For the software testing tools, see xUnit. 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 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 xu(Mo Kα1) = . See also Ångström Ångström star References Units of length Customary units of measurement X-rays