The x unit (symbol xu) is a unit of length approximately equal to 0.1 pm (10−13 m).
[1] It is used to quote the wavelength of X-rays and gamma rays.
Originally defined by the Swedish physicist Manne Siegbahn (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 calcite crystals used in the measuring apparatus.
One x unit was set at 1⁄3029.04 of the spacing of the (200) planes of calcite at 18 °C.
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:[2] The 2006 CODATA recommended values for these units are:[3]