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}} {{math|ℓ_P}}
   {{val|1.8937|e=-3}} a₀

| 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 (10⁻¹³ m).{{GoldBookRef|file=X06719|title=x unit}} 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:{{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}}

• 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:{{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}}

: 1 xu(Cu Kα₁) = {{val|1.00207699|(28)|e=-13|u=m}},

: 1 xu(Mo Kα₁) = {{val|1.00209955|(53)|e=-13|u=m}}.