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 (10⁻¹³ 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 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:^[2]

• 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:^[3]

1 xu(Cu Kα₁) = 1.00207699(28)×10⁻¹³ m,

1 xu(Mo Kα₁) = 1.00209955(53)×10⁻¹³ m.

• Ångström
• Ångström star