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What is an Å* (angstrom star)?



A very small unit of length used to measure the wavelengths of X rays and the distances between atoms in crystals (such distances are measured by shining X rays through the crystal). Symbol, Å* (no period). The unit is read as “angstrom star”.

  1. A. Bearden¹ introduced this unit in 1965to replace the X unit, defining its length by taking the wavelength of the tungstenKα1 line as exactly 0.209 010 0 Å*, a value chosen to make 1 Å* equal to 1 angstrom within 5 parts per million. To quote Bearden:

This numerical value of the wavelength is now proposed for use with the W Kα₁ line to define the x-ray wavelength standard by the relation

λkα1 = 0.20290 10 0 Å*.

This is a new unit of length which may differ from the angstrom by ±5 ppm (probable error), but as a wavelength standard it has no error. In order to clearly indicate that this unit is not exactly an angstrom, it is suggested that it be designated Å*.

Later work has shown that the Å* unit is about 15 parts per million bigger than an angstrom. The value of the unit according to the 1986 CODATA recommendations is 1.000 014 81 × 10⁻¹⁰ meters, with a one-standard-deviation uncertainty of ± 0.000 000 92 × 10⁻¹⁰ meters.²

By 1970, this unit was clearly obsolescent, because:

(1) the availability of nearly perfect synthetic crystals such as silicon which have lattice parameters uniform to parts in 10⁸ or better throughout a macroscopic portion of the crystal, and (2) combined optical and X-ray interferometers which enable a crystal lattice spacing to be determined directly in metres. The potential accuracy of lattice parameter measurements with such combined interferometers is at least parts in 10⁸.