: ''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). It is used to quote the
wavelength
In physics and mathematics, wavelength or spatial period of a wave or periodic function is the distance over which the wave's shape repeats.
In other words, it is the distance between consecutive corresponding points of the same ''phase (waves ...
of
X-ray
An X-ray (also known in many languages as Röntgen radiation) is a form of high-energy electromagnetic radiation with a wavelength shorter than those of ultraviolet rays and longer than those of gamma rays. Roughly, X-rays have a wavelength ran ...
s and
gamma ray
A gamma ray, also known as gamma radiation (symbol ), is a penetrating form of electromagnetic radiation arising from high energy interactions like the radioactive decay of atomic nuclei or astronomical events like solar flares. It consists o ...
s.
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
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.
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 of determining the atomic and molecular structure of a crystal, in which the crystalline structure causes a beam of incident X-rays to Diffraction, diffract in specific directions. By measuring th ...
:
* the copper x unit (symbol xu(Cu Kα
1)) is defined so that the wavelength of the
Kα1 line of
copper
Copper is a chemical element; it has symbol Cu (from Latin ) 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 pinkish-orang ...
is exactly 1537.400 xu(Cu Kα
1);
* the molybdenum x unit (symbol xu(Mo Kα
1)) is defined so that the wavelength of the Kα
1 line of
molybdenum
Molybdenum is a chemical element; it has Symbol (chemistry), symbol Mo (from Neo-Latin ''molybdaenum'') and atomic number 42. The name derived from Ancient Greek ', meaning lead, since its ores were confused with lead ores. Molybdenum minerals hav ...
is exactly 707.831 xu(Mo Kα
1).
The 2006
CODATA recommended values for these units are:
: 1 xu(Cu Kα
1) = ,
: 1 xu(Mo Kα
1) = .
See also
*
Ångström
*
Ångström star
References
{{reflist
Units of length
Customary units of measurement
X-rays