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Swedish physicist Kai Siegbahn won the 1981 Nobel Prize in physics. He was awarded the prize for his work in developing high-resolution electron spectroscopy.

Karl Manne Georg Siegbahn (1886-1978), Swedish physicist and Nobel laureate. Siegbahn's research in X-rayspectroscopy and his development of instruments for precise measurement of X-ray wavelengths advanced the exploration of atomic structure (see Atom). For his discoveries and research in the field of X-ray spectroscopy, Siegbahn was awarded the 1924 Nobel Prize in physics.

Born in Örebro, Sweden, Siegbahn attended the Physics Institute at the University of Lund, where he received a B.S. degree in 1908, an M.S. degree in 1910, and a Ph.D. degree in 1911. He became a lecturer at Lund in 1911 and from 1922 to 1937 was a professor of physics at the University of Uppsala. From 1937 until his retirement in 1964, he was Research Professor of Experimental Physics at the Royal Swedish Academy of Sciences in Stockholm and the first director of its Nobel Institute of Experimental Physics.

Siegbahn's early work was in electricity and magnetism (see Physics), but by 1914 his attention had turned to X-ray spectroscopy, a technique for investigating the portion of the electromagnetic spectrum that contains X rays—electromagnetic radiation with shorter wavelengths and higher frequencies than those of visible light. X-ray spectroscopy is based on the fact that each element, when bombarded by fast-moving electrons, emits X rays of a characteristic wavelength and frequency. For example, the X rays emitted by calcium will be different from those emitted by iron. The X-ray spectrometer is an instrument that measures and records the wavelengths of the emitted X rays. Siegbahn, a talented instrument designer, improved the X-ray spectrometer in various ways so it detected and measured X rays with more precision and allowed him to discover previously unknown series of X rays.

Siegbahn's work contributed to the understanding of the atom and its structure and supported the prevailing model that electrons were arranged in spherical shells around the nucleus of an atom. His research yielded information about virtually all the elements from sodium to uranium and made possible the analysis of unknown substances. His 1923 Spectroscopy of X Rays was a standard reference, his measurements of X-ray wavelengths were relied upon for their precision, and other physicists adopted his precise instrumentation. Siegbahn's research ultimately resulted in many current applications of X-ray spectroscopy in such diverse fields as nuclear physics, chemistry, astrophysics, and medicine.