Hahn, Otto

Hahn, Otto

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

radiochemistry.

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.

BIBLIOGRAPHY

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