The X-unit (symbol xu) is a unit of length formerly used to measure the wavelength of X-rays and gamma rays. The X-unit is approximately 1.0021×10-13 metres. Defined by the Swedish physicist Karl Manne Georg 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.
The copper X-unit (symbol xu(CuKα1)) is calibrated by setting the wavelength of the Kα1 line of copper as exactly 1537.400 xu. This works out to 1.002 077 89×10−13 ± 7.0×10−20 m.
Similarly, the molybdenum X-unit (symbol xu(MoKα1)) sets the Kα1 line of molybdenum as exactly 707.831 xu, which works out to 1.002 099 38×10-13 ± 4.5×10−20 m.
Siegbahn, who received the Nobel Prize in Physics in 1924 for his work in X-ray spectroscopy, aimed to define a unit equal to approximately 10-13 metres, and he succeeded admirably. In his honour the unit was also called the Siegbahn.
Around 1965 the old Ångström (Å*) started to replace the Siegbahn in common usage.