More accurate X-ray measurements

US scientists have developed a new method to reduce uncertainty in X-ray wavelength measurement, in what is said to be the first major advance since the 1970s in reducing sources of error common in X-ray angle measurement.

The ability to sense X-rays at precise wavelengths has a wide range of applications - from allowing law enforcement to detect and identify trace explosives to helping astrophysicists better understand cosmic phenomena. It all comes down to looking very closely at the X-ray spectrum and measuring the precise position of wavelengths being emitted by the subject. Each material has its own unique X-ray ‘fingerprint’ - but a slight error in angle measurement can skew the results.

“While many fields need good X-ray reference data, many of the measurements that presently fill standard reference databases are not great - most data were taken in the 1970s and are often imprecise,” said Larry Hudson from the National Institute of Standards and Technology (NIST), who co-authored the new study.

X-ray wavelengths are measured by passing the beam through special crystals and very carefully measuring the angle that exiting rays make with the original beam. The crystal is typically mounted on a rotating device that spins the crystal to two different positions where a spectral line is observed.

The angle between the two positions is measured, in a geometry trick which determines the line’s position more precisely than a single measurement would while also cancelling out some potential errors. However, the digital encoder - the device that translates the rotation of the crystal to an angle measurement - remains prone to error.

Writing in the journal Metrologia, Hudson and his colleagues put forth a new approach which uses laser beams bouncing off a mirrored polygon that is rotated on the same shaft that would carry the crystal. In combination with NIST sensing instrumentation and analysis, the method can measure X-ray angles with an uncertainty of 0.06 arcseconds, which is at least three times more accurate than the uncalibrated encoder.

A laser from the NIST-designed autocollimator (square device at top) is beamed at the mirrored polygon in the grey 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 bottom takes measurements that minimises the wobbling the polygon experiences while spinning. Credit: Hudson/NIST.

“If a giant windshield wiper stretched from Washington DC 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,” Hudson explained.

Calibrating measurement devices to greater precision will provide better understanding of a host of newly designed materials, which often have complicated crystal structures that give rise to unusual effects such as high-temperature superconductivity. The new method will also permit better understanding of the relationship between the structures and properties of novel materials.

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