Manne Siegbahn - his research

Research activities

• Short personal CV
• Academic positions and work at Lund 1906-1923
• Early work on X-ray spectroscopy
• Development of precision spectroscopy
• Refraction, dispersion and absorption
• Work at Uppsala 1923-1927
• Ruled gratings: The wavelength scale
• Very soft X-rays and the extreme ultraviolet
• The Nobel Prize
• The move to Stockholm
• The Research Institute for Physics: 1937-1964
• Ruling of diffraction gratings
• The electron microscope

The description below of Manne Siegbahn's research is quoted from his Biographical Memoirs of the Royal Society written by Prof. Hugo Atterling, Volume 37, 1991. Throughout the text we have omitted the references to the bibliography that is included in the original article.

Short personal CV

Manne Siegbahn was married and he and his wife Karin had two sons. The elder son, Bo, became a member of parliament and ambassador, and the younger, Kai, became a physicist like his father and received the Nobel Prize of physics in 1981.

Manne Siegbahn died in 1978 at and age of 92 years.

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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.

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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.

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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.

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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.'

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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.

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Ruled gratings: The wavelength scale

In the latter half of the 1920s Siegbahn introduced ruled gratings in his research programme, following work by Compton and R.L. Doan in 1925, and later work by J. Thibaud. E. Bäcklin's thesis (1928) describes the initial Uppsala thechniques, which were based on ordinary optical gratings. Bäcklin then designed a new spectrograph using a grating originally used by A.J. Ångström, F.R.S., in his famous investigations of the solar spectrum about 60 years earlier. This grating was ruled on glass with about 220 lines to the millimetre. With this new spectrograph, Bäcklin concentrated on the precise value of the wavelength of the Al Kα-line, for which he found an absolute value of 8.333Å (± 0.1%). This measurement revealed the calibration error in the X-units of wavelength, and gave an improved value for the lattice constant of calcite. From this Bäcklin then found Avogadro's number, and finally a value for the electronic charge e. He obtained a value e = 4.793 ´ 10^-10 esu (± 0.3%); this was approximately 0.4% larger than the value R.A. Millikan had published in 1917, which was the most recent at that time.

Siegbahn's group at Uppsala pursued this discrepancy through into the 1930s, not only making more accurate grating measurements but attempting a revaluation of Millikan's old experimental data. G. Kellström made a very careful determination of the density of air, and in 1935 he reported a value that was greater than that used by Millikan. This Uppsala work completely resolved the long-standing discrepancy between the oil-drop method and the X-ray measurements.

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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.

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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'.

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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'.

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The Research Institute for Physics: 1937-1964

From the beginning, the new institute was planned and equipped for research in nuclear physics, although other activities were included in the initial programme. Bohr's institute in Copenhagen was already planning for a cyclotron, and in September 1936 Siegbahn made enquiries about several details concerning the magnet. These two cyclotrons were to be two of the first in Europe. Siegbahn sent a member of his group, S. von Friesen, to the U.S.A. to visit several accelerator laboratories, including Lawrence's laboratory at Berkeley. Construction of a cyclotron with a pole diameter of 80 cm started in September 1937, with the help of a generous grant from the Wallenberg Foundation.

During the first eight years of the Institute the resources and staff numbers were small; a photograph of physicists and technical staff taken in May 1943 shows only 20 people. Nevertheless the cyclotron was ready for preliminary beam tests in the autumn of 1939. Von Friesen recalls that he observed the first 'reasonance', i.e. the first indication of a beam, in October during a quick test before he had to join up for military service.  He eventually left the Institute in December 1940 and returned to Uppsala, later becoming Professor of Physics at the Lund University.

The first beam of deuterons was obtained in December 1941, after a major redesign by Siegbahn of the vacuum chamber and the accelerator 'dees'. From 1942 the cyclotron was used for production of radionuclides for the β- and γ-ray spectroscopy work at the Institute and, to a great extent, for medical and biological purposes. In a further remodelling in the early 1950s a deflector system was incorporated to provide an external beam; this work was done by K.G. Malmfors. The machine had its final run in April 1977.

A much larger cyclotron, with pole pieces 225 cm in diameter, was constructed at the Institute in the years 1947-51. For this major development Siegbahn again obtained the help of the Wallenberg Foundation, after the Rockefeller Foundation had given a grant of 500 000 Swedish Crowns (about $100 000). It was built in a new laboratory named 'Wallenbergstiftelsens Cyklotronlaboratorium'. This machine played an important part in the research programme for many years. It was extensively remodelled in the early 1970s, and served as a basic research tool until the mid-1980s, when it was dismantled to make room for a new accelerator project.

One of the small but select group of physicists who moved with Siegbahn from Uppsala in 1937 was T. Magnusson, for many years Siegbahn's assistant. He completed his doctoral thesis on absorption spectra in the extremely soft X-ray region during his first year in Stockholm, and became Siegbahn's right-hand man in the Institute. In 1941 a new institute of Military Physics was established in Sweden, its work being distributed to various locations. Siegbahn put part of his new laboratory at its disposal, and Magnusson became engaged in the management of the new organization. He became its head in 1944, with his office at the Research Institute, and eventually left in 1945 when he was appointed head of the physics division of a new defence research organization, the 'Försvarets Forskningsanstalt'. He later became its Director General.

Among the first experiments done at the new Institute were H. Alfvén's studies of cosmic radiation. Alfvén had been a student under Siegbahn at Uppsala, and had joined the Stockholm group in 1937. His expertise in electronics was of great value in the design and construction of equipment. He suggested a new and simple method of stabilizing high voltages, which was used for the electron microscope. S. Eklund, another graduate physicist who had moved from Uppsala, was an important contributor to instrumentation at this time; he constructed a large Wilson cloud chamber, and a 300 kV neutron generator. Some years later he built a 400 kV cascade generator with acceleration tube for electrons and positive ions. Manne Siegbahn's youngest son, Kai, was also involved in instrumentation at the Institute, building a magnetic spectrograph for β- and γ-ray spectroscopy.  Work in this field up to the mid-1950s is described in Beta- and gamma-ray spectroscopy, edited by Kai Siegbahn.

Manne Siegbahn strongly supported the scientific nuclear projects started at the Institute, but he did not himself take an active part in nuclear physics research. His own research work was directed to the continued development of ruling machines, assisted by L. Lundin, and to the design of an electron microscope. He was also involved in improvements to the molecular pump and to the design of X-ray tubes.

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Ruling of diffraction gratings

Two basement rooms of the institute were designed to take ruling machines. The first was transferred from Uppsala; it produced gratings with 576 lines per millimetre and a total of 50 000 lines. A new and larger machine was completed in 1938, which ruled as many as 1140 lines per millimetre. These machines produced gratings of the finest quality. Many of these gratings were sold to Hilger and Watts in London, and this firm eventually bought the Uppsala machine in 1957. A copy of this machine was later sold to China.

Among the gratings ruled on this last machine, before it was sold to China, were several gratings made for NASA in the U.S.A. It must have given Siegbahn great satisfaction when NASA, after very careful tests, selected two of these gratings. One was mounted in a rocket spectrometer that, on a flight on 30 September 1961, recorded the solar spectrum in the wavelength range 100-400Å. The other was used in the first Orbiting Solar Observatory OSO-1, launched on 7 March 1962, which obtained many thousand spectra of the Sun.

The machine that was first built at the Stockholm institute is now in the 'Sveriges Tekniska Museum' (the National Museum of Science and Technology) in Stockholm.

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The electron microscope

Among the first pieces of equipment built at the Research Institute was an electron microscope designed and developed by Siegbahn. In all its versions the microscope had electromagnetic lenses. A unique feature was its horizontal mounting. A Siegbahn molecular pump, of the type developed at Uppsala, was used for evacuation. An interesting improvement in 1942 was an arrangement for taking stereoscopic pictures. Another modification was made in 1944 or 1945 when the microscope was used for electron-diffraction experiments.

Siegbahn's electron microscope became of great importance to Swedish scientists and, after the War, to many visitors from abroad; during the War it was the only electron microscope available in Sweden. It was used extensively for research in physics, chemistry, biology, medicine, and in various areas of technology. Magnifications of up to about 20 000 could be obtained.

With the exception of a few pages and a picture in the Swedish publication Nordisk Familjeboks Månadskrönika in 1939, and pieces in the annual reports to the Academy of Sciences, Siegbahn did not publish anything on these instruments. In 1959, in reply to R. Rüdenberg, who was 'compiling a story about the inception of the electron microscope', Siegbahn wrote that he started construction of it in 1938 and that it 'was a laboratory type just to allow design experiments ... As there was a continuous development of the design I did not publish anything during the first years and then came the commercial instrument, which made a publication unnecessary. Some copies of our elmicroscope were at that time brought on to the market by a Swedish industrial firm.'

The firm that Siegbahn refered to was Georg Schönander, a company specializing in the manufacture of X-ray equipment and with which the research Institute collaborated a great deal. After the 1940s this company built several microscopes following the pattern developed at Siegbahn's laboratory."

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