A unit of length used to specify the wavelength of x rays. Symbol, X, Xu or xu. Also known as a siegbahn. The length of the X-unit was set at 1/3029.04 of the spacing of the (200) planes of calcite at 18°C.1 This value was chosen with the intention of making the X unit approximately 10-13 meters, but more recent measurements show it is about 2 parts per thousand larger than that.
The copper x unit is based on taking the wavelength of the Kα1 line of copper to be exactly 1537.400 xu. According to the 1986 CODATA recommendations, it is equal to 1.002 077 89 × 10−13 meters with a one-standard-deviation uncertainty of ± 0.000 000 70 × 10-13 meter (relative uncertainty, 0.70 parts per million).2 Its symbol is xu(CuKα1).
The molybdenum x unit is based on taking the Kα1 line of molybdenum to be exactly 707.831 xu. According to the 1986 CODATA recommendations, it is equal to 1.002 099 38 × 10-13 meters with an uncertainty of ± 0.000 000 45 × 10−13 meter (relative uncertainty, 0.45 parts per million).2 Its symbol is xu(MoKα1).
Around 1965 some workers began to use the A* unit in place of the X unit.
According to the current national standard in the United States3, the X unit is not to be used. The meter should be used instead.
1. Manne Siegbahn.
“Röntgenspektroskopische Präzionsmessungen. (Erste Mitteilung).”
Annalen der Physik. 4th series, volume 59, page 56.
Leipzig: Verlag von Johann Ambrosius Barth, 1919.
M. Siegbahn.
Spektroskopie der Röntgenstrahlen.
Berlin: Springer-Verlag, 1931.
Pages 42-44.
2. E. Richard Cohen and Barry N. Taylor.
“The 1986 CODATA recommended values of the fundamental physical constants.”
Journal of Research of the National Bureau of Standards, volume 92,
no. 2, page 1. (March-April 1987)
Table 3.
3. IEEE/ASTM SI 10™-2002.
American National Standard for Use of the International System of Units (SI): The Modern Metric System.
New York: IEEE, 30 December 2002.
See Section 3.3.3.
The symbol “Xu” is found in
National Research Council.
A Glossary of Terms in Nuclear Science and Technology.
New York: American Society of Mechanical Engineers, 1955.
Page 187.
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