Analytical and Laboratory

Improving the Accuracy of X-Ray Measurements

12 May 2017

NIST’s X-ray machine for the measurement of the Copper Alpha spectrum. Source: NISTNIST’s X-ray machine for the measurement of the Copper Alpha spectrum. Source: NISTResearchers at the National Institute of Standards and Technology (NIST) have developed a new machine that can precisely measure a spectrum of X-rays.

The machine, which took 20 years to develop, will make accurate measurements of materials for use in everything from infrastructure to new drugs in development. The device will also ensure measurements of materials from other labs are reliable.

“The wavelength of an X-ray is a ruler by which we can measure spacings of atoms in crystals," says Marcus Mendenhall, a researcher at NIST. “We now know the length of our ruler better, and all kinds of materials can now be measured with improved accuracy.”

The device will allow researchers to link measurements of the lattice spacings with greater confidence to the definition of the meter in the International System of Units (SI), which allow for quality assurance at the smallest levels.

The measurements were consistent with results from the past 40 years while researchers were able to find new details about the X-ray spectrum. X-rays are used for more than just health care and can help identify and characterize common substances such as cement, metals, ceramics, electronics, and medicines. X-rays are also used to view atomic structures of substances, a method called diffraction.

The high-precision machine will play a large role in future industry and academic research at NIST in the realm of quality assurance programs and to verify the accuracy of specific measurements. The machine will also help in developing reference values needed for calibrating laboratory X-ray instruments worldwide, NIST says.

How It Works

The new device produces K-alpha lines of copper X-rays that are no different from those produced by other machines. They are produced by firing electrons at a copper target. The difference is that NIST’s machine can scan a full circle around the sample with improved accuracy. The machine is also equipped with an X-ray camera to give more information than traditional X-ray machines. It also provides self-consistency checks for alignment of the sample and reduces systemic uncertainties, researchers say.

The machine was constructed in a subterranean laboratory with closely controlled temperature, which researchers say allows for accurate measurements. A goniometer was also installed in the machine for use in measuring the angles between the faces of crystals that make up typical samples of solid materials.

Using the circle closure method repeatedly—that uses multiple comparisons of the differences between two or more angular scales—the process is rotated with respect to each other to determine the measurement uncertainties in each scale. This, in combination with the wide scan range, brings accuracy measurements of the angle between the crystals and the X-ray spectrum, without disturbing crystal alignment, researchers say.

"The goal was not to make a machine that the rest of the world and commercial entities can imitate and make themselves, but rather, to make a machine that can give everyone the best answer to measurement questions," Mendenhall says.

The complete research findings can be found in the Journal of Physics B: Atomic, Molecular and Optical Physics.

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