Machine Design

NIST Ruler for Measuring X-ray Diffraction is the Most Accurate in the World

National Institute of Standards and Technology, www.nist.gov

To give crystallographers a reference for calibrating instruments, the National Institute of Standards and Technology developed a measuring device or “meter stick” good down to a femtometer (or 0.000000000000001 m, roughly the size of a neutron). The ruler is actually a square multilayer silicon chip that is 25 mm on a side. Each is measured and certified by NIST for spacing and angles of the crystal planes of silicon atoms in the base crystal. The device, which costs about $1,500, will also measure angles with accuracy better than an arc second. That’s about the angle made by the diameter of a quarter — if you’re looking at it from two miles away.

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