The addition of five copper ridges to existing superconducting transition-edge sensors (TES) could prove of great value to astronomers and materials scientists. These enhanced experimental X-ray sensors will help determine the temperature and motion of matter in space, and, in semiconductor material analysis, assist in differentiating among nanoscale contaminants on silicon wafers.
Developed by researchers at NIST, the sensors can measure X-ray energies with an uncertainty of only 2.4 eV. They absorb individual X-rays and derive a measurement of their energy from the resulting rise in temperature. The temperature is measured with a bilayer of copper and molybdenum, a superconducting metal whose resistance changes in response to radiational heating. A coating of bismuth prevents the X-rays from passing through the sensor. The new design offers a performance ~40 × superior to that of conventional X-ray sensors made of silicon and lithium.
Adding copper ridges to a transition-edge sensor increases its sensitivity
The copper ridges are placed on the sensor perpendicular to the current flow, reducing the change in resistance from superconducting to normal. This softer transition serves to damp unexplained noise that degrades measurement precision. Another design alteration reduced the TES size from 400 to 250 mm2 , raising the temperature caused by the X-rays to better match the broader temperature range of the resistance change.
The investigators anticipate further improvements that will allow the sensor to achieve the 2 eV resolution goal set by NASA, which plans to mount TES devices on a space telescope still under development.
NIST holds U.S. Patent No. 6239431 on the sensor design concept. The research was supported by NIST and NASA and described in Applied Physics Letters, November 7, 2005.