X unit

X unit
(Xu)
   a unit of distance formerly used for measuring the wavelength of x-rays and gamma rays. The X unit is approximately 1.0021 x 10-13 meter, 0.001 0021 angstrom, or 100.21 femtometers. The wavelength of these powerful forms of radiation is now measured in picometers (pm) or femtometers (fm). The unit was defined by the Swedish physicist K. M. G. Siegbahn in 1925, at a time when the wavelengths could not be measured directly. The definition was made in terms of the spacing between planes of the calcite crystals used in the measuring apparatus. Siegbahn, who received the Nobel Prize in Physics in 1924 for his work in X-ray spectroscopy, aimed to define a unit equal to approximately 10-13 meter, and he succeeded admirably. In his honor the unit was also called the Siegbahn unit.

Dictionary of units of measurement. 2015.

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