The National Institute of Standards and Technology (NIST) has issued a new ruler. Designed to be the most accurate commercially available "meter stick" for the nano world, the new measuring tool — a calibration standard for X-ray diffraction — boasts uncertainties below a femtometer. That's roughly the size of a neutron.
The new ruler is in the form of a thin, multilayer silicon chip 25 millimeters square (just under an inch). Each one is individually measured and certified by NIST for the spacing and angles of the crystal planes of silicon atoms in the base crystal.
Formally NIST Standard Reference Material (SRM) 2000, "Calibration Standard for High-Resolution X-Ray Diffraction", the new ruler gives crystallographers an extremely well-known crystal sample for calibrating their precision instruments. It was made possible by the development of a unique parallel beam diffractometer at NIST that makes measurements traceable to international measurement standards and is believed to be the most accurate angle measuring device of its kind in the world. The NIST instrument can measure angles with an accuracy better than an arc second, 1/3600 of a degree. "Our accuracy is at about the angle made by the diameter of a quarter — if you're looking at it from two miles away," explains NIST materials scientist Donald Windover, "The precision is better, about the size of Washington's nose."
Because the crystal lattice values for SRM 2000 — spacing, tilt, orientation — are traceable to SI units, the new material provides an absolute reference for high-precision calibrations.