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 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:
- the copper X-unit (symbol xu(Cu Kα1)) is defined so that the wavelength of the Kα1 line of copper 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 molybdenum is exactly 707.831 xu(Mo Kα1).
The 2006 CODATA recommended values for these units are:
- 1 xu(Cu Kα1) = 1.00207699(28)×10−13 m
- 1 xu(Mo Kα1) = 1.00209955(53)×10−13 m
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