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For the software testing tools, see .

The x unit (symbol xu) is a unit of length approximately equal to 0.1 pm (10−13 m). It is used to quote the of and .

Originally defined by the physicist (1886–1978) in 1925, the x unit could not at that time be measured directly; the definition was instead made in terms of the spacing between planes of the crystals used in the measuring apparatus. One x unit was set at of the spacing of the of calcite at 18 .

In modern usage, there are two separate x units, which are defined in terms of the wavelengths of the two most commonly used X-ray lines in X-ray crystallography:

  • the copper x unit (symbol xu(Cu Kα1)) is defined so that the wavelength of the of is exactly 1537.400 xu(Cu Kα1);
  • the molybdenum x unit (symbol xu(Mo Kα1)) is defined so that the wavelength of the Kα1 line of is exactly 707.831 xu(Mo Kα1).

The 2006 CODATA recommended values for these units are:

1 xu(Cu Kα1) = ,
1 xu(Mo Kα1) = .


See also
  • Ångström
  • Ångström star

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