X unit
The x unit is a unit of length approximately equal to 0.1 pm. It is used to quote the wavelength of X-rays and gamma rays.
Originally defined by the Swedish physicist Manne Siegbahn 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 of the spacing of the 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:
- the copper x unit is defined so that the wavelength of the Kα1 line of copper is exactly 1537.400 xu;
- the molybdenum x unit is defined so that the wavelength of the Kα1 line of molybdenum is exactly 707.831 xu.
The 2006 CODATA recommended values for these units are:
