X unit

1 =
SI units
100.21×10^−15 m 100.21×10^−6 nm
Natural units
6.2007×10^21 ℓP 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 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 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

See also

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: 633–730. Bibcode 2008RvMP...80..633M. doi:10.1103/RevModPhys.80.633. http://physics.nist.gov/cuu/Constants/codata.pdf.