Otto Hahn

How to use this course

Otto Hahn received the 1944 Nobel Prize in Chemistry for the discovery of nuclear fission. That discovery, made with Fritz Strassmann in December 1938, was the last chapter of a career of chemical work on radioactive substances that began in 1904. Hahn discovered more radioactive species than almost anyone of his generation, developed the methods for handling invisible quantities of matter, and spent thirty years in partnership with the physicist Lise Meitner. After 1945 he became the public face of German science and an opponent of nuclear weapons.

Each technical lesson opens with a paragraph for the general reader, then gives precise statements, derivations and exercises with folded solutions. The companion course on Lise Meitner covers the physics of the same work, in particular beta spectra and the liquid-drop interpretation of fission. Both courses give the same account of the discovery of fission and of the 1944 Nobel Prize.

Timeline

Frankfurt and Marburg, 1879–1904

Otto Hahn was born in Frankfurt am Main on 8 March 1879, the youngest son of Heinrich Hahn, a glazier who built up a prosperous glazing business, and Charlotte Hahn, née Giese. As a schoolboy he did chemistry experiments in the family laundry room. His father hoped he would become an architect.

He began studying chemistry at the University of Marburg in 1897 and spent two semesters in Munich, where he heard the organic chemist Adolf von Baeyer. He took his doctorate at Marburg in 1901 with Theodor Zincke, on a problem in organic chemistry. After a year of military service he returned to Marburg as Zincke’s assistant for two years. Nothing so far pointed toward radioactivity. He planned a career in the chemical industry, and to improve his English he arranged a stay in London.

London and Montreal, 1904–1906

In the autumn of 1904 Hahn arrived at University College London, where William Ramsay, the discoverer of the noble gases, gave him a dish of barium salt containing a small amount of radium to purify. Hahn had never worked with radioactive substances. In the course of the work he found an activity that did not belong to radium: a new thorium product, far more active than thorium itself, which he called radiothorium (Lesson 04). In the same period, with Otto Sackur, he showed that actinium and the “emanium” of Friedrich Giesel were the same substance.

Ramsay advised him to stay with radioactivity and to learn its physics from Ernest Rutherford. From the autumn of 1905 to the summer of 1906 Hahn worked at McGill University in Montreal. There he found radioactinium, a new member of the actinium series, and studied alpha rays with Rutherford. Rutherford’s quantitative style, measuring everything by electroscope and decay curve, became Hahn’s own. Bertram Boltwood at Yale, working on the same series, was a rival.

Hahn returned to Germany in 1906 with an international reputation and no position. Emil Fischer took him into the Chemical Institute of the University of Berlin.

Berlin, Dahlem and the war, 1906–1918

Radioactivity was not a respectable subject for German chemists, and Hahn worked largely on his own in Fischer’s institute. In spring 1907 he habilitated and became a Privatdozent. That year he discovered mesothorium, the long-lived link in the thorium series, and ionium, the parent of radium in the uranium series (now thorium-230). Mesothorium was chemically inseparable from radium, and the firm of Knöfler sold it as a cheaper substitute, sometimes called German radium.

In 1907 the Austrian physicist Lise Meitner arrived in Berlin. At the physics colloquium Heinrich Rubens introduced them. Fischer did not admit women to his laboratories, so they set up in a former woodworking shop in the basement with its own entrance. The division of labour lasted thirty years: Hahn did the chemistry, Meitner the physics. In 1909 they introduced the recoil method of separating decay products.

The Kaiser Wilhelm Institute

The Kaiser Wilhelm Institute for Chemistry opened in Berlin-Dahlem on 23 October 1912, and Hahn moved there as head of a small radioactivity section; Meitner joined him. A clean building allowed measurements of weak activities, and Hahn turned to the faint radioactivity of potassium and rubidium (Lesson 08). He married Edith Junghans, an art student, on 22 March 1913.

War

In August 1914 Hahn was called up as a reserve officer and won the Iron Cross at the first battle of Ypres. In 1915 Fritz Haber recruited him to the new gas-warfare unit, along with physicists including James Franck and Gustav Hertz. Hahn helped plan and observe gas attacks on several fronts, tested gas masks, and was poisoned by phosgene in 1917. During home leaves he resumed the search for the parent of actinium, which Meitner carried forward in Berlin; they announced protactinium in 1918 (Lesson 05).

Radiothorium and mesothorium

For the general reader. Thorium is weakly radioactive, but its radioactivity is not a single process. It passes through a family of short- and long-lived descendants. Hahn’s first discoveries were two members of this family. Their story shows a basic phenomenon: a freshly purified substance changes its activity over years as its descendants decay away and grow back.

Radiothorium is an isotope of thorium, so ordinary chemical thorium contains it in equilibrium amounts. Hahn found it in 1905 because his radium residue came from a thorium mineral, and it was separated with the thorium fraction. Its activity per gram is enormous compared with thorium-232: the ratio of specific activities is the inverse ratio of half-lives, about \(7\times10^{9}\).

The minimum

Commercial thorium salts are separated from radium in the factory. At that moment the salt contains thorium-232 and radiothorium (which is thorium), but no mesothorium (radium). The radiothorium then decays with its own half-life of 1.91 years, while new mesothorium slowly grows from thorium-232 and feeds new radiothorium. The total activity of the radiothorium group first falls and later recovers.

Derivation of the bracket. Constant source \(S\): \(\dot N_1=S-\lambda_1N_1\), \(\dot N_2=\lambda_1N_1-\lambda_2N_2\), with \(N_1(0)=N_2(0)=0\). Then \(\lambda_1N_1=S(1-e^{-\lambda_1t})\). Solving the second equation with an integrating factor gives \(\lambda_2N_2/S=1-\frac{\lambda_2e^{-\lambda_1t}-\lambda_1e^{-\lambda_2t}}{\lambda_2-\lambda_1}\), which equals the bracket since \(-(\lambda_2-\lambda_1)=\lambda_1-\lambda_2\).

Hahn and, independently, Bertram Boltwood noticed that thorium preparations of different ages had different radiothorium contents. The explanation required a long-lived, chemically separable link between thorium and radiothorium. Hahn isolated it in 1907 and called it mesothorium; later he found that it was itself two substances, the 5.75-year mesothorium 1 and the 6.15-hour mesothorium 2.

Radioelements, isotopes and protactinium

For the general reader. By 1910 some forty radioactive “elements” had been found, but there was room in the periodic table for only a dozen. Many of them, including Hahn’s radiothorium and mesothorium, could not be separated chemically from known elements. Frederick Soddy’s explanation was that they were the same elements with different atomic masses: isotopes. The rules that followed made it possible to predict where a missing radioelement must sit, and in 1917 Hahn and Meitner used them to find element 91.

Hahn’s own observations supplied evidence. Mesothorium 1 and radium could not be separated, nor radiothorium and thorium, nor ionium and thorium. Inseparability was costly for industry and essential for theory. Soddy coined the word isotope in 1913.

The parent of actinium

Actinium (\(Z=89\)) occurs in uranium ores in constant proportion, so it descends from uranium, but its long-lived parent was unknown. By the displacement laws the parent was either a radium isotope (\(Z=88\), beta emitter) or an isotope of element 91 (alpha emitter). Hahn and Meitner chose element 91, expected to behave like tantalum. They extracted pitchblende residues with acid, carried the trace with tantalum compounds, and watched for the growth of actinium’s short-lived descendants over many months. Meitner did most of the 1917 work while Hahn was on war service. The 1918 paper, “Die Muttersubstanz des Actiniums, ein neues radioaktives Element von langer Lebensdauer”, announced the element, named protactinium. Soddy and John Cranston reached the same conclusion independently and acknowledged the Berlin priority. The International Union of Pure and Applied Chemistry confirmed the name and the discovery in 1949. The companion Meitner course treats the decay chain and its numbers in full.

Uranium Z and nuclear isomerism

For the general reader. In 1921 Hahn found a weak radioactivity in uranium with a half-life of about 6.7 hours that matched no known substance. Chemically it was element 91, and its mass had to be 234, exactly like the 1.2-minute uranium X\(_2\). Two nuclei with the same protons and the same neutrons but different lifetimes: Hahn had found the first nuclear isomer, which he considered his best piece of work. A theory had to wait fifteen years.

The branching

Uranium X\(_1\) (thorium-234, 24.1 days) beta-decays to uranium X\(_2\), an excited state of protactinium-234 at 74 keV now written protactinium-234m (half-life 1.17 minutes). Most of the time this state beta-decays directly to uranium-234. In about 0.16% of decays it instead drops by gamma emission to the ground state of protactinium-234. That ground state is uranium Z (half-life 6.70 hours), which then beta-decays to uranium-234.

Hahn found uranium Z by working with large amounts of uranium salts and separating protactinium-like fractions, where a 6.7-hour component appeared alongside the known activities. He reported it in February 1921.

In protactinium-234 the excited state is so hindered that beta decay, normally slower, wins the competition 99.84% of the time. Carl Friedrich von Weizsäcker gave this explanation in 1936, after the neutron and the shell picture of nuclear spins had made it possible. Isomers are now routine: technetium-99m, with a 6-hour half-life, is the most used isotope in nuclear medicine.

Applied radiochemistry

For the general reader. Radiochemists work with substances in amounts too small to see or weigh. Whether a trace is carried down with a precipitate depends on rules that Kasimir Fajans, Friedrich Paneth and Hahn worked out. In the 1920s Hahn turned the problem around: radioactive atoms could be used to study ordinary solids. He called the field applied radiochemistry, and his 1933 Cornell lectures on it became a textbook in 1936.

Fractional crystallization

Marie Curie separated radium from barium by repeated crystallization. If crystals grow slowly so that each new layer is in equilibrium with the solution, the trace and carrier are distributed according to the logarithmic law of Doerner and Hoskins (1925):

where \(x_0,x\) are the amounts of trace in solution before and after, \(y_0,y\) the amounts of carrier, and \(D\) the distribution coefficient. Derivation. In each small step the ratio of trace to carrier entering the crystal is \(D\) times their ratio in solution: \(dx/dy=D\,x/y\). Integrate. If \(D>1\) the trace is enriched in the crystals. For radium in barium bromide and chloride, \(D\) is well above 1, which is why fractional crystallization enriches radium. This rule played the central part in December 1938 (Lesson 09).

The emanation method

If a solid contains a trace of radium or thorium X, it produces the noble gas radon or thoron. Part of the gas escapes from the solid; the fraction escaping is its emanating power. Escape happens by recoil from atoms near the surface and by diffusion along pores and grain boundaries. The emanating power therefore measures the internal surface and its changes, for example during ageing of precipitates, dehydration, sintering or phase transitions. Hahn developed the method in the 1920s.

Radioactive clocks

For the general reader. A slowly decaying element in a rock steadily produces its daughter, so the ratio of daughter to parent tells the age of the rock. At Dahlem from 1912 Hahn studied the faint radioactivity of potassium and rubidium, which Norman Campbell and Alexander Wood had discovered in 1906. This work led him to propose dating minerals by the strontium formed from rubidium, a method that became one of the main tools of geochronology.

For rubidium-87, \(^{87}\mathrm{Rb}\to{}^{87}\mathrm{Sr}+e^-+\bar\nu\), with half-life \(4.92\times10^{10}\) years. Dividing by the stable, non-radiogenic isotope strontium-86 gives the isochron equation

Minerals from one rock that crystallized together share the initial ratio but have different rubidium contents. Plotted against each other, the two measured ratios lie on a straight line whose slope is \(e^{\lambda t}-1\), so the unknown initial strontium need not be guessed. The isochron method was developed by geochemists in the 1950s and 1960s, using the mass spectrometers that made the method practical. Hahn’s contribution, with Strassmann and others at Dahlem, was to show with the chemist’s tools that rubidium-rich minerals contain strontium formed by the decay of rubidium, and so to propose the method.

Barium, December 1938

For the general reader. For four years Hahn, Meitner and Fritz Strassmann had assumed that neutrons turn uranium into heavier elements. In late 1938 Hahn and Strassmann thought they had found radium among the products, an element four places lighter, which was already hard to explain. When they tried to separate this “radium” from the barium they had added as a carrier, it would not separate. The substance was barium. Hahn, the chemist, trusted his chemistry and published it; he asked Meitner, the physicist, what it could mean.

From transuranics to “radium”

The programme began in 1934, when Meitner persuaded Hahn to repeat Enrico Fermi’s neutron experiments on uranium, and Strassmann, an analytical chemist who had joined the institute in 1929, took part. The team assigned many activities to supposed elements 93–96 (the Meitner course, Lesson 08, describes this scheme). In 1938 Irène Joliot-Curie and Pavel Savitch in Paris reported a 3.5-hour activity resembling lanthanum. Repeating related separations in the autumn, Hahn and Strassmann found activities that came down with barium carrier, which they took to be radium isotopes formed by two successive alpha emissions, decaying into “actinium” isotopes. Meitner left Germany in July. At a meeting in Copenhagen on 13 November 1938 she told Hahn the radium claim was physically implausible and urged him to check it.

The test

On 19 December Hahn wrote to Meitner that the radium isotopes behaved like barium. He and Strassmann submitted their paper to Die Naturwissenschaften on 22 December, and it appeared on 6 January 1939 under the title “Über den Nachweis und das Verhalten der bei der Bestrahlung des Urans mittels Neutronen entstehenden Erdalkalimetalle”. The text stated, cautiously, that as chemists they had to name barium, while as nuclear chemists close to physics they could not yet take a step so contrary to all previous experience. A second paper, published on 10 February 1939, gave further evidence, used the word Uranspaltung (uranium splitting), and anticipated the release of extra neutrons. Meanwhile Meitner and Otto Robert Frisch, at Kungälv over Christmas, had explained the process as the division of a nuclear liquid drop, with about 200 MeV released; their note in Nature appeared on 11 February 1939.

Fission products

For the general reader. After 1939 Hahn and Strassmann spent the war years identifying the products of uranium fission. The fragments come in a range of masses, with two peaks, and each starts with too many neutrons. They become stable by a chain of beta decays, which explains why the Berlin group had seen so many half-lives. This is the chemistry behind reactor waste, fallout and medical isotopes.

Mass distribution

Thermal-neutron fission of uranium-235 is usually asymmetric. The yield curve has a light peak near \(A\approx95\) and a heavy peak near \(A\approx138\), each with a cumulative yield of about 6% per fission, and a deep valley at symmetric division (\(A\approx117\)). The heavy peak is anchored by the stability of nuclei near the doubly magic tin-132, an effect beyond the liquid-drop model.

For \(A=140\): \(Z_{\rm UCD}\approx54.6\), while the stable isobar is cerium-140 (\(Z=58\)). The chain is

The barium-140 / lanthanum-140 pair was among the activities in the December 1938 experiments: the “radium” and its “actinium” daughter.

Dahlem under Nazi rule, Farm Hall and the Nobel Prize, 1928–1946

Hahn became director of the Kaiser Wilhelm Institute for Chemistry in 1928. In 1933 he was visiting professor at Cornell University, where his Baker lectures became Applied Radiochemistry (1936). When the Nazi government began expelling Jewish scientists that year, Hahn protested privately and tried to help dismissed colleagues; he did not join the Nazi Party. He resigned his professorship at the University of Berlin on 31 January 1934. The institute, endowed privately, gave him some protection, and he kept Meitner, still an Austrian citizen, and Strassmann, who had refused to join Nazi organizations.

After the annexation of Austria in March 1938, Meitner was in danger. Hahn helped organize her escape in July, with Dirk Coster and Paul Rosbaud, and gave her a diamond ring that had been his mother’s, in case she needed money. They kept up a constant correspondence. Hahn and Meitner met in Copenhagen on 13 November 1938. After the barium result, Hahn wrote to Meitner on 19 December and sent the paper on 22 December (Lesson 09). Meitner, now in Sweden, was not an author of the chemical papers of 1939, which appeared under Hahn and Strassmann alone.

The war

From 1939 the institute took part in the German nuclear research programme, the Uranverein; Hahn’s section studied fission products. Historians now judge that Hahn, while never a Nazi, became part of the regime’s scientific establishment, travelling abroad and accepting honours; after the war he described his wartime work as pure research, which overstates the separation. The institute was bombed in February 1944 and moved to Tailfingen in Württemberg, where Allied troops arrested Hahn in April 1945. With nine other German scientists, among them Werner Heisenberg, Max von Laue and Carl Friedrich von Weizsäcker, he was interned at Farm Hall near Cambridge from July 1945 to January 1946, where their conversations were secretly recorded. When the news of Hiroshima arrived on 6 August 1945, Hahn was so shaken by the use of his discovery that his colleagues feared for him.

The 1944 Nobel Prize

In 1944 the Royal Swedish Academy of Sciences, on the advice of its chemistry committee, reserved the chemistry prize for a year. In November 1945 it awarded the 1944 prize to Otto Hahn alone, “for his discovery of the fission of heavy nuclei”. Hahn learned of it while interned. He received the prize in Stockholm on 10 December 1946 and gave his Nobel lecture, “From the natural transmutations of uranium to its artificial fission”, on 13 December 1946.

The prize did not include Strassmann, whom the committee regarded as the junior partner, nor Meitner and Frisch, whose interpretation the committee treated as secondary to the chemical discovery. Meitner was nominated 49 times between 1924 and 1967, for chemistry and for physics, and never received a prize. When the committee papers were opened, the historians Elisabeth Crawford, Ruth Lewin Sime and Mark Walker concluded in 1997 that her exclusion reflected disciplinary bias, political obtuseness, ignorance and haste. In the postwar years Hahn presented fission as a discovery of chemistry, made after Meitner had left, which hurt her. Meitner wrote that Hahn fully deserved the prize, while maintaining that she and Frisch had contributed something not insignificant. Their friendship lasted until their deaths in the same year.

The Max Planck Society and the bomb, 1946–1968

Hahn returned to Germany in January 1946 and settled in Göttingen in the British zone. On 1 April 1946 he became president of the Kaiser Wilhelm Society, which the Allies intended to dissolve. Its institutes were refounded as the Max Planck Society on 26 February 1948, with Hahn as founding president until May 1960 and honorary president thereafter. Rebuilding German research under his leadership was a long effort, and his international standing helped restore contacts; in 1959 he led a delegation to the Weizmann Institute in Israel. He shared the Max Planck Medal with Meitner in 1949. Einstein, who refused most contact with German institutions after the war, wrote in 1949 that Hahn was among the few who had remained upright during those years.

Against nuclear weapons

On 13 February 1955 Hahn gave a radio lecture, “Cobalt 60: danger or blessing for mankind?”, on the radioactive fallout of nuclear weapons, repeated in English on the BBC. In July 1955, at the Lindau meeting of Nobel laureates, he and Max Born initiated the Mainau Declaration warning that nuclear war could destroy humanity. On 12 April 1957 he was one of eighteen West German nuclear scientists, the Göttingen Eighteen, who declared that they would take no part in making or testing nuclear weapons and opposed arming the Bundeswehr with them. His standing gave the statement weight with the German public. He co-founded the Federation of German Scientists in 1959.

His son Hanno, an art historian, was killed with his wife in a car accident in France in 1960. Hahn received the Enrico Fermi Award of the United States Atomic Energy Commission in 1966, jointly with Meitner and Strassmann. He died in Göttingen on 28 July 1968, aged 89, three months before Meitner. He is buried in the Stadtfriedhof in Göttingen.

Legacy

• Radiochemistry: radiothorium, radioactinium, mesothorium, ionium, protactinium (with Meitner), and the first nuclear isomer; the rules and methods for handling invisible quantities.
• Applied radiochemistry: the emanation method, radioactive tracers in the study of solids, and rubidium–strontium dating.
• Fission: the chemical identification of barium and lanthanum among the products of irradiated uranium, and the cataloguing of fission products.
• Institutions: the Max Planck Society; the Otto Hahn Medal for young researchers; the Hahn-Meitner-Institut in Berlin (1959), named for both.
• Public role: the Mainau Declaration and the Göttingen Eighteen. Element 105 was proposed as “hahnium” in the 1970s; the name was not adopted, and element 105 is dubnium.

Sources and further reading

Biography and history

1. Otto Hahn, Wikipedia (with extensive references); German edition.
2. The Nobel Prize, The Nobel Prize in Chemistry 1944; Otto Hahn, biographical; Nobel lecture, 13 December 1946.
3. Max Planck Society, Otto Hahn and the Mainau Declaration.
4. Harnack House, Otto Hahn.
5. Hahn, Otto, Complete Dictionary of Scientific Biography (via Encyclopedia.com).
6. E. Crawford, R. L. Sime, M. Walker, A Nobel tale of postwar injustice, Physics Today 50(9) (1997) 26.
7. R. L. Sime, Lise Meitner: A Life in Physics, University of California Press, 1996.
8. O. Hahn, Otto Hahn: A Scientific Autobiography, trans. W. Ley, Scribner, 1966.
9. Operation Epsilon (Farm Hall), Wikipedia.
10. Discovery of nuclear fission, Wikipedia.

Chemistry and physics

1. O. Hahn, L. Meitner, Die Muttersubstanz des Actiniums, ein neues radioaktives Element von langer Lebensdauer, Physikalische Zeitschrift 19 (1918).
2. O. Hahn, Über eine neue radioaktive Substanz im Uran, Berichte der Deutschen Chemischen Gesellschaft 54 (1921) (uranium Z).
3. O. Hahn, Applied Radiochemistry, Cornell University Press, 1936.
4. O. Hahn, F. Strassmann, Über den Nachweis und das Verhalten der bei der Bestrahlung des Urans mittels Neutronen entstehenden Erdalkalimetalle, Naturwissenschaften 27 (1939) 11–15.
5. O. Hahn, F. Strassmann, Nachweis der Entstehung aktiver Bariumisotope aus Uran und Thorium durch Neutronenbestrahlung, Naturwissenschaften 27 (1939) 89–95.
6. L. Meitner, O. R. Frisch, Disintegration of uranium by neutrons: a new type of nuclear reaction, Nature 143 (1939) 239–240.
7. C. F. von Weizsäcker, Metastabile Zustände der Atomkerne, Naturwissenschaften 24 (1936) (theory of isomers).
8. Fajans–Paneth–Hahn law, Wikipedia.
9. G. Friedlander, J. W. Kennedy, E. S. Macias, J. M. Miller, Nuclear and Radiochemistry, 3rd ed., Wiley, 1981.
10. G. Faure, T. M. Mensing, Isotopes: Principles and Applications, 3rd ed., Wiley, 2005 (Rb–Sr dating).