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This effort specializes in designing, fabricating, and testing
innovative crystal spectrometers for a variety of scientific and technological
applications. One recent design that has found a variety of uses is shown
schematically above. X-rays from a source are dispersed and focussed by a
curved crystal used in transmission through a central, background-suppressing
aperture onto a fixed image plane. Two mirror-symmetric spectra are imaged, in
this case on a size 2 CCD dental sensor, and a spectrum is constituted by
pixel column summing as shown to the right. The distance between identical
features can be converted to energy via a predetermined plate function of the
form E=1/sqrt(a/(x*x) +b) where a and b are constants and x is the
distance from the center of the detector. Relative to a flat crystal instrument,
the curved-crystal design provides insensitivity to source size, enhanced
spectral dispersion and, because of the focusing action of the bent crystal,
increased efficiency.
We recently patented this design for the noninvasive determination of the
applied high voltage of mammographic x-ray sources which is accomplished by
measuring the high-energy cutoff of the continuum spectrum. Since the entire
spectral distribution is registered, the effects of added filtration and target
and window contamination can also be studied. Designs have also been adopted as
core plasma diagnostics for high-power laser studies at the Laboratory for
Laser Energetics
(LLE)
in Rochester, NY and at the National Ignition Facility
(NIF)
at Lawrence Livermore National Laboratory in collaboration with NRL. For more
detailed information see the
NRL
spectroscopy web site. The design has also been applied by other researchers
toward the measurement of the 1s Lamb shift in hydrogenic uranium at
GSI
in Darmstadt, Germany. Additional applications are being pursued, including
characterization of pulsed laser sources used in medical applications. |
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Top: False color image from a quartz-crystal spectrometer acquired
with a size-2 dental CCD with 524 × 760 pixels. A pinhole-camera
image of the focal spot is visible in the center of the image.
Bottom: The pixel column sum plot or spectrum of the image shown
above. This shows the molybdenum spectrum (the alpha lines are
saturated), broad-band continuum radiation, and the image of the
focal spot in the center (shown with an expanded vertical scale).
The high-energy cutoff of the spectrum is marked by arrows and,
through the dispersion function, gives the high-voltage applied to
the x-ray tube. (Larger view) |
Spectra acquired at the NIST EBIT of Ne-like Xe and, using a metal vapor
vacuum arc (MEVVA), He-like Ti.
Inquiries or comments:
larry.hudson@nist.gov