X unit explained

Symbol:	xu
Quantity:	length
Units1:	SI units
Units2:	Natural units
Inunits2:	a0
Units3:	imperial/US units

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:^[1]

• 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:^[2]

1 xu(Cu Kα₁) =,

1 xu(Mo Kα₁) = .

See also

• Ångström
• Ångström star

Notes and References

1. Peter J. . Mohr . Barry N. . Taylor . CODATA recommended values of the fundamental physical constants: 1998 . . 28 . 6 . 1713–1852 . 1999JPCRD..28.1713M . 10.1063/1.556049 . 1999 . https://web.archive.org/web/20171001122752/https://www.nist.gov/sites/default/files/documents/pml/div684/fcdc/rmp1998-2.pdf. 2017-10-01.
2. Mohr . Peter J. . Barry N. . Taylor . David B. . Newell . 2008 . 2 . CODATA Recommended Values of the Fundamental Physical Constants: 2006 . . 80 . 10.1103/RevModPhys.80.633 . 633–730 . 2008RvMP...80..633M . 0801.0028 . https://web.archive.org/web/20171001121533/https://www.nist.gov/sites/default/files/documents/pml/div684/fcdc/rmp2006-2.pdf. 2017-10-01.