: ''For the software testing tools, see

xUnit xUnit is the collective name for several unit testing frameworks that derive their structure and functionality from Smalltalk's SUnit. ''SUnit'', designed by Kent Beck in 1998, was written in a highly structured object-oriented style, which lent ...

.'' The x unit (symbol xu) is a

unit Unit may refer to: Arts and entertainment * UNIT, a fictional military organization in the science fiction television series ''Doctor Who'' * Unit of action, a discrete piece of action (or beat) in a theatrical presentation Music * ''Unit'' (alb ...

of length approximately equal to 0.1 pm (10⁻¹³ m). It is used to quote the

wavelength In physics, the wavelength is the spatial period of a periodic wave—the distance over which the wave's shape repeats. It is the distance between consecutive corresponding points of the same phase on the wave, such as two adjacent crests, tro ...

X-ray An X-ray, or, much less commonly, X-radiation, is a penetrating form of high-energy electromagnetic radiation. Most X-rays have a wavelength ranging from 10 picometers to 10 nanometers, corresponding to frequencies in the range 30&nb ...

s and

gamma ray A gamma ray, also known as gamma radiation (symbol γ or \gamma), is a penetrating form of electromagnetic radiation arising from the radioactive decay of atomic nuclei. It consists of the shortest wavelength electromagnetic waves, typically ...

s. Originally defined by the

Swedish Swedish or ' may refer to: Anything from or related to Sweden, a country in Northern Europe. Or, specifically: * Swedish language, a North Germanic language spoken primarily in Sweden and Finland ** Swedish alphabet, the official alphabet used by ...

physicist

Manne Siegbahn Karl Manne Georg Siegbahn FRS(For) HFRSE (3 December 1886 – 26 September 1978) was a Swedish physicist who was awarded the Nobel Prize in Physics in 1924 "for his discoveries and research in the field of X-ray spectroscopy". Biography 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 Calcite is a Carbonate minerals, carbonate mineral and the most stable Polymorphism (materials science), polymorph of calcium carbonate (CaCO3). It is a very common mineral, particularly as a component of limestone. Calcite defines hardness 3 on ...

crystals used in the measuring apparatus. One x unit was set at of the spacing of the (200) planes of calcite at 18

°C The degree Celsius is the unit of temperature on the Celsius scale (originally known as the centigrade scale outside Sweden), one of two temperature scales used in the International System of Units (SI), the other being the Kelvin scale. The ...

. 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 X-ray crystallography is the experimental science determining the atomic and molecular structure of a crystal, in which the crystalline structure causes a beam of incident X-rays to diffract into many specific directions. By measuring the angles ...

: * the copper x unit (symbol xu(Cu Kα₁)) is defined so that the wavelength of the Kα₁ line of

copper Copper is a chemical element with the symbol Cu (from la, cuprum) and atomic number 29. It is a soft, malleable, and ductile metal with very high thermal and electrical conductivity. A freshly exposed surface of pure copper has a pinkis ...

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 Molybdenum is a chemical element with the symbol Mo and atomic number 42 which is located in period 5 and group 6. The name is from Neo-Latin ''molybdaenum'', which is based on Ancient Greek ', meaning lead, since its ores were confused with lea ...

is exactly 707.831 xu(Mo Kα₁). The 2006 CODATA recommended values for these units are: :1 xu(Cu Kα₁) = , :1 xu(Mo Kα₁) = .

References

{{reflist Units of length Customary units of measurement X-rays

See also

References