Science Constants - Unit, Siegbahn Symbol: UX X Value: 1.00020778970 × 10-13 m Category: - Units of Length Description: The Siegbahn Unit was originally used in X-Ray diffractometry and atomic spectroscopy. The value was defined by K.N.G. Siegbahn (1886 - 1977) in 1925 as the distance between lattice planes with the Miller index (200) of a calcite crystal (Island Spath) measured by X-Ray diffraction using a CuKα1 spectral line. This distance is equal to 3029.45 UX