Otto Hahn

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Hahn, Otto

(b. Frankfurt am Main, Germany, 8 March 1879; d. Göttingen, Germany, 28 July 1968)


Hahn was one of the first of the numerous great figures in Ernest Rutherford’s circle, although his first fame dates from work performed even before their meeting. Early in the twentieth century he became a pioneer in radiochemistry, along with Frederick Soddy, Bertram Boltwood, and Kasimir Fajans. His long and distinguished career extended through the discovery of nuclear fission to the study of fission fragments and to the rebirth of German science following World War II.

His father, Heinrich, was descended from Rhenish peasant stock, but he was disinclined to follow the family tradition of farming. Instead, he pursued the family avocation and became a glazier, buying his own shop after settling in Frankfurt. His advance from artisan to businessman coincided with the building boom in his city which followed the Franco-Prussian War, and prosperity enabled the Hahn family to rise to middle-class respectability. Otto’s mother, Charlotte Stutzmann, née Giese, had north German ancestry; most of her family were merchants, although a few were in the professions. In 1913 Otto married Edith Junghans, by whom he had one son.

Otto was a sickly youth, but after the age of fourteen he was quite healthy. At the local high school he was a good but not outstanding student. His interest in chemistry arose from some dabbling in the subject with a classmate and increased when he attended a series of lectures given to an adult audience. His father wished him to become an architect, but Otto prevailed and entered Marburg University in 1897. His autobiographical reminiscences suggest that he spent more time in the beer halls than in studying, and he expresses regret at his inattention to physics and mathematics. But he must have absorbed a respectable amount of chemistry; after receiving his doctorate in 1901 and following a year’s infantry service, he returned to Marburg as assistant to his principal professor, Theodor Zincke.

This post was coveted, since one could obtain the professor’s recommendation to any of the large chemical companies in Germany, which led the world in application of scientific talent to industry. For Hahn this was an important step since he had no thoughts of pursuing an academic career. Near the end of his two years with Zincke, Hahn was advised of a possible job which required command of a foreign language, since the firm might have need to send him abroad occasionally. At his own expense he went to England in September 1904, and Zincke, who did not want him to be idle, obtained a place for him in Sir William Ramsay’s laboratory at University College, London.

Ramsay, famous for his discovery of several “inert” gases, developed an interest in radioactivity which was furthered by Soddy, who had first worked with Rutherford and then spent a year with Ramsay. The latter was without radiochemical help so, handing his young German visitor a dish containing about 100 grams of barium salt, he asked him to extract the few milligrams of radium in it according to Marie Curie’s method. Hahn, an organic chemist whose dissertation had dealt with bromine derivatives of isoeugenol, was unfamiliar with this subject, but Ramsay observed that he would approach the work without preconceived ideas. Because the sample was small, Ramsay proposed that Hahn confirm Marie Curie’s determination of the atomic weight of radium by preparing it in some organic compounds (thereby greatly increasing the total amount being examined) and calculating the atomic weight from the measured molecular weights.

Chance sometimes favors the unprepared mind, and Hahn, who familiarized himself with only the basics of radioactivity, followed the prescribed separations technique and found himself the discoverer of a new radioelement: radiothorium. The explanation was that the material given to him came from an ore which contained a large percentage of thorium in addition to the uranium. Thus, upon completion of the chemical procedure, not all the activity was confined in the radium-containing fraction; indeed, the new substance in the remainder was several hundred thousand times more active than thorium and ultimately yielded the characteristic one-minute half-life of thorium emanation. In this same year, with another young German, Otto Sackur, he examined A.L. Debierne’s actinium and F. O. Giesel’s emanium, showing them to be identical and resolving what was then a controversial issue.

Ramsay felt such research ability would be wasted in industry and urged his visitor to take a post which he secured for him in Emil Fischer’s chemical institute at the University of Berlin. By this time Hahn’s interest in organic chemistry had receded before the fascination of radioactivity, and he was amenable to the proposal. But first he wished to attain greater mastery over radioactivity by working under the leading figure in the field, Rutherford. Thus, in September 1905, he crossed the Atlantic to spend the next year at McGill University in Montreal. His reception was cordial but reserved, for Rutherford had a low opinion of Ramsay’s competence in radioactivity and distrusted such work as came from his laboratory. Moreover, the New Zealander’s good friend and prominent radiochemist at Yale, Boltwood, had characterized radiothorium as a “compound of Th-X and stupidity.” Hahn, however, soon convinced the skeptics of the reality of his substance, established warm friendships with them, and again exhibited his talent for discovering radioelements by soon finding radioactinium. Such work was the means by which the constituents and their sequence in the radioactive decay series were determined.

Hahn arrived at Fischer’s institute in the fall of 1906 and in order to continue these investigations he established a mutually profitable relationship with Knöfler and Company, producers of thorium preparations. While in Canada, he had measured a half-life for radiothorium of about two years; but Boltwood— who had tested a number of commercially prepared thorium salts, had found them deficient in radiothorium and had tried unsuccessfully to detect its growth—argued for a much longer half-life. From Knöfler, Hahn obtained samples prepared a number of years earlier and found that their activities decreased at first and then gradually increased. This was proof of his belief in a long-lived radioelement between thorium and radiothorium, which he separated in 1907 and named mesothorium. Because it was chemically inseparable from radium, which was difficult to obtain in Germany, and owing to the rising medical demand for radium, Knöfler successfully marketed high-activity mesothorium as “German radium.”

Within a year of his return to his homeland, Hahn was appointed a Privatdozent in Fischer’s institute, thereby joining the teaching faculty of the University of Berlin; he became a professor in 1910. He became friendly with physics professors Rubens, Nernst, and Warburg, and such younger colleagues as Max von Laue, Otto von Baeyer, James Franck, Gustav Hertz, Peter Pringsheim, and Erich Regener. But the most important physicist to enter his life was Lise Meitner, who came from Vienna in 1907 to do theoretical work under Max Planck and wished also to pursue some studies in experimental radioactivity. Thus began a fruitful collaboration that lasted thirty years. Since Hahn had an almost complete collection of known radioelements, they decided to examine all their beta radiations. This led to the proof that several elements, thought not to radiate as they decayed, actually were weak beta emitters. Further work on the magnetic deflection of the beta rays added much to the ultimate explanation of their continuous and line spectra. Hahn also pioneered the method of radioactive recoil in 1909 (done independently by Russ and Makower), with which he and Meitner found a few more radioelements.

When the new Kaiser Wilhelm Gesellschaft opened its research Institut für Chemie in Berlin-Dahlem, in late 1912, Hahn was made head of a small, independent department of radioactivity and invited Meitner to join him. Since this new laboratory was uncontaminated, he was able to study such weakly radioactive substances as rubidium and potassium and developed an enduring interest in the geological dating of rocks by means of these elements. This was also the time of the most profound theoretical advances in Hahn’s own field of radiochemistry, but he seems to have taken little part in them. Fajans and Soddy independently in 1913 announced the group displacement laws, which placed the radioelements in appropriate boxes of the periodic table, and the concept of isotopy, which held that inseparable radioelements were not only similar but chemically identical. Like other radiochemists, Hahn had long been familiar with such facts as the inseparability of mesothorium and radium, and of radiothorium and thorium. But generalizations to explain these puzzles—and theoretical speculation in general—were not his style; Hahn was simply a superb experimentalist.

During World War I, Hahn served in the gaswarfare corps, under the scientific leadership of Fritz Haber. He was involved in research, development, testing, manufacturing, and using the new weapons. Even before the armistice, having had the opportunity to visit his laboratory in Berlin-Dahlem, Haha and Meitner in 1917 discovered the most stable isotope of the element 91, which they named protactinium (the original discoverers of this element, Fajans and Göhring in 1913, had named their short-lived isotope brevium). This parent of actinium helped resolve the uncertain sequence in the actinium series, although recognition that it was entirely independent of the uranium series (descended from U238) did not come until the discovery of actinouranium (U235) (the existence of which was inferred from Aston’s mass-spectrographic work in 1929), the ultimate source of this series. After the discovery of protactinium, Hahn believed that it descended, through uranium Y (Th231), from primordial uranium in a branch parallel with the well-known uranium series. His subsequent examination of uranium and its products turned up in 1921 a small, but persistent and inexplicable, activity in the uranium series’ protactinium isotope. Here was a case of branching, but not the one Hahn was looking for. He had found that the first example of nuclear isomerism, i.e., uranium Z, has the same parent and the same daughter product as uranium X2 and both these protactinium isotopes are formed by, and decay by, beta emission. But their nuclei are at different energy levels and decay with different half-lives.

By the early 1920’s almost all of the naturally occurring radioelements were known, and opportunities for basic research in radiochemistry were limited. Hahn turned toward applications of his specialty and developed the “emanation” method, by which changes in the surfaces and the formation of surfaces in finely divided precipitates could be studied. He also worked with tracer techniques, developed by Hevesy and Paneth, and extended the rules of Fajans and Paneth for the precipitation and adsorption of small quantities of matter.

Radiochemistry was resurrected and transformed into nuclear chemistry with the great events of the early 1930’s Chadwick’s discovery of the neutron, the Joliot-Curies’ discovery of artificial radioactivity, and Fermi’s use of neutron bombardment to produce additional radioactive materials, including some thought to be new elements beyond uranium in the periodic table. There was much work now for nuclear chemists, and Hahn was deeply involved in identifying the many products and their decay patterns. The “transuranium” elements in particular excited his interest and, with Meitner and Fritz Strassmann, he endeavored to determine their chemical and physical properties. Along with these transuranium elements, the neutron bombardment of uranium seemed to produce several radioactive bodies which separated with barium and could only be, they thought, isotopes of radium. It was difficult enough to explain how uranium (element 92) changed to radium (88), especially as no alpha particles were observed, and virtually no thought was given to the possibility that these bodies were actually barium, an element in the middle of the periodic table. But when they next attempted to separate the “radium” from the barium carrier, the activity remained with the barium fraction.

At the end of 1938, writing as nuclear chemists, Hahn and Strassmann insisted upon the accuracy of their identification. As scientists familiar with nuclear physics, however, they could scarcely believe in a transmutation from uranium to barium. Hahn sent news of these findings to Meitner in Sweden, where she had fled to escape the Nazis. With her nephew, Otto Frisch, she correctly interpreted the phenomenon as a splitting of the uranium nucleus and named it “fission.”

Hahn was little concerned with the energy released in fission and played no part in the German atomic bomb and reactor project during World War II. Instead, he devoted most of his efforts to the study of fission fragments. When the chemical institute, of which he had become director in 1928, was destroyed in an air raid, he moved his usable equipment to southern Germany and resumed work there. With several other nuclear physicists and chemists he was arrested in the spring of 1945 by Allied troops and interned for over half a year in England. There, to his profound dismay, he heard of the application of his discovery when nuclear weapons were detonated over Hiroshima and Nagasaki. He learned also of the award to him of the 1944 Nobel Prize in chemistry, and he received a request to become president of the Kaiser Wilhelm Gesellschaft.

On his release and return to Germany in early 1946, Hahn accepted leadership of this society, which was soon renamed the Max Planck Gesellschaft, at the instance of the occupation authorities. He played a major role in reestablishing not only the society’s research institutes but German science as a whole. He also was responsible for the 1955 “Mainau Declaration” of Nobel laureates, warning of the danger in misuses of atomic energy, and was one of eighteen eminent German scientists who in 1957 protested publicly any German acquisition of nuclear arms.


I. Original Works. Hahn’s only full-sized scientific text consists of his 1933 Baker Lectures at Cornell University, published as Applied Radiochemistry (Ithaca, 1936). He was, however, prolific in recording his reminiscences: “Einige persönliche Erinnerungen aus der Geschichte der natürlichen Radioaktivität,” in Die Naturwissenschaften, 35 (1948), 67–74; New Atoms, Progress and Some Memories, W. Gaade, ed. (New York, 1950); a collection of papers; “Personal Reminiscences of a Radiochemist,” in Journal of the Chemical Society (1956), 3997–4003, the Faraday Lecture; “The Discovery of Fission,” in Scientific American, 198 (1958), 76–84; Otto Hahn: A Scientific Autobiography (New York, 1966), translated and edited by Willy Ley from the original Vom Radiothor zur Uranspaltung (Brunswick, 1962) Otto Hahn: My Life (London, 1970), which was translated by Ernst Kaiser and Eithne Wilkins from the original Mein Leben (Munich, 1968). An extensive bibliography of his scientific and other papers appears in the Biographical Memoirs of Fellows of the Royal Society reference below.

II. Secondary Literature. Through Hahn’s long and active life there appeared numerous articles about him, often on the occasion of a major birthday. Examples of this literature are Stefan Meyer, “Zur Erinnerung an die Jugendzeit der Radioaktivität,” in Die Naturwissenschaften, 35 (1948), 161–163; Erich Regener, “Otto Hahn 70 Jahre,” in Zeitschrift für Elektrochemie, 53 (1949), 51–53; O. R. Frisch, et al., eds., Trends in Atomic Physics; Essays Dedicated to Lise Meitner, Otto Hahn, Max von Laue on the Occasion of Their 80th Birthday (New York, 1959).

The most extensive obituary notice in English is by R. Spence, in Biographical Memoirs of Fellows of the Royal Society, 16 (1970), 279–313. Concerning Hahn’s greatest discovery, Hans G. Graetzer and David L. Anderson reprint numerous papers and furnish connecting narrative in The Discovery of Nuclear Fission (New York, 1971); an analysis is Esther B. Sparberg, “A Study of the Discovery of Fission,” in American Journal of Physics, 32 (1964), 2–8.

Lawrence Badash

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(b. Frankfurt am Main, Germany, 8 March 1879, d. Göttingen, Germany, 28 July 1968)

radiochemistry. For the original article on Hahn see DSB, vol. 6.

Hahn was an exceptionally prominent public figure in postwar Germany, known for the discovery of nuclear fission, for his principled conduct during National Socialism, and for his leadership in the rehabilitation of German science. For decades he was the subject of admiring media attention and much biographical material, including his own two autobiographies and a posthumously published memoir, but significant aspects of his life and work remained largely exempt from critical historical examination until well after his death. This supplement focuses on the scientific and political context for the fission discovery, Hahn’s wartime involvement in the German nuclear fission project, and his postwar advocacy for the German scientific community.

The Discovery of Nuclear Fission: December 1938 . Like many conservative academics in the interwar period, Hahn was dismayed by the loss of the imperial monarchy and dismissive of the Weimar Republic. His first reaction to the National Socialist regime was to hope for a national revival, but he was appalled by the purge of Jewish scientists and soon found himself, probably for the first time, in a nonconformist political position. He was concerned for dismissed Jewish colleagues, maintained contact with émigré friends, and at the end tried to intervene on behalf of several Jewish friends who faced deportation. It helped that the Kaiser Wilhelm Institute (KWI) for Chemistry, of which Hahn was director, was privately funded by the chemical industry and therefore was somewhat insulated, at least at first, from direct control by the government. Thus Lise Meitner, Hahn’s closest colleague and head of the institute’s nuclear physics section, was not dismissed in 1933, although she was of Jewish origin, and Hahn was able to retain Fritz Strassmann, a promising young chemist whose anti-Nazi position made him virtually unemployable outside the institute. From 1934 to 1938 Hahn, Meitner, and Strassmann investigated the radioactive species, thought to be transuranium elements, which were produced by the neutron bombardment of uranium. The work was interdisciplinary, requiring nuclear physics for the reaction processes and chemistry and radiochemistry for analyzing the many radioactive products.

Meitner fled Germany for Sweden in July 1938, but she and Hahn were able to maintain contact by mail. In November 1938 they met secretly in Copenhagen, soon after Hahn and Strassmann found several new activities among the uranium products, which they attributed to isotopes of radium. To Meitner and other physicists it seemed impossible that slow neutrons could cause uranium to lose two alpha particles and form radium, and Meitner pressed Hahn to rigorously reexamine the new activities. Hahn and Strassmann then began a series of fractional crystallization and indicator experiments designed to verify the radium by separating it from the barium that they had used as a carrier. When they could not separate it, the chemists concluded that their “radium” was in fact an isotope of the much lighter element barium—the first indication that uranium nuclei had split. Hahn informed Meitner toward the end of December

1938, asking her to find some “fantastic explanation” for the surprising result. She and her physicist nephew Otto Robert Frisch provided the first theoretical interpretation, calculated the energy released, and named the process nuclear fission.

In Germany the discovery came at a time of heightened political tension, and Hahn, regarded as politically unreliable, was suddenly threatened with the loss of his institute. When he and Strassmann published the barium finding in early January 1939, physicists worldwide greeted it as sensational news, but for Hahn it was also a “heaven-sent gift” that he urgently hoped would protect him and his institute. Afraid that others would learn that he had continued to collaborate with Meitner in exile and that he had informed her of the barium before publication, he insisted that the discovery resulted solely from chemical experiments that he and Strassmann had done in December, and that physics had played no part. In the end, Hahn and his institute were safe. But historians have come to regard his effort to distance the discovery from physics and himself from Meitner as an injustice to her and a misrepresentation of the interdisciplinary nature of the scientific work. At the very least, this can be seen as an instance in which normal standards of scientific attribution were compromised by the effects of anti-semitism, forced emigration, and fear.

Wartime Fission Research in Germany . In March 1939 Frédéric Joliot and his group in Paris reported that secondary neutrons are released during uranium fission, raising the possibility of an energy-producing chain reaction.

The German military took notice, and just after the war began in September 1939 the Army Ordnance unit for high explosives convened a group of leading atomic scientists to explore the military potential of nuclear fission. Hahn, still concerned for his institute, was eager to participate and committed the KWI for Chemistry to the fission project. Hahn later recalled that the first mention of an atomic bomb gave him a “terrible fright” but that he resolved to go on with his research as before.

The field was new, and much fundamental research was indeed necessary, but German scientists, like their Allied counterparts, quickly understood that weapons could in principle be made from two fissile nuclides: the rare isotope of uranium, uranium-235, and the transuranium element 94. Accordingly, the German fission project focused on building a nuclear reactor, for energy and to breed element 94, and on developing methods for separating uranium-235 from natural uranium. Neither goal was met. The German project was active and well supported, but it was far smaller in scale than the corresponding Allied effort.

The KWI for Chemistry was involved in every major aspect of the project and was classified at the highest level of importance to the war effort until the end. Institute physicists investigated neutron reaction processes and the properties of moderators, essential for bomb physics and for the theory and design of the reactor under construction at the nearby KWI for Physics. A sizable group worked on mass spectroscopic methods for isotope separation. Chemists analyzed and purified uranium and its compounds for the reactor, prepared a small amount of element 93 (later named neptunium) and attempted to find element 94 (plutonium). Hahn and Strassmann characterized a large number of fission fragments, data that would have been essential for the operation of a working reactor. With few exceptions, the results were unpublished, circulating as secret reports within the fission project.

After the war Hahn repeatedly stated that he had done only basic research, citing his work on fission fragments, most of which was openly published, and the work on elements 93 and 94. No doubt Hahn preferred to think of himself as a simple scientist engaged in fundamental research, but during the war his primary role was to head an institute that made its scientific expertise available to the state. The institute thrived, and Hahn became one of the Nazi regime’s technocratic and military elites, was permitted to travel in occupied Eastern Europe to promote German “cultural influence,” and was even allowed to visit neutral Sweden, where he was made a foreign member of the Royal Academy of Sciences, a prelude to the Nobel Prize. In 1943 he received a high-ranking civilian award for his contributions to the war effort. After air raids destroyed the institute buildings in Berlin Dahlem in February–March 1944, Hahn and the KWI for Chemistry relocated to southern Germany, where the work continued until the war’s end.

Postwar . Hahn dedicated himself to the rebuilding effort, serving as president of the Kaiser Wilhelm Gesellschaft (KWG) and its successor, the Max Planck Gesellschaft, from 1946 to 1960. To an entire generation of German scientists, he was an iconic figure, the prototype of the decent German, a Nobel laureate whose most famous discovery was the result of basic research, a man known for his upright stance during National Socialism. As spokesperson for the KWG and, by extension, for the scientific community, Hahn projected an image of German science as undiminished in excellence and uninvolved in politics or the war. Particularly in the precarious early postwar years, his leadership succeeded in fostering solidarity among scientists and drawing support from the Allied occupation authorities.

As with most Germans of his generation, however, Hahn’s advocacy meant rewriting the history of the recent past. In his public statements and autobiographical writings, Hahn described his wartime work as unfettered fundamental research that was unrelated to the war effort, never examining his part in the fission project as a whole, the secret research in his institute, or its ties to industry, government, and the military. Like other fission scientists, he misrepresented the objective of the fission project, claiming that it was never directed toward a bomb but only to a nuclear reactor for energy production. Similarly, Hahn depicted the KWG under National Socialism as a haven for free, independent science, even though he was quite aware of the KWG’s ties to the Hitler regime and the military, and its opportunistic expansion into German-occupied Europe.

Hahn’s postwar efforts to distance himself and his institutions from National Socialism were typical of the self-portrayals of his generation. With his prominence, reputation, and the sheer quantity of his writings, he created a widely accepted narrative that obscured rather than illuminated the realities of scientific structures in the National Socialist state. It was only after the late twentieth century that historians had the necessary documentation and critical distance to explore the accommodation and collaboration of science in this period.


By far the most important archival source for Otto Hahn is the collection of his personal and professional papers in the Archiv zur Geschichte der Max-Planck-Gesellschaft, Berlin. A list of Hahn’s publications and selected secondary literature has been assembled by his grandson and published in Otto Hahn: Erlebnisse und Erkenntnisse, edited by Dietrich Hahn, Düsseldorf: Econ Verlag, 1975, which includes Hahn’s frankest memoir, written in 1945 and published posthumously, together with selected correspondence and postwar writings.


A Scientific Autobiography, translated and edited by Willy Ley. London: MacGibbon & Kee, 1967.

My Life. Translated by Ernst Kaiser and Eithne Wilkins. New York: Herder and Herder, 1968.


Berninger, Ernst, ed. Otto Hahn—Eine Bilddokumentation: Persönlichkeit, wissenschaftliche Leistung, Öffentliches Wirken. Munich: H. Moos Verlag, 1969.

Crawford, Elisabeth, Ruth Lewin Sime, and Mark Walker. “A Nobel Tale of Wartime Injustice.” Nature 382 (1996): 393–395.

Gerlach, Walther, and Dietrich Hahn, ed. Otto Hahn: Ein Forscherleben unserer Zeit. Stuttgart: Wissenschaftliche Verlagsgesellschaft, 1984.

Hahn, Dietrich, ed. Otto Hahn Begründer des Atomzeitalters, Eine Biographie in Bildern und Dokumenten. Munich: List Verlag, 1979.

———. Otto Hahn: Leben und Werken in Texten und Bildern. Frankfurt/Main: Insel, 1988.

Krafft, Fritz. Im Schatten der Sensation: Leben und Wirken von Fritz Straæmann. Weinheim: Verlag Chemie 1981.

Sime, Ruth Lewin. Lise Meitner: A Life in Physics. Berkeley: University of California Press, 1996.

———. “The Politics of Memory: Otto Hahn and the Third Reich.” Physics in Perspective 8 (2006): 3–51.

———. “Otto Hahn and the Kaiser-Wilhelm-Institut für Chemie in World War II.” In Gemeinschaftsforschung, Bevollmächtigte und der Wissenstransfer. Die Organisation kriegsrelevanter Forschung und die Kaiser-Wilhelm-Gesellschaft im NS-System, edited by Helmut Maier. Wallstein: Göttingen, 2007.

Walker, Mark. German National Socialism and the Quest for Nuclear Power 1939–1949. Cambridge, U.K.: Cambridge University Press, 1989.

———. “Otto Hahn: Responsibility and Repression.” Physics in Perspective 8 (2006): 116–163.

Ruth Lewin Sime

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Otto Hahn


German Chemist

Often regarded as the leading nuclear and radiochemical experimentalist of the twentieth century, Otto Hahn won the 1944 Nobel Prize for his discovery of nuclear fission. Widely respected for both his scientific research and personal integrity, he also played a leading role in reestablishing scientific research in Germany following the destruction of World War II.

Hahn was one of four children born to a professional glazier, and was initially attracted to organic chemistry in college, taking his doctorate at Marburg University in 1901 under Theodor Zincke. After a year of infantry service, he returned to work as Zincke's assistant in 1903. Dissuading Hahn from his intention to work in industry, in 1904 Zincke obtained a position for him as a research assistant in London with William Ramsay (1852-1916), where he isolated radiothorium, a radioactive isotope of thorium, by chemical analysis of a radioactive mineral blend. In 1905 Hahn left to spend a year with Ernest Rutherford (1871-1937) at McGill University in Montreal, where he repeated his success under Ramsay by discovering radioactinium.

In 1906 Hahn returned to Germany to work in the Berlin Chemical Institute headed by Emil Fischer (1852-1919), where in 1907 he was promoted to Privatdozent (non-stipendiary lecturer) and in 1910 to a professorship. Two other significant developments also occurred in 1907: Hahn identified mesothorium, an intermediate radioactive isotope between thorium and radiothorium, and he began a 30-year collaboration with the brilliant female physicist Lise Meitner (1878-1968). In 1912 Hahn and Meitner moved to positions at the newly established Kaiser Wilhelm Institute for Chemistry, for which Hahn later served as president in 1928.

At the Institute, Hahn was initially engaged in supporting the German military effort in World War I by research under Fritz Haber (1868-1934) on poisonous gases, a role he later greatly regretted. During the 1920s he studied emissions of beta particles (electrons ejected from a nuclear proton that changes into a neutron) by extremely weak radioactive substances, particularly radioisotopes of potassium and rubidium. His method of determining the production of strontium by the rate of radioactive decay and half-life of rubidium was subsequently utilized as a new method for dating geological strata and artifacts. In 1921 he also discovered "uranium-Z," the first nuclear isomer, though its nature remained unexplained for more than a decade until the discovery of the neutron and of artificially induced radioactivity by neutron bombardment in the 1930s.

Another important area of research was Hahn's development of the "emanation method" to study the character of and changes in the surfaces of finely divided solution precipitates. A radioactive "tracer" element was added to the precipitate, and the rate of diffusion was then measured by tracking the path and rate of emanation of a rare gas due to radioactive decay. The method proved particularly useful for working with minute quantities of matter insufficient for measurement by other techniques, and also provided information on temperature changes and crystal lattice structures. Hahn showed a direct correlation between the surface/volume ratio of the precipitate and the emanation rate.

During the 1930s, with the discovery of the neutron by James Chadwick (1891-1974), the development of neutron bombardment techniques by Enrico Fermi (1901-1954), and the creation of artificial radioactivity using Fermi's techniques by Frédéric Joilot-Curie (1900-1958) and Irène Joilot-Curie (1897-1956), Hahn's research interests shifted to the study of decay patterns and products of nuclear isotopes, particularly the transuranium elements, using electrolysis and precipitation techniques. His refusal to cooperate with the new Nazi regime made his position at the Institute increasingly difficult, especially after Lise Meitner, an Austrian Jew, was forced to flee to Sweden in 1938. Later that year, Hahn and his colleague Fritz Strassmann sent a letter to Meitner with the baffling report that neutron bombardment of a uranium sample had not produced radium as expected, but barium instead. Meitner supplied the correct interpretation of the result as the first observed example of nuclear fission.

During World War II, Hahn concentrated on research of fission products, compiling a table of over 100 nuclear isotopes by 1945. Captured by Allied troops and interred with other leading German scientists in England, Hahn learned of the belated award of the 1944 Nobel Prize for his work on fission, but heard with disbelief and despair the news of the dropping of atomic bombs on Hiroshima and Nagasaki. As Germany's most prestigious physical scientist who had not been involved in atomic weapons research during the war, Hahn was asked to take direction of the newly re-founded Kaiser Wilhelm Institute, renamed the Max Planck Institute in 1948. Despite his advanced years, Hahn worked energetically to reestablish scientific research in Germany, and became an outspoken opponent of nuclear weapons and a cautionary critic of nuclear power.


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Otto Hahn

The German chemist Otto Hahn (1879-1968) was a joint discoverer of nuclear fission and a Nobel Prize winner in chemistry.

Otto Hahn was born in Frankfurt am Main on March 8, 1879. He was the youngest son of the owner of a prosperous glazing business. After leaving school in Frankfurt, he went to Marburg University with the intention of entering the chemical industry. Research on bromine derivatives of isoeugenol led to a doctorate in 1901, and after a year's military service he returned to Marburg to continue his research.

The turning point in Hahn's career came in 1904. He had in mind an industrial post for which knowledge of a foreign language was desirable, so he worked under Sir William Ramsay at University College, London. His task was to separate radium from a sample of impure barium chloride. Within a few months he showed that another radioactive substance was present and named it radiothorium. Urged by Ramsay to continue academic research in radioactivity, Hahn moved to Montreal, Canada, in 1905 to work with Ernest Rutherford. Here again success came quickly, and within a year he had recognized two other radioactive species, which he called thorium-C and radioactinium.

In 1906 Hahn returned to Germany, obtaining a place in Emil Fischer's Chemical Institute at Berlin University. Beginning work in a converted woodshop in the basement, he was soon joined by Lise Meitner, with whom he was to collaborate for 30 years. Here he discovered the radioelement mesothorium, studied beta emissions, and recognized the phenomenon known as radioactive recoil.

In 1913 Hahn was appointed head of radioactivity research in the new Kaiser Wilhelm Institute for Chemistry. Despite the interruptions of war service, Hahn made many major discoveries in the next 25 years. In an investigation of the radioactivity of rubidium he established a method for determining the geological ages of minerals that was in many cases more reliable than the traditional one using the radioactivity of uranium. A study of the radioactive precursors of actinium led to the discovery of the element protoactinium.

Following the discovery of artificial radioactivity by the Joliot-Curies in 1934, Meitner and Hahn repeated Enrico Fermi's experiment of bombarding uranium atoms with neutrons and agreed with his conclusion that new (transuranic) elements had been produced. Among the products isolated appeared to be new isotopes of radium; the suggestion that the "radium" was in fact barium by Hahn and Fritz Strassmann in January 1939 was the first indication that the atomic nucleus had been split. This discovery of nuclear fission became, of course, the basis for the production of nuclear weapons, a development which Hahn always deplored.

Hahn was a prisoner of war in England for a few months in 1945, and the next year he received the Nobel Prize for chemistry, which he had been awarded for 1944. Twenty years later Germany's first nuclear vessel was appropriately named Otto Hahn.

Further Reading

A primary source is Hahn's A Scientific Autobiography (1962; trans. 1966). A detailed biographical profile of Hahn is in the Royal Society, Biographical Memoirs of Fellows of the Royal Society (vol. 16, 1970). See also Otto Robert Frisch, ed., Trends in Atomic Physics:Essays Dedicated to Lise Meitner, Otto Hahn, Max von Laue on the Occasion of Their 80th Birthday (1959), and Eduard Farber, Nobel Prize Winners in Chemistry, 1901-1961 (1953; rev. ed. 1963). □

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Hahn, Otto (1879–1968) German physical chemist. He worked on radioactivity with William Ramsay and Ernest Rutherford. In 1906, he returned to Germany to work with Lise Meitner. In 1917, they discovered protactinium. Hahn and Meitner investigated Enrico Fermi's work on the neutron bombardment of uranium. He received the 1944 Nobel Prize in chemistry.

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Otto Hahn (ô´tō hän), 1879–1968, German chemist and physicist. His important contributions in the field of radioactivity include the discovery of several radioactive substances, the development of methods of separating radioactive particles and of studying chemical problems by the use of radioactive indicators, and the formation of artificial radioactive elements by bombarding uranium and thorium with neutrons. He received the 1944 Nobel Prize in Chemistry for splitting the uranium atom (1939) and discovering the possibility of chain reactions. The development of the atomic bomb was based on this work. Hahn was a member of the Kaiser Wilhelm Institute of Chemistry, Berlin, from 1912 and director from 1928 to 1944. He was in Allied custody (1944–46) and on his return to Germany became head of the Kaiser Wilhelm Gesellschaft, Göttingen (later reorganized as the Max Planck Gesellschaft).