Karl Manne Georg The Nobel Prize in Physics 1924

biography

Karl Manne Georg Siegbahn was born on the 3rd of December, 1886, at Örebro in Sweden. His father was Nils Reinhold Georg Siegbahn, a stationmaster of the State Railways, and his mother was Emma Sofia Mathilda Zetterberg. After receiving a high-school education he entered the University of Lund in 1906, where he obtained his doctor's degree, in 1911, on the thesis "Magnetische Feldmessung". From 1907 to 1911 he served as Assistant to Professor J. R. Rydberg in the Physics Institute of the University, afterwards he was appointed lecturer and (in 1915) Deputy Professor of Physics. On the death of Rydberg, he was appointed Professor (1920). In 1923 he became Professor of Physics at the University of Uppsala. In 1937 came his appointment as Research Professor of Experimental Physics, at the Royal Swedish Academy of Sciences. When the Physics Department of the Nobel Institute of the Academy came into being, that same year, Siegbahn was made its first Director.

Siegbahn's early work (1908-1912) was concerned with problems of electricity and magnetism.

From 1912 to 1937 his research work was mainly devoted to X-ray spectroscopy. He developed new methods, and designed instruments for this purpose. His improvements and new constructions of air pumps and X-ray tubes enabled a considerable increase of the radiation intensity, and the numerous spectrographs and crystal or linear gratings which he constructed, have resulted in a highly increased accuracy of his measurements. In this way, a large number of new series within the characteristic X-radiations of elements could be discovered. The new precision technique thus developer by Siegbahn led to a practically complete knowledge of the energy and radiation conditions in the electron shells of the atoms, while at the same I time a solid empirical foundation was created for the quantum-theoretical interpretation of attendant phenomena. Siegbahn's findings in this field havt been summarized by him in his book Spektroskopie der Röntgenstrahlen, 1923 (rev. ed., 1931; ed. in English, 1924), a classic in scientific literature. As a measure of the high precision achieved by Siegbahn's spectrographs (which are held at a constant temperature and read, in tenths of seconds, by means of two microscopes mounted diametrically opposite one another on a precision goniometer) may be mentioned the fact that his energy-level values, arrived at thirty years ago, still serve for many purposes.

The research activity in the Institute under Siegbahn's leadership was directed towards problems of nuclear physics. For this purpose a cyclotron was constructed capable of accelerating deuterons of up to 5 to 6 MeV (1939), which was soon to make place for a larger one for deuteron energies of up to 30 MeV. In addition to this, a high-tension generator for 400,000 volts was built, as a provisional measure, during the War (transformed into a plant for 1.5 million volts in 1962). For the purpose of studying the energy and radiation of the different radioactive isotopes an electromagnetic separator has been constructed at the Institute, and several new types of ß-spectrographs for various purposes have been designed and built. With these technical resources, and after suitable methods had been developed, a number of important projects for research were taken up. The radiation processes of unstable atomic nuclei and nuclear reactions of various kinds have been studied and exact measurements made of the magnetic properties of atomic nuclei. Other projects tackled by Siegbahn and his staff include the construction of an electron microscope of a new pattern and an automatically working ruling-engine for scratching well-defined gratings (with up to 1,800 lines per mm), especially for X-rays and the extreme ultraviolet field. A large number of young scientists, including many from foreign countries, have taken part in the progressively developed research work to study the atomic nucleus and its radioactive properties. Siegbahn travelled a great deal and visited practically all important centres of scientific activity in Europe (1908-1922), Canada and the United States (1924-1925), where he, on invitation of the Rockefeller Foundation, gave lectures at the Universities of Columbia, Yale, Harvard, Cornell, Chicago, Berkeley, Pasadena, Montreal, and several other universities. After World War II, he visited the main nuclear research institutes in the U.S.A. during the years 1946 and 1953 (Berkeley, Pasadena, Los Angeles, St. Louis, Chicago, M.I.T. Boston, Brookhaven, Columbia, etc.).

As member of the Commission Internationale des Poids et Mesures (1937) he took part in annual meetings of this Commission in Paris; he was elected honorary member of this Commission when he left his membership ( 1956). Siegbahn was President of the International Union of Physics, during the period 1938-1947. Other honours, in addition to the Nobel Prize in Physics (1924) awarded to Professor Siegbahn included the Hughes Medal (1934) and the Rumford Medal (1940) from the Royal Society, London; the Duddel Medal from the Physical Society, London (1948). He is honorary doctor in Freiburg (1931), Bukarest (1942), Oslo (1946), Paris (1952) and the Technical Faculty in Stockholm (1957). He is Member of the Royal Society, London and Edinburgh, of the Academie des Sciences, Paris, and of several other academies. Professor Siegbahn married Karin Högbom in 1914. They have two sons: Bo (b. 1915), at present (1964) Ambassador at Marocco; and Kai (b. 1918), since 1954 Professor of Physics

Academic positions and work at Lund 1906-1923

At the end of his first year as an undergraduate, Siegbahn was appointed as an 'extraordinarie amanuensis' at the Physics Institute at Lund, followed by 'amanuensis' in 1908, 'assistent' in 1910, and 'docent' (senior lecturer) in May 1911. The docent post carried no salary, but he continued his assistant post and also received a 'docent fellowship' from October 1911. Thus, at the end of the autumn term of 1911, he reported to the Dean of the Philosophical Faculty's section of science that, during the last two months of term, he, as docent, had given one lecture per week on optics and, as assistant, had helped in practical classes and given three lectures per week to the propeadeutic course for admittance to the Institute. Early in 1913 Siegbahn enquired about a professorship in theoretical electrotechnology at the Royal Institute of Technology in Stockholm, and in 1915 about a professorship in electricity at the National Swedish Telecommunications Administration. Although he was himself confident of his qualifications for these chairs, he was not encouraged to apply. Instead he became the natural successor to J. Rydberg (For. Mem. R.S. 1919), Professor of Physics at Lund. Rydberg, famous for his numerical analysis of the regularities in optical spectra, worked more as an individual than as a leader of a school; he was, nevertheless, a leading figure and unifying force in the Physics Society of Lund. In 1913 and 1914 Siegbahn deputized for him during periods of illhealth. This duty became permanent in 1915, and in 1920 he formally suceeded Rydberg, who died in 1919. One of the three experts who recommended the appointment was Svante Arrhenius, F.R.S., Head of the Nobel Institute of Physical Chemistry in Stockholm. He congratulated the University on appointing 'a man so extremely competent and with such a well-earned reputation'. Siegbahn's inaugural lecture was on 'The problem of matter in the light of X-ray research'. Whithin the next few years he had developed the new lines of research which would make the Institute at Lund a leading centre for work in X-ray spectroscopy. Siegbahn's early work in the domain of electricity and magnetism included such subjects as the use of the electric arc as a michrophone, high-frequency generators for measuring purposes, the oscillations of telephone membranes and the use of the telephone as an oscillograph. His inventiveness and ability in the design of instruments were apparent from the very beginning of his career. He found himself well poised to enter the newly opened field of X-ray spectroscopy. Following a proposal by M. Laue, X-ray diffraction by a crystal had been demonstrated by W. Friedrich and P. Knipping in 1912. W.H. Bragg, F.R.S., and his son W.L. Bragg, F.R.S., introduced the concept of selective reflection by crystal planes (the Bragg Law) and in 1913 described the first X-ray spectrometer. At about the same time (before mid-1913), similar work was being done at Rutherford's laboratory in Manchester by H. Moseley, F.R.S., and C.G. Darwin, F.R.S. In subsequent work done in 1913 and in 1914, Moseley made his comprehensive investigations of the X-ray spectra of a series of elements from aluminium to gold, leading to his crucial discovery of the correlation between the characteristic X-ray frequencies and the atomic numbers (Moseley's Law). Also in 1913, M. de Broglie, F.R.S., introduced the turnable crystal method for X-ray analysis. Although the outbreak of World War I practically put a stop to these pioneering research efforts in England and in France, there was no easy way for Lund to gain the advantage. There was only a small laboratory with meagre resources, and it is remarkable that Siegbahn, only shortly after the new research field had been opened, managed to start and accomplish a comprehensive programme, engaging enthusiastic assistants, and in the space of a few years placing the Institute at Lund in the lead of research in X-ray spectroscopy. The key to this advance was precision: Siegbahn, with his skills as an instrument designer, accepted the challenge of achieveing higher resolution and accuracy, and an extension of the available wavelength range.

Early work on X-ray spectroscopy

The possibility of using the spectrum to identify elements was demonstrated in Siegbahn's first X-ray paper in 1914. A crystal of sodium chloride was used to diffract X-rays from a tube with a platinum-coated anticathode. Siegbahn found two unexpected lines, which turned out to be first and second order lines from a silver layer underneath the platinum coating. He further found that, in addition to the Ag line Kα reported by Moseley there was a second weaker line Kβ. Siegbahn stated that this could be expected on the analogy of the spectra of Pd and Rh. This paper also contained a discussion of absorption, including an attempt to find empirical expressions for atomic absorption coefficients. The first of a large series of doctoral theses initiated by Siegbahn was that of I. Malmer in 1915. He studied the K-series for a total of 19 elements, increasing the known number of lines and resolving many lines into doublets. These early investigations used simple apparatus adapted from ordinary optical spectrometers. Malmer developed de Broglie's turnable crystal method, using a rock-salt crystal turned by clockwork at 15° per hour. He used a glass X-ray tube with a changeable anticathode, cooled by water, and pumped by a Gaede molecular pump. The current was regulated by adjusting the vacuum. For some of his work Malmer used secondary radiation, which required a high-intensity X-ray tube of metal which Siegbahn had designed for the purpose. The first vacuum spectrograph was completed early in 1916, and a second, improved version was demonstrated at a Scandinavian meeting of scientists in Kristiania (Oslo) in July of that year. Improved X-ray tubes were also developed, including a tube whose cathode consisted of a heated filament (a platinum ribbon or a tungsten filament) coated with calcium oxide. From December 1915 to June 1916 Siegbahn, partly in collaboration with E. Friman and W. Stenström, published many papers with new X-ray spectroscopy data. Investigations of the K-series had been extended below the region of the periodic table studied by Malmer. Measurements of the L-series for many elements were published in several of these papers and in Friman's doctoral thesis. Early in 1916 Siegbahn reported a new line closely related to the L-series, which he designated l. He was looking for lines outside the L-series, on the long-wave side, expecting to find an M-series which had been predicted by Wagner and others. The M-series itself, however, required the vacuum spectrograph, which extended spectral work to wavelengths up to 12 Å. He was now able to report the discovery of the M-series for uranium and some other elements down to gold.

Development of precision spectroscopy

To provide his laboratory with precision instruments suitable for various wavelenght ranges, Siegbahn designed three spectrographs, described in four papers. A vacuum spectrograph was required for wavelengths above 2Å; this instrument was less suitable for shorter wavelenghts because of the broadening of lines occurring as a result of the penetration of the X-rays into the crystal, and the other instruments were designed for wavelengths of 0.5Å to 2Å and below 0.5Å respectively. New X-ray tubes were also designed and constructed, especially for the longer wavelenghts where it was difficult to obtain sufficient intensity. A further instrument, a double spectrometer, was designed for measurements of the absorption of X-rays. In this device, an X-ray was divided into two monochromatic beams by reflection from two separate crystals. After reflection the tow beams entered two separate ionization chambers, allowing a comparison of the two intensities. The vacuum spectrograph was equipped with vernier scales reading to 5 arcminutes, giving improved accuracy. Another improvement involved measuring the glancing diffraction angles by photographing the spectrum with crystal and plate in two positions, giving a spectrum first to one side of the direct beam and then to the other. A good illustration of the improved accuracy now obtained with the new instruments is provided by measurements of the Kα1-line of Cu. Moseley's 1913 value for the unresolved Kα line was 1.549Å. In 1916 Siegbahn and Stenström found a value 1.539 ±0.003Å for Kα1, and in 1918 Siegbahn found 1.537358 ±0.000033Å. At this level of accuracy, the absolute calibration of X-ray wavelengths became doubtful, and wavelengths were quoted in X-units, approximately 10-3Å. The value of the X-unit was eventually agreed to one part in 105 in 1945, after international discussions involving W.L. Bragg and Siegbahn. Siegbahn's 1918 value should properly be quoted as 1537.358 ±0.033 X-units; this may be compared with his report in 1929 of the value obtained from measurements by his pupils S. Eriksson and A. Larsson, which was 1537.396 X-units. The continued development of instruments and techniques resulted in a major collection of spectroscopy data by Siegbahn and his pupils. Extensive measurements of K- and L-lines were made by E. Hjalmar, by A. Leide, and by D. Coster, a Dutch physicist who worked at Siegbahn's laboratory from 1920 to 1922. M-series measurements were published in theses by W. Stenström and E. Hjalmar. There were many attempts to find lines belonging to the N-series. Hjalmar found some lines in this series, following earlier work by V. Dolejšek. The satellite, or non-diagram lines were first observed by Siegbahn and Stenström in 1916.

Refraction, dispersion, and absorption

In the high precision measurements of Siegbahn and his colleagues, it was already necessary to take account of deviations from the Bragg Law. C.G. Darwin had published correction formulae in 1914, and Stenström's thesis showed that corrections were needed for X-rays of longer wavelengths. Stenström interpreted his results as refraction, indicating that the index of refraction was less than unity. The new spectrographs at Lund allowed Siegbahn to extend these measurements to shorter wavelengths, and in 1924 Hjalmar and Siegbahn used these accurate instruments in their discovery of anomalous dispersion. Investigations of refraction and dispersion of X-rays became an essential part of the research programme at Uppsala after Siegbahn's move in 1923. The important phenomenon of total reflection at grazing incidence, demonstrated by A.H. Compton, F.R.S., in 1922, was verified by Siegbahn and O. Lundquist in the following year. Refraction in a glass prism was demonstrated by Siegbahn and A. Larsson in 1924. Absorption of X-rays in matter in the form of K-band edges was discovered by M. de Broglie. Contributions from Lund to this subject started in 1919 with the publication by Siegbahn and E. Jönsson of high-resolution absorption spectra, showing that the K-series had one absorption edge, and the L-series three edges, as theory predicted. The existence of five absorption edges for the M-series was also verified at Lund for uranium and thorium. Fine structure in the absorption edges was discovered by Stenström. The high-precision investigations at Lund of X-ray spectra and absorption played a very important role as support to Bohr's atomic theory, as acknowledged by N. Bohr, F.R.S., in his Nobel lecture in December 1922. In 1919 Siegbahn organized a conference at Lund on atomic physics; Bohr and Sommerfeld were invited as principal speakers. Correspondence with Sommerfeld between 1918 and 1922 again demonstrates the importance of the fine structure of X-ray spectra. The doublet structure of the strongest line in the K-series was explained through the hypothesis of electron spin by G.E. Uhlenbeck and S. Goudsmit in 1925. Siegbahn wrote in 1956: 'During the first decade after Laue's discovery, research in this field practically completely cleared up the general features of the X-ray spectra and thereby also the electronic structure of all the atoms.'

Work at Uppsala 1923-1927

The achievements at Lund were all the more remarkable for the primitive conditions under which they were done, and the limited finance available to Siegbahn. The impossibility of purchasing equipment from abroad led in 1917 to the start of an instrument factory at Lund, known as 'Aktiebolaget Vetenskapliga Instrument' (Scientific Instruments Ltd). The articles of association were signed by six people, among whom were Siegbahn and Borelius. Two others, Bruno and Hill, had already been running a small workshop to provide instruments for the University. Siegbahn was the first managing director, and the firm started with a capital of 25 000 Swedish crowns, obtained according to G. Borelius by 'begging'. Although the firm was initially successful, competition after the War led to its closure in 1921. It was therefore an attractive prospect for Siegbahn when in 1922 he was offered the Chair of Physics in Uppsala, which became vacant on the death of G. Granqvist. As in the appointment at Lund, Svante Arrhenius was one of the three supporting experts recommending Siegbahn's appointment: he referred to Siegbahn's achievements in establishing 'an axtraordinary school of Swedish and foreign pupils, the like of which has not existed since the days of Linnaeus'. A new Physics Department building at Uppsala had been completed in 1908. It was bigger, more modern and better equipped than that at Lund. Siegbahn nevertheless took the opportunity for expansion, expecially in the staff. Even so, expansion was modest in modern terms: the teaching staff of two was expanded to three. A precision-instrument maker was added to the one machinist, and Siegbhan obtained a secretary. He also asked for a reduction in the number of lectures he was supposed to give each year, replacing some with seminars and spending more time conducting the students' experimental work. Instrumental improvements in X-ray spectroscopy continued. Geiger counters were introduced in 1924 by Siegbahn's pupil K. Molin. Vacuum spectrographs, which gave an extension to longer wavelenghths, were developed; with R. Thoraeus the measurements were extended to about 25 Å. In these spectrographs a notable invention was the O-ring vacuum seal, which was needed for demountable spectrographs where the detector was a photographic plate within the vacuum chamber. Siegbahn also made notable advances in vacuum pumps: he developed the Gaede molecular pump by substituting a disc-shaped rotor for the cylindrical rotor employed by W. Gaede and by Holweck. During the mid-1920s Siegbahn designed several instruments called tube spectrometers. A non-vacuum model of such a spectrometer, intended for wavelenghts up to about 2.5Å, was described in 1925. In principle it was similar to an instrument built for the same wavelength range at Lund some years earlier. The beam-defining slit was mounted at one end of a tube which at the other end carried the photographic plate holder. A later model, also intended for wavelengths up to about 2.5Å, was designed by Siegbahn with the assistance of A. Larsson (Nordhult) and described by the latter in 1927 and later by Siegbahn. High-vacuum instruments of the tube type were also built. a model described by Siegbahn and Thoraeus in 1926 was a multipurpose spectrometer, 'intended for general X-ray spectroscopic purposes where fairly high precision is needed'. It had a very small internal volume. Somewhat later Siegbahn designed a high-precision high-vacuum instrument according to the same principle but with many new design features. Around 1930 the bent-crystal spectrograph introduced by J.W.M. DuMond and H.A. Kirkpatrick, was applied at Uppsala by A.E. Sandström and by E. Ingelstam.

Very soft X-rays and the extreme ultraviolet

The longest wavelengths that could be measured by the crystal technique were around 20Å. In the extreme ultraviolet, using concave ruled gratings, R.A. Millikan and I.S. Bowen had measured wavelengths down to about 160Å when investigating spectra from highly ionized ions. Siegbahn set about closing this gap by using concave gratings in his vacuum spectrographs. The critical component in this new development was the ruled concave grating. B. Edlén, one of Siegbahn's students, described the first instrument of this type in a conference paper of 1929. The grating had a radius of 101 cm and was mounted at a glancing angle of 10°. This instrument was designed both for X-ray and for optical spectra; the light source in the latter was a spark gap in high vacuum, separated from the spectrograph only by the entrance slit. The ruled gratings used at this time were all produced abroad. A particular treasure was one of Rowland's large concave gratings. A young 'amanuens' was given the task of cleaning this grating, which he attempted by cooling it on ice to condense water on the surface. To his dismay, it cracked into two parts. With shaky knees he went to his professor and told him what had happened. Siegbahn calmly said: 'Isn't that what I always have said, we must make gratings of our own.' According to E. Hulthén, Siegbahn had planned to build gratings while at Lund: he asked A.A. Michelson in 1917 about the supply of concave gratings, but in 1919 Michelson had to tell him that the ruling machine in Chicago was not working well enough. The first ruling machine, constructed in 1929 by I. Lindell in the Uppsala laboratory, produced several high-quality plane gratings, having from 300 to 1800 lines to the millimetre. So that these gratings could be used at glancing incidence, the ruling was very light, avoiding 'plough-ridges' which would cut off part of the radiation. A larger ruling engine was built in 1932 by E. Tingvall, a mechanic in the institute's workshop who became a specialist in this field. This machine produced gratings up to 10 cm diameter, although the optimum width for concave gratings was 20-40 mm. The number of lines to the millimeter was 288, 576 or 1152. The concave-grating technique available at Uppsala around 1932 made possible a further extension into the very soft X-ray region. Thus the L-series could be followed to 250Å, the M-series to 192Å, and the N-series to 190Å. Already in 1929, Siegbahn's pupils B. Edlén and A. Ericson reported optical measurements reaching wavelengths down to about 75Å. The gap between X-ray and optical spectra was now bridged. Thanks in particular to Edlén, later Professor of Physics at Lund, the institute at Uppsala soon became the leading laboratory in this branch of spectroscopy. Edlén's work before 1934 was published in his doctoral thesis, which Siegbahn later described as masterly. Edlén later stated that a particularly significant development at this time was the discovery of the helium-like spectra of some light elements. This provided a test for the calculations of E. Hylleraas of the energy of the ground configuration of two-electron systems. This was the fist quantum-mechanical calculation that had yielded an accurate result for a system containing more than one electron. Edlén also highlighted investigations of resonance lines in the cobalt-like spectra of ions up to Sn XXIV, which implied a record in ionization that stood unsurpassed for som 30 years. Again of primary importance was the analysis of the spectra of Fe X and Fe XI, which yielded results that were to give the first clue to the interpretation of the spectrum of the solar corona. Calibrations of wavelengths in this region of the spectrum were improved by arranging a concave grating spectrograph to record optical and X-ray spectra side by side. A new technique was also developed to investigate absorption in the very soft X-ray region, by using optical spark sources with spectra very rich in lines in place of the very low intensities available in continuous X-ray sources

The Nobel prize

The period 1924-25 was highlighted by some memorable events. In 1924 the first edition of Siegbahn's book, Spektroskopie der Röntgenstrahlen, appeared from Verlag Julius Springer, followed by an English edition in 1925. In December 1924 Siegbahn started, with his wife, his first visit to U.S.A. and Canada. The English edition of his book was delivered to the Clarendon Press at Oxford while he was preparing for the journey, but publication was later than he had hoped." ... "About six months after their return to Uppsala, Siegbahn reached the summit of his fame, when in the autumn of 1925 the Royal Swedish Academy announced that the withheld 1924 Nobel Prize in physics had been awarded to him 'for his discoveries and research in X-ray spectroscopy'.

The move to Stockholm

From the start of his work in 1914 Siegbahn was in the lead for two decades in the field of X-ray spectroscopy. Over the same period nuclear physics had similarly emerged as a rapidly growing branch of physics, starting with the first controlled nuclear transmutation by E. Rutherford, F.R.S., in 1919. Siegbahn was now to become a research organizer in this new field, through the appointment to head of a new institute in Stockholm. Some 80 years earlier, in 1849, the Swedish Academy of Scineces had decided to establish a physics and a chemistry institute, and early in 1850 the first physicist and the first chemist had been appointed. The latter office was abolished in 1904, but the post of physicist existed until 1922, when the last holder died. In fact, owing to financial problems, the Physics Institute had ceased activity in 1918. A new physics institute was now proposed to the Academy, based on investigations in which Siegbahn played an essential role. In May 1930 he had visited the Swedish banker K.A. Wallenberg (founder of the Knut and Alice Wallenberg Foundation) and discussed with him the foundation of a new physics institute under the auspices of the Academy. Wallenberg offered half of the estimated cost of three million Swedish Crowns, provided that the other half could be found from other sources before the end of the year. Unfortunately this was not achieved, and the definitive proposal was not made until March 1935. The proposal for a new research institute for physics included a request to the Riksdag (the Swedish Parliament) to establish a personal research professorship in experimental physics for Siegbahn. Buildings for the institute were to be erected on a site already available. Capital was raised through a Nobel Foundation fund that was at the Academy's disposal. For the purchase of instruments and equipment, and for operating expenses, considerable grants were promised by the Wallenberg Foundation. The Government responded by requesting the Chancellor of the Swedish Universities to investigate an alternative plan, in which Siegbahn would be given improved working conditions at Uppsala. This stirred up a lively debate, but eventually the riksdag approved the Academy's request and in June 1936 Siegbahn was appointed Professor of Experimental Physics at the Academy of Sciences. Buildings were erected at Frescati, close to the Academy buildings, on a site belonging to the Nobel Foundation. Siegbahn started as Director on 1 July 1937, and the buildings were completed in October. Underground premises for a cyclotron were finished in March of the following year. The 1935 proposal was for an institute organized as a Nobel Institute of experimental physics. However, when the final decision was taken, it was stated that the personal professorship funded from the Government was not to be incorporated with the Academy's Nobel Institute. The intention was, however, that at the expiration of the time for the personal professorship the institute was to become a department of the Nobel Institute. There was already a separate Department of physics, whose Director, the theoretical physicist C.W. Oseen, died in 1944. This department was then placed at the disposal of the Academy's Institute for Physics. Those working at Siegbahn's institute simply regarded the two names as synonymous. From the end of the 1940s, the name 'Nobel Institute of Physics' was used in all scientific papers issuing from the laboratory, although within the Academy it was referred to as the 'Research Institute for Physics'. On 1 January 1953 Siegbahn become Professor Emeritus but he served as Director of the Research Institute for eleven years and six months more. When his directorship ended the Institute's status was changed, but not in the way indicated by the Academy in 1935. Instead, an arrangement was made with the Government and the Riksdag to transform the Institute, whose operation had become too heavy a burden on the Academy, both financially and otherwise, into a government organization with an unchanged programme. The new organization came into effect on 1 July 1964, on which day Siegbahn retired at the age of 77. With the change of status, the name was changed to 'Forskningsinstitutet för Atomfysik' (with the shortened English translation the 'Research Institute of Physics'). In July 1988 the institute was again renamed, becoming the 'Manne Siegbahn Institute of Physics'.