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news release:
Copper Ridges Nearly Double X-ray Sensor
Performance
Gaithersburg, MD, Nov 17: A series of copper ridges nearly doubles the
resolution of experimental X-ray sensors, enabling more precise identification
of the X-ray "fingerprints" of different atoms, researchers at the
National Institute of Standards and Technology (NIST) report. The sensors are
expected to be powerful tools for astronomy, such as in determining the
temperature and motion of matter in space, and for semiconductor materials
analysis, helping to differentiate between nanoscale contaminant particles on
silicon wafers.
The new design, described in the Nov. 7 issue of Applied Physics Letters,* can
measure X-ray energies with an uncertainty of only 2.4 electron volts (eV),
breaking through a long-standing 4.5 eV plateau in the performance of
superconducting "transition edge" sensors (TES). The cryogenic sensors
absorb individual X-rays and measure the energy based on the resulting rise in
temperature. The temperature is measured with a bilayer of normal metal (copper)
and superconducting metal (molybdenum) that changes resistance in response to
the heat from the radiation. The new TES design performs about 40 times better
than conventional X-ray sensors made of silicon and lithium.
The primary design change was the addition of five copper ridges patterned on
the sensor, perpendicular to the current flow, which blunts or softens the
change in resistance from superconducting to normal. NIST holds a patent on the
sensor design concept.** The gentler transition reduces unexplained
"noise" that degrades measurement precision. A second change was a
reduction in device size from 400 to 250 micrometers square, which increases the
rise in temperature caused by the X-rays, to better match the broader
temperature range of the change in resistance.
NIST researchers expect to further improve sensor performance to reach the 2 eV
resolution goal set by the National Aeronautics and Space Administration (NASA).
NASA plans to mount TES sensors on a telescope sent into space on a satellite
mission still under development. Improved sensor performance will enable
scientists to better measure tiny shifts in X-ray frequency caused by the motion
of atoms in space, for example. The latest NIST work was supported in part by
NASA and the NIST Office of Microelectronics Programs.
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