The Siegbahn notation is used in X-ray spectroscopy to name the spectral lines that are characteristic to elements.
For these reasons, International Union of Pure and Applied Chemistry (IUPAC) recommends another nomenclature.
The use of the letters K and L to denote X-rays originates in a 1911 paper by Charles Glover Barkla, titled The Spectra of the Fluorescent Röntgen Radiations[1] ("Röntgen radiation" is an archaic name for "X-rays").
By 1913, Henry Moseley had clearly differentiated two types of X-ray lines for each element, naming them α and β.
[2] In 1914, as part of his thesis, Ivar Malmer (sv:Ivar Malmer), a student of Manne Siegbahn, discovered that the α and β lines were not single lines, but doublets.