Editing X unit

{{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}}&nbsp;{{math|[[Planck length|<var>ℓ</var><sub>P</sub>]]}}<br /><!--
    -->&emsp;&emsp;&emsp;{{val|1.8937|e=-3}}&nbsp;[[Bohr radius|''a''<sub>0</sub>]]
| units3   = [[imperial units|imperial]]/[[US customary units|US]]&nbsp;units
| inunits3 = {{val|3.9453|e=-12|ul=in}}
}}
: ''For the software testing tools, see [[xUnit]].''

The '''x unit''' (symbol '''xu''') is a [[Units of measurement|unit]] 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]] of [[X-ray]]s and [[gamma ray]]s.

Originally defined by the [[Sweden|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 [[Miller index|(200) planes]] of calcite at 18 [[Celsius|°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 [[K-alpha|Kα<sub>1</sub> line]] of [[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]] is exactly 707.831&nbsp;xu(Mo&nbsp;Kα<sub>1</sub>).

The 2006 [[Committee on Data for Science and Technology|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]]
* [[Angstrom#Angstrom_star|Ångström star]]

== References ==
{{reflist}}

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