 the x unit (symbol xu) is a unit of length approximately equal to 0.1&nbsp;pm (10&minus;13&nbsp;m). it is used to quote the wavelength of x-rays and gamma rays. originally defined by the swedish physicist manne siegbahn (1886â1978) 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 (200) planes of calcite at 18 Â°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 (symbol xu(cu kÎ±1)) is defined so that the wavelength of the kÎ±1 line of copper is exactly 1537.400 xu(cu kÎ±1); * the molybdenum x unit (symbol xu(mo kÎ±1)) is defined so that the wavelength of the kÎ±1 line of molybdenum is exactly 707.831 xu(mo kÎ±1). the 2006 codata recommended values for these units are: :1 xu(cu kÎ±1) = , :1 xu(mo kÎ±1) = . == see also == * ÃngstrÃ¶m == references == 
Customary_units_of_measurement Units_of_length