# X unit

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1 =
SI units
100.21000157262×10−15 m 100.21×10−6 nm
Natural units
6.2006999026909×1021 P 1.8937×10−3 a0
US customary units (Imperial units)
328.77000515947×10−15 ft 3.9452998924652×10−12 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.

## References

1. ^ IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version:  (2006–) "x unit".
2. ^ Mohr, Peter J.; Taylor, Barry N. (1999). "CODATA recommended values of the fundamental physical constants: 1998". J. Phys. Chem. Ref. Data 28 (6): 1713–1852. doi:10.1103/RevModPhys.72.351.
3. ^ Mohr, Peter J.; Taylor, Barry N.; Newell, David B. (2008). "CODATA Recommended Values of the Fundamental Physical Constants: 2006". Rev. Mod. Phys. 80 (2): 633–730. arXiv:0801.0028. Bibcode:2008RvMP...80..633M. doi:10.1103/RevModPhys.80.633.