐ray detector based on a superconducting transition‐edge thermometer operated near 100 mK. A superconducting quantum interference device is used to measure the current through the thermometer, and negative electrothermal feedback is used to improve the energy resolution and shorten the thermal time constant. We have used a detector mounted on a scanning electron microscope to measure the energy of titanium Kα (4.5 keV) fluorescence x rays with a resolution better than 14 eV full width at half‐maximum. Using two other devices, we have measured an energy resolution for Joule heat pulses of 2.6 eV at 1 keV and 0.2 eV at 4 eV, the best reported for any calorimeter. An electrical noise equivalent power of 3×10−18 W/√Hz was also measured, suggesting the use of these detectors as infrared bolometers.