# X unit

1 =
SI units
100.21×10−15 m 100.21×10−6 nm
Natural units
6.2007×1021 P 1.8937×10−3 a0
US customary / Imperial units
328.77×10−15 ft 3.9453×10−12 in

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 13029.04 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 copper X-unit (symbol xu(Cu Kα1)) is defined so that the wavelength of the 1 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:[3]

1 xu(Cu Kα1) = 1.00207699(28)×10−13 m
1 xu(Mo Kα1) = 1.00209955(53)×10−13 m