Siegbahn, Karl Manne Georg
SIEGBAHN, KARL MANNE GEORG
(b. Örebro, Sweden, 3 December 1886; d. Stockholm, Sweden, 24 September 1978
Manne Siegbahn modernized Swedish physics. He established experimental research schools at Lund, Uppsala, and Stockholm that commanded international attention; he also helped introduce the organization and methods of “big science” into Sweden. The research program that dominated most of his career and through which Swedish experimental physics was brought into the mainstream of international physics required perfecting X-ray spectroscopic instrumentation and measurement. His research problem may have been narrowly defined, but he chose it well and executed it to near perfection. His career also shows the importance for Swedish science of the Nobel Prize.
Siegbahn’s father, Nils Reinhold Georg Siegbahn, a stationmaster for the Swedish state railway, came from a family with strong traditions in civil service and in high-precision work. His mother was Emma Zetterberg. Siegbahn first considered an education leading to a career as a military engineer but found military life not to his liking. After his father retired, the family moved to Lund, where in 1906 Siegbahn enrolled at the university; he received the candidate’s degree in 1908, the licentiate degree in 1910, and in 1911, at the age of twenty-five, a doctorate in physics.
Siegbahn was immediately named a docent in physics; from 1915 to 1920 he assumed the duties of professor during Johannes (Janne) Rydberg’s prolonged illness. In January 1920, after Rydberg’s death, he was named professor. By this time he had established a research group working on X-ray spectroscopy. He also had married Karin Evelina Högbom in 1914; they had two sons.
When the professor of physics at Uppsala University, Gustaf Granqvist, died suddenly in 1922, Siegbahn was confronted with a choice: to remain in Lund, close to Copenhagen and the Continent, or move to Uppsala, then Sweden’s primary university and physics department. He chose Uppsala, but within a few years recognized the importance of having both a well-staffed modern laboratory and a research position free from teaching and administrative chores.
Siegbahn finally achieved his goal in 1937, when he received a personal research professorship with which to lead the Royal Swedish Academy of Sciences’ new Nobel Institute of Experimental Physics. He held the post of professor until 1952 and remained as director until he retired in 1964. Here he developed a school of nuclear physics. He joined the Nobel Physics Committee when he moved to Uppsala in 1923 and served as its chairman from 1945 to 1947. In 1925 he was awarded the previously withheld 1924 Nobel Prize in physics. (There had been no suitable candidates in 1924.) Several foreign universities and academies bestowed honors upon him. He received honorary doctorates from the universities of Freiburg, Bucharest, Oslo, and Paris; was elected member of the royal societies of London and Edinburgh, the Paris Academy of Sciences, and the U.S.S.R. Academy of Sciences; and was awarded the Hughes Medal (1934), the Rumford Medal (1940), and the Duddell Medal (1948). From 1938 to 1947 he was president of the International Union of Physics.
Siegbahn began his research career as an assistant to Rydberg but did not develop a disciple-master relationship with the rather reclusive professor. His early choice of problems reflected his technological predilections; he began with a series of electrotechnical projects. In his doctoral dissertation he treated methods for measuring magnetic fields. Siegbahn early recognized the importance of international contacts; he studied at Göttingen (1908) and Munich (1909). He made shorter study trips to Paris and Berlin (1911) and to Paris and Heidelberg (1914).
At Munich, Siegbahn met Arnold Sommerfeld, who introduced him to problems related to X radiation. In 1914 he published preliminary articles in Swedish and German defining the area that he staked out for his future endeavors: X-ray spectroscopy. Although he published six articles on electrotechnical studies immediately thereafter, in 1915 he became totally devoted to X-ray spectroscopy. Before leaving Lund in 1923, he had published more than thirty articles in this area, either alone or with his assistants.
Although laboratory facilities at Lund were meager, Siegbahn was nevertheless well placed for assuming a major position in X-ray spectroscopic studies. Rydberg, originally a mathematician, had devoted much of his career to the relations between spectra and Dmitri Mendeleev’s periodic system for chemical elements. Moreover, Anders and Knut Ångström, Robert Thalén, and Bernhard Hasselberg had contributed to making spectroscopy the dominant feature of Swedish physics.
Beginning in 1906, Charles Glover Barkla had studied the polarization of X rays and discovered characteristic radiation from different elements. That is, when substances are exposed to X rays, they emit a secondary radiation with a specific penetrative power characteristic of the element concerned. Barkla distinguished two components in this secondary radiation that he called K and L. The significance of these radiations became apparent once new instruments for studying X radiation and their spectra were devised by the Braggs and Maurice de Broglie. William Henry Bragg and William Lawrence Bragg used an ordinary X-ray tube with a goniometer, in which a crystal of rock salt was mounted on the rotating table. To register the “reflected” X-ray beam, they placed an ionization chamber on a turntable arm. By devising focusing methods, they showed the distribution of the intensity as a function of the angle of incidence, in two or three orders, three broad peaks on a background of the “white” X-ray spectrum.
In France, Maurice de Broglie used a fixed photographic plate to register the X rays reflected by a carefully rotated crystal. He thereby produced X-ray spectra with sharp, well-defined lines similar to those obtained from optical spectra. These spectral lines could be identified with Barkla’s K and L radiations.
Just prior to World War I, Henry Moseley showed in his studies of the K and L spectra of sets of consecutive elements that the square root of the frequencies of the lines progressed linearly with the atomic number. To record the spectra of the softer L radiation, which is easily absorbed in air, Moseley introduced a vacuum spectrometer.
The implications of the new techniques for physics and chemistry were further emphasized in 1914 when Walther Kossel offered an interpretation of the spectral lines in light of Niels Bohr’s new atomic model. Because X-ray spectra are so much simpler than the thousands of lines that characterize optical spectra, they could be better instruments for identifying chemical elements. However, to fulfill X-ray spectroscopy’s promise as a tool for atomic and chemical research, an increased resolution in the registered spectra would be necessary, as would an increase in the precision of wavelength measurement for both the emission spectra and the absorption discontinuities, or edges.
Starting with de Broglie’s method and Moseley’s program for mapping spectral lines, Siegbahn set out to perfect X-ray spectroscopy. In this research he was assisted in becoming the leading investigator by two factors. First, the war interrupted the work of most investigators in the field, including the Braggs and de Broglie, who did war-related research; Moseley, the most promising of all, was killed at Gallipoli. Second. Siegbahn possessed an extraordinary ability to improve, design, and build instruments. Although Siegbahn was in early contact with Bohr and had planned to go to Rutherford’s laboratory at Manchester in 1915, he showed little interest in engaging in the theoretical implications or physical interpretation of X-ray spectra. He did, however, understand the significance of mapping the X-ray spectrum. When Siegbahn received a copy of Sommerfeld’s 1916 article, in which elliptical orbits and additional quantum conditions were introduced into an atomic model, he immediately sought and found the predicted relativistic doublet K transitions. Siegbahn’s continued contact with Sommerfeld, as well as with Bohr and Kossel, provided impetus to repeated improvement of the precision of wavelength measurements and to extension of these measurements to as wide a range of elements as possible.
Siegbahn began by constructing a metal X-ray tube that he and Ivar Malmer used in a study of the K series of heavy elements (zirconium to neodymium). They showed in 1915 that each of the two K lines identified by Mosely (Kα and Kβ) actually consists of a doublet. Karl Wilhelm Stenström extended the K series in 1916 to lighter elements (down to sodium), using improved Siegbahn tubes. To extend Moseley’s study of the L series, for which four lines had been identified, Siegbahn constructed improved vacuum spectrometers and X-ray tubes to reduce absorption by air and, especially for lighter elements, by the wall of the tube. He worked with Einar Friman (1916) in a study of the L series for zinc to uranium. They extended the longest recorded wavelength from Moseley’s 6 Ångstrom units to 12.8 Ångström units, and increased the precision by two orders of magnitude. The surprisingly complicated line structure was systematically analyzed in Friman’s 1916 doctoral dissertation.
In 1916 Siegbahn also identified a new series of lines for heavy elements that, following the Barkla system, he called the M series. Stenström then began a detailed investigation of this series, which was repeated with a still better X-ray tube and spectrograph by Elis Hjalmar for his dissertation (1923). Hjalmar also measured five N lines for uranium and thorium, and one for bismuth, after this series had been identified in 1922 by Siegbahn and V. Dolejsek (as predicted by Bohr).
Additional important discoveries by Siegbahn’s Lund school included J. Bergengren’s work (1920) showing that the position of the K absorption edge of phosphorus depends upon the atom’s allotropic modifications. Axel Lindh continued this line of inquiry. Torsten Heurlinger commenced pioneering work on band spectra (1918) that was continued after his illness by Erik Hulthén. Dirk Coster, one of the many foreigners who came to Lund, learned the methods of X-ray spectroscopy while working there from 1920 to 1922. He made major contributions by clarifying the relation of the X-ray spectral lines to Bohr’s theory of atomic structure and the periodic table of elements. Coster then joined Bohr’s group in Copenhagen (1922-1923), where he and György Hevesy used Siegbahn-built equipment to identify element 72 (hafnium).
To increase substantially the accuracy in measuring wavelengths so as to meet the needs of atomic theory, Siegbahn began in 1918 to build spectrometers for different wavelength regions. These spectrometers increased precision and certainty to the point that the greatest errors in the measurements arose from uncertainty in the lattice constants of the crystals used. As long as the same crystal is used in all measurements, the relative values of the wavelengths are not influenced by this uncertainty. By measuring the angle of reflection with high accuracy, using a double-angle method, Siegbahn increased the precision about a hundredfold over the earlier wavelength determinations.
Siegbahn introduced a new unit of wavelength, the X unit, which is roughly one one-thousandth of an Ångström unit. First defined on the basis of the lattice constant of rock salt, the unit was later (1919) specified more precisely by the lattice constant of calcite for the cleavage surface (3029.04 X units at 18°C). Siegbahn used this value as the basis for his laboratory’s program, beginning in the early 1920’s, to remeasure the K, L, and M series and the K and L absorption edges with extreme precision.
Siegbahn was assisted in the construction of instruments first by the “old mechanic” Alfred Ahl-strom, who lived in a one-room work shed and charged 1.25 crowns for any repair, regardless of how much time was required. Eventually the Physics Institute hired A. L. Pedersen, a metalworker who was able to build X-ray tubes and spectrometers. Siegbahn sketched design diagrams for instruments and then discussed construction details at the workshop. At times he made modifications himself. He repeatedly improved X-ray tubes, spectrometers, and vacuum systems. He was able to increase the intensity of radiation considerably by building metal X-ray tubes with hot cathodes; Lindh perfected these in the early 1920’s.
Two additional factors enabled Siegbahn to establish a vital school in Lund. After a series of reforms in intermediate schools in the early 1900’s, enrollment at the university increased dramatically. At the Physics Institute the number of students taking laboratory instruction rose from about fifteen in 1906 to sixty-two in 1912. While Siegbahn was assisting Rydberg, he instilled enthusiasm for physics in many of these students. They followed Siegbahn, choosing to write doctoral dissertations under his supervision. After 1914 the number of undergraduate students dropped as dramatically as it had risen. At this point, room could be made in the modest laboratory for research projects; moreover, when Siegbahn assumed professorial duties, he broke with Rydberg’s tradition of lecturing. He delegated teaching duties to an assistant while he supervised the advanced doctoral investigations.
After the war Siegbahn made study trips abroad to obtain insight into the requirements for better X-ray spectroscopic measurements and to bring his improvements and results to the attention of others. Although he did not direct his school toward theoretical problems, he believed it important for experimental physicists to follow the relevant advances in theory. Unlike most Swedish physicists, he was not hostile to the new atomic physics. In 1919 he organized a conference in Lund at which he brought Bohr and Sommerfeld together. Siegbahn participated in the third international Solvay Conference in 1921. Even as he and his group started remeasuring all the known lines and edges, he began in 1923 to prepare a compendium of his school’s work; he was now the international leader in the field. The German University at Prague offered him a professorship. Siegbahn declined even though the crowded conditions in the Lund laboratory created obstacles. He could not, however, so easily decline the call to Uppsala in 1922.
In a remarkably short period of time following his move in June 1923, Siegbahn set his research program in motion at the larger, more modern physics institute at Uppsala. He brought to Uppsala not only an experimental research program that could command international attention but also a new style of leadership and a new attitude toward research. As he had done at Lund, he tried to minimize formalities, fuss, and meaningless rituals, and to squeeze as much research as possible from the relatively meager resources. Obtaining results was all that mattered. He did not hesitate to give the responsibility for an important problem to a promising doctoral student; if parts and materials were not otherwise at hand, he allowed their being scavenged from beautifully constructed older instruments.
His style of leadership enabled Siegbahn to attract a constant stream of students and assistants. The institute’s tool shop manufactured remarkably accurate and innovative instruments that made possible the fruitful continuation of Siegbahn’s program for remeasuring X-ray wavelengths. Machinists and precision metalworkers who constructed the instruments, such as John Amberntsson, whom Siegbahn brought from Lund, and Ernst Tingval were crucial in the continued march toward greater precision and further extensions of the spectral wavelengths.
In its drive toward ever greater precision, Siegbahn’s school was helped by a number of technical facilities instituted by its leader: a recording microphotometer for registering a spectrogram’s intensity, a ruling engine for obtaining gratings up to five millimeters, a machine for making gratings up to ten centimeters wide, and high-vacuum pumps. Increased intensity revealed more lines in each series; Siegbahn’s school also measured the frequencies of spark lines and of “forbidden” transitions between the M and N series. Considerably greater detail in the absorption spectra was found, most notably by Edvin Jönsson. Using Siegbahn’s gratings and suggestion, Bengt Edlén and others at Uppsala studied the optical spark spectra in the ultraviolet region, photographically recording down to 10 Ångström units. Siegbahn’s team managed to extend the long-wave limit of X-ray spectroscopic registrations in the K, L, M, and N series to 400 Ångström units. The two spectral regions were thereby bridged.
In 1924, acting on the initiative of the young Uppsala physicist Ivar Waller, Siegbahn and several co-workers demonstrated the long-sought refraction of X rays through a prism. Axel Larsson (Nordhult) continued Stenström’s earlier studies of deviations from Bragg’s equation, now with sufficient precision to be able to calculate the deviations, and thereby the dispersion. In this manner they could show that the expression obtained according to the theory of X-ray interference in crystals does not cover the whole deviation, and that an anomalous dispersion varying with wavelength also occurs.
This work was connected with Siegbahn’s efforts toward the absolute determination of the X-ray scale. In a related effort, Erik Bäcklin used gratings to obtain absolute values for selected wavelengths that proved to be 0.15 percent lower than the corresponding values produced using crystals. This result implied that some factors used in calculating the crystal lattice constant had to be in error. Using Gunnar Kellström’s new determination of the viscosity of air and Sten von Friesen’s measurements of electron wavelengths, Bäcklin was able to show that Robert Millikan’s determination of the electron charge, e, was too low because of an erroneous value for the coefficient of the air’s inner friction. Using a new determination for e in calculating lattice constants, Bäcklin then obtained, within the calculated limits of error, the same values for X-ray wavelengths determined by crystal as by absolute measurements. Consequently, Siegbahn’s earlier wavelength scale now lacked justification.
About 1930 Siegbahn recognized that his research program had reached a point of professional diminishing returns. He had brought Swedish experimental physics a long way toward internationalization and recognition; he understood that further institutional and organizational changes would be necessary to continue this process and to allow renewal as the frontiers and problems of physics changed. He was, however, frustrated over administrative burdens, teaching duties, and rather limited resources.
After receiving the Nobel Prize in physics, Siegbahn used his increased authority within the five-member Nobel Physics Committee to obtain resources for his research. Eventually he obtained a new institute, but this exercise in amassing resources and authority did not go unnoticed or unchallenged. First, his Nobel Prize was not without controversy. Siegbahn was nominated in 1923 by O. D. Khvol’son, but he withdrew his name from competition. In 1925 he was nominated by Stephan Meyer, Max von Laue, and David Starr Jordan (acting on David Locke Webster’s recommendation) for his precision X-ray spectroscopic measurements. Committee members Svante Arrhenius and Vilhelm Carlheim-Gyllenskold opposed awarding the prize to Siegbahn, not because they felt his work was not significant but rather (they alleged) because the nomination did not meet the statutory requirements: a prize can be given only for a new invention or discovery (or an old one newly shown to be important). Also, since Siegbahn and his school were in the midst of remeasuring wavelengths, an award would have violated the committee’s tradition of waiting for work to be completed before assessing its full significance. In fact, Arrhenius and Carlheim-Gyllenskold hoped to limit the authority of Siegbahn and Uppsala physics as well as to withhold the prize and divert the prize money to the committee’s own fund. Uppsala Nobel Committee members C. W. Oseen (chair) and Allvar Gullstrand pressed successfully on behalf of their colleague.
After receiving the Nobel Prize, Siegbahn was able to claim a major portion of the committee’s fund for his research. As important as these grants may have been for buying instruments or paying assistants, however, Siegbahn required considerably greater resources. Having watched with some envy how The Svedberg had used his 1926 Nobel Prize for chemistry to attract large sums of money for his ultracentrifuge and a new institute of physical chemistry, Siegbahn launched a similar plan in 1930.
The Knut and Alice Wallenberg Foundation was willing to donate half of the 3 million crowns needed if the rest could be obtained from other sources. Svedberg had been able to attract Rockefeller Foundation funding; Siegbahn could not. He hoped to establish a Nobel Institute for Experimental Physics, but the funds available to the committee were too meager. He therefore supported, partly as a substitute, the creation in 1933 of the Nobel Institute for Theoretical Physics for his colleague Oseen.
A new effort was begun in 1935; Oseen was then the Swedish Academy’s president, and Henning Pleijel, another member of the Nobel Physics Committee, was the academy’s secretary. With the assistance of academy member Gösta Forssell, who used X rays and radioactive substances for therapeutic purposes, they convinced the academy to petition the government to establish a personal research professorship for Siegbahn. The academy had terminated its physics institute and professorship more than a decade earlier for financial reasons; here was an opportunity to revive them.
Further, Siegbahn’s supporters arranged that grants from funds at the disposal of the Nobel Physics Committee should be used for planning and building the institute. Some observers in the Swedish Academy protested that the diversion in 1935 of the money from the reserved 1934 prize to the committee and the Nobel Foundation’s funds was based not on the lack of qualified candidates for the prize but on a desire by the committee and Siegbahn’s supporters to funnel as much Nobel money as possible into the project. The Wallenberg Foundation agreed to pay much of the remaining costs: instruments, furnishings, and operating expenses. In 1937 the Nobel Institute for Experimental Physics was established under Siegbahn’s leadership.
Interest in supporting the plan was perhaps heightened by Siegbahn’s aims for the new institute: he hoped to introduce nuclear physics into Sweden. Even before the institute opened, he sent Sten von Friesen to Cornell University and the University of California at Berkeley for a four-month study of cyclotron construction. By 1939, thanks to an additional grant from the Wallenberg Foundation, they inaugurated a 7 MeV deuteron cyclotron with which they hoped to pursue research and to produce radioactive isotopes for medical use.
Sweden’s postwar commitment to atomic energy enabled Siegbahn to develop the institute into one of the major nuclear research facilities in Europe. In 1946 Hugo Atterling and Gunnar Lindström began construction of a larger cyclotron, aided by Rockefeller Foundation money; when completed in 1951, it was able to accelerate deuterons up to 30 MeV. A provisional high-tension generator capable of producing 400,000 volts was built during the war, and then transformed into a plant with 1.5 million volts. Other facilities that were added to the institute included an electromagnetic isotope separator and various nuclear spectrographs.
Although Siegbahn was increasingly becoming an administrator, both of his institute and of several national committees, he still found time to construct new instruments, including an electron microscope. He directed his group to the study of nuclear radiations, to the exact measurement of the magnetic properties of atomic nuclei, and to such other projects as Hannes Alfvén’s experiments on cosmic radiation. Siegbahn’s son Kai instituted a research program in β spectroscopy that eventually made Stockholm the international center for such studies.
Using his international contacts, Manne Siegbahn was able to send Swedish students and assistants to major foreign universities and laboratories; he also attracted or invited foreign researchers to Sweden. In the course of his career, and largely as a result of it, Swedish physics emerged as an important component of the international discipline.
I. Orginal Works. Bibliographies of Siegbahn’s writings are in Åke Dintler and J. C. Sune Lindquist, eds., Uppsala universitets Matrikel 1937-1950 (Uppsala, 1953); and in Poggendorff, V, 1162-1163, VI. 2443, and VIIb, 4857-4858. His Spektroskopie tier Röntgenstrahlen (Berlin, 1924; 2nd, rev. ed. 1931), translated by George A. Lindsay as The Spectroscopy of X-Rays (Oxford, 1925; reiss. Ann Arbor, Mich., 1976), contains many of his school’s results, including a chronologically arranged bibliography of these and related advances in X-ray spectroscopy.
II. Secondary Literature. Olle Edqvist, “Manne Siegbahn”, in Kosmos (1987), 163-176; Sten von Friesen, “Manne Siegbahn: Minnesteckning”, in Kungligka fysiografiska sällskapets i Lund årsbok (1979), 75-81; Arvid Leide, Fysiska institutionen vid Lunds imivershet (Lund, 1968), 136-146; Axel Lindh, “En svensk nobelpristagare”, in Kosmos (1925-1926), 5-63 (a detailed account of the Siegbahn school’s instruments and methods); and Torsten Magnusson, “Marine Siegbahn,” in Swedish Men of Science 1650-1950 (Stockholm, 1950), 280-291, and Manne Siegbahn 1886-3/12/ 1951 (Uppsala, 1951). Further historical background into early X-ray spectroscopy can be found in J. L. Heilbron, H. G. J. Moseley: The Life and Letters of an English Physicist 1887-1915 (Berkeley, 1974); and B. R. Wheaton, The Tiger and the Shark: Empirical Roots of Wave-Particle Dualism (Cambridge, 1983).
Robert Marc Friedman