X unit

For the software testing tools, see xUnit.

The x unit (symbol xu) is a unit of length approximately equal to 0.1 pm (10 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 ⁄_3029.04 of the spacing of the (200) planes of calcite at 18 °C.