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) = 1.002 076 99(28) × 10−13 m
- 1 xu(Mo Kα1) = 1.002 099 55(53) × 10−13 m
See also
- Ångström star
References
- ^ International Union of Pure and Applied Chemistry. "x unit". Compendium of Chemical Terminology Internet edition.
- ^ 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:.
- ^ 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. doi:. http://physics.nist.gov/cuu/Constants/codata.pdf.
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