X unit
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The X-unit (symbol xu) is a unit of length formerly used to measure the wavelength of X-rays and gamma rays. The X-unit is approximately 1.0021×10−13 metre. Defined by the Swedish physicist Karl Manne Georg 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 1/3029.04 of the spacing of the (200) planes of calcite at 18 °C.
The copper X-unit (symbol xu(CuKα1)) is calibrated by setting the wavelength of the Kα1 line of copper at exactly 1537.400 xu. This works out to 1.002 077 89×10−13 ± 7.0×10−20 m. Similarly, the molybdenum X-unit (symbol xu(MoKα1)) sets the Kα1 line of molybdenum at exactly 707.831 xu, which works out to 1.002 099 38×10−13 ± 4.5×10−20 m.
Manne 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 metres, and he succeeded admirably. In his honour, the unit was also called the siegbahn.
Around 1965 the old ångström (Å) started to replace the siegbahn in common usage. Today, in SI units, 1 Å is 0.1 nanometre or 100 picometres or 10−10 metres.
