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A D V A N C E D M A T E R I A L S & P R O C E S S E S | A P R I L 2 0 1 5
1 0 TESTING | CHARACTERIZATION
NEW TECHNIQUE REDUCES
UNCERTAINTY IN X-RAY
MEASUREMENTS
Scientists at the National Insti-
tute of Standards and Technology
(NIST), Gaithersburg, Md., developed
a new method to reduce uncertainty
in x-ray wavelength measurement that
could provide much anticipated im-
provements. Accurate measurement
of x-ray wavelengths depends on the
ability to measure angles very precise-
ly and with very little margin for error.
The new approach is reportedly the
first major advance since the 1970s in
reducing certain sources of error com-
mon in x-ray angle measurement.
X-ray wavelengths are measured
by passing a beam through special
A laser from the NIST-designed autocollimator (square device at top) is beamed at
the mirrored polygon in the gray circle at left, and its reflection allows the angle of
the polygon’s faces to be precisely determined while the polygon rotates. The black
device at the bottom takes measurements that minimize wobbling by the polygon
while spinning. Courtesy of Hudson/NIST.
crystals and carefully measuring the
angle that exiting rays make with
the original beam. The crystal is typ-
ically mounted on a rotating device
that spins the crystal to two different
positions where a spectral line is ob-
served. The angle between the two
is measured, which determines the
line’s position more precisely than a
single measurement would and also
cancels out some potential errors.
However, one limit is the accuracy
of the digital encoder, which trans-
lates the crystal’s rotation to an angle
measurement.
Larry Hudson and his team found
a way to dramatically reduce the er-
ror in that measurement. Their new
approach uses laser beams bouncing
off a mirrored polygon that is rotated
on the same shaft that would carry
the crystal. The approach allows the
team to use additional mathematical
shortcuts to their advantage. With new
NIST sensing instrumentation and
analysis, x-ray angles can now be mea-
sured routinely with an uncertainty of
0.06 arcsec—an accuracy more than
three times better than an uncalibrat-
ed encoder.
Hudson describes this reduction
as significant enough to set world re-
cords in x-ray wavelength measure-
ment. “If a giant windshield wiper
stretched from Washington to New
York City (364 km) and were to sweep
out the angle of one of these errors, its
tip would move less than the width of
a DVD,” he says. Calibrating measure-
ment devices to greater precision will
Granta Design, UK, released
new software and services that
support rigorous and auditable
composite qualification process-
es. New data were added to the
existing GRANTA MI: Composites
software package, developed with
input from leading aerospace
companies. The software enables
management of complex data as-
sociated with composite materials,
including layup, cure cycles, direc-
tional properties, and the history
and properties of constituent parts.
BRIEFS
Francisco Rangel won FEI Co.’s (Hillsboro, Ore.) 2014 Image Contest grand
prize for “Expanded Vermiculite,” a hydrated magnesium-aluminum-iron
silicate. Rangel, who works at the National Institute of Technology-INT/
MCTI Characterization Center for Nanotechnology Materials and Ca-
talysis, won airline tickets to London or Washington, a hotel stay, travel
stipend, and two tickets to the IMAX movie, Mysteries of the Unseen World.
.