# X unit

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{{Short description|Unit of length}}
{{Infobox unit
| symbol   = xu
| quantity = [length](/source/length)
| units1   = [SI units](/source/SI_units)
| inunits1 = {{val|1.0021|e=-13|ul=m}}
| units2   = [Natural units](/source/Natural_units)
| inunits2 = {{val|6.2007|e=21}}&nbsp;{{math|[<var>ℓ</var><sub>P</sub>](/source/Planck_length)}}<br /><!--
    -->&emsp;&emsp;&emsp;{{val|1.8937|e=-3}}&nbsp;[''a''<sub>0</sub>](/source/Bohr_radius)
| units3   = [imperial](/source/imperial_units)/[US](/source/US_customary_units)&nbsp;units
| inunits3 = {{val|3.9453|e=-12|ul=in}}
}}
: ''For the software testing tools, see [xUnit](/source/xUnit).''

The '''x unit''' (symbol '''xu''') is a [unit](/source/Units_of_measurement) of length approximately equal to 0.1&nbsp;pm (10<sup>&minus;13</sup>&nbsp;m).<ref>{{GoldBookRef|file=X06719|title=x unit}}</ref> It is used to quote the [wavelength](/source/wavelength) of [X-ray](/source/X-ray)s and [gamma ray](/source/gamma_ray)s.

Originally defined by the [Swedish](/source/Sweden) physicist [Manne Siegbahn](/source/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](/source/calcite) crystals used in the measuring apparatus. One x unit was set at {{frac|3029.04}} of the spacing of the [(200) planes](/source/Miller_index) of calcite at 18 [°C](/source/Celsius).

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](/source/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](/source/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](/source/K-alpha) of [copper](/source/copper) is exactly 1537.400&nbsp;xu(Cu&nbsp;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](/source/molybdenum) is exactly 707.831&nbsp;xu(Mo&nbsp;Kα<sub>1</sub>).

The 2006 [CODATA](/source/Committee_on_Data_for_Science_and_Technology) 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](/source/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](/source/%C3%85ngstr%C3%B6m)
* [Ångström star](/source/Angstrom)

== References ==
{{reflist}}

Category:Units of length
Category:Customary units of measurement
Category:X-rays

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Adapted from the Wikipedia article [X unit](https://en.wikipedia.org/wiki/X_unit) by Wikipedia contributors ([contributor history](https://en.wikipedia.org/wiki/X_unit?action=history)). Available under [Creative Commons Attribution-ShareAlike 4.0 International](https://creativecommons.org/licenses/by-sa/4.0/). Changes may have been made.