{{Short description|Unit of length}} {{Infobox unit | symbol = xu | quantity = length | units1 = SI units | inunits1 = {{val|1.0021|e=-13|ul=m}} | units2 = Natural units | inunits2 = {{val|6.2007|e=21}} {{math|<var>ℓ</var><sub>P</sub>}}<br /><!-- -->   {{val|1.8937|e=-3}} ''a''<sub>0</sub> | units3 = imperial/US units | inunits3 = {{val|3.9453|e=-12|ul=in}} }} : ''For the software testing tools, see xUnit.''
The '''x unit''' (symbol '''xu''') is a unit of length approximately equal to 0.1 pm (10<sup>−13</sup> m).<ref>{{GoldBookRef|file=X06719|title=x unit}}</ref> 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 {{frac|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:<ref>{{cite journal |author-first1=Peter J. |author-last1=Mohr |author-first2=Barry N. |author-last2=Taylor |title=CODATA recommended values of the fundamental physical constants: 1998 |journal=Journal of Physical and Chemical Reference Data |volume=28 |issue=6 |pages=1713–1852 |bibcode=1999JPCRD..28.1713M |doi=10.1063/1.556049 |year=1999 |url=https://www.nist.gov/pml/div684/fcdc/upload/rmp1998-2.pdf |archive-url=https://web.archive.org/web/20171001122752/https://www.nist.gov/sites/default/files/documents/pml/div684/fcdc/rmp1998-2.pdf|archive-date=2017-10-01}}</ref> * the '''copper x unit''' (symbol '''xu(Cu Kα<sub>1</sub>)''') is defined so that the wavelength of the Kα<sub>1</sub> line of copper is exactly 1537.400 xu(Cu Kα<sub>1</sub>); * the '''molybdenum x unit''' (symbol '''xu(Mo Kα<sub>1</sub>)''') is defined so that the wavelength of the Kα<sub>1</sub> line of molybdenum is exactly 707.831 xu(Mo Kα<sub>1</sub>).
The 2006 CODATA recommended values for these units are:<ref>{{cite journal |author-last1=Mohr |author-first1=Peter J. |author-first2=Barry N. |author-last2=Taylor |author-first3=David B. |author-last3=Newell |year=2008 |issue=2 |title=CODATA Recommended Values of the Fundamental Physical Constants: 2006 |url=https://www.nist.gov/pml/div684/fcdc/upload/rmp2006-2.pdf |journal=Reviews of Modern Physics |volume=80 |doi=10.1103/RevModPhys.80.633 |pages=633–730 |bibcode=2008RvMP...80..633M |arxiv=0801.0028 |archive-url=https://web.archive.org/web/20171001121533/https://www.nist.gov/sites/default/files/documents/pml/div684/fcdc/rmp2006-2.pdf|archive-date=2017-10-01}}</ref> : 1 xu(Cu Kα<sub>1</sub>) = {{val|1.00207699|(28)|e=-13|u=m}}, : 1 xu(Mo Kα<sub>1</sub>) = {{val|1.00209955|(53)|e=-13|u=m}}.
== See also == * Ångström * Ångström star
== References == {{reflist}}
Category:Units of length Category:Customary units of measurement Category:X-rays