High Quality Content by WIKIPEDIA articles! The x unit (symbol xu) is a unit of length approximately equal to 0.1 pm (10?13 m). It is used to quote the wavelength of X-rays and gamma rays. Originally defined by the Swedish physicist Manne 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. A unit of measurement is a definite magnitude of a physical quantity, defined and adopted by convention and/or by law, that is used as a standard for measurement of the same physical quantity. Any other value of the physical quantity can be expressed as a simple multiple of the unit of measurement.

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