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 ("Röntgen radiation" is an
archaic name for "X-rays"). These letters, in the middle of the alphabet, were chosen over A and B to allow for the possibility that further series of X-rays, both more and less penetrating, would subsequently be discovered. By 1913,
Henry Moseley had clearly differentiated two types of X-ray lines for each element, naming them α and β. 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. In 1916, Siegbahn published this result in the journal
Nature, using what would come to be known as the Siegbahn notation. ==Correspondence between the Siegbahn and IUPAC notations==