Otto Hahn


Otto Hahn was a German chemist and pioneer in the fields of radioactivity and radiochemistry. Hahn is referred to as the father of nuclear chemistry. He discovered radioactive isotopes of radium, thorium, protactinium and uranium. He also discovered the phenomena of radioactive recoil and nuclear isomerism. In 1938, Hahn and Fritz Strassmann discovered nuclear fission, for which Hahn received the 1944 Nobel Prize for Chemistry. Nuclear fission was the basis for nuclear reactors and nuclear weapons that were developed by the Manhattan Project during World War II.
Hahn was an opponent of national socialism and the persecution of Jews by the Nazi Party that caused Meitner to flee Germany. During World War II, he worked on the German nuclear weapons program, and as a consequence was incarcerated by the Allied forces in Farm Hall with nine other scientists.
Hahn served as the last president of the Kaiser Wilhelm Society for the Advancement of Science in 1946 and as the founding president of its successor, the Max Planck Society from 1948 to 1960. Considered by many to be a model for scholarly excellence and personal integrity, he became one of the most influential and respected citizens of the postwar West Germany.

Early life

Otto Hahn was born in Frankfurt am Main on 8 March 1879, the youngest son of Heinrich Hahn, a prosperous glazier, and Charlotte Hahn Giese. He had an older half-brother Karl, his mother's son from her previous marriage, and two older brothers, Heiner and Julius. The family lived above his father's workshop. The younger three boys were educated at Klinger Oberrealschule in Frankfurt. At the age of 15, he began to take a special interest in chemistry, and carried out simple experiments in the laundry room of the family home. His father wanted Otto to study architecture, as he had built or acquired several residential and business properties, but Otto persuaded him that his ambition was to become an industrial chemist.
In 1897, after taking his Abitur at the, Hahn began to study chemistry at the University of Marburg. His subsidiary subjects were mathematics, physics, mineralogy and philosophy. Hahn joined the Students' Association of Natural Sciences and Medicine, a student fraternity and a forerunner of today's Landsmannschaft Nibelungi. He spent his third and fourth semesters at the University of Munich, studying organic chemistry under Adolf von Baeyer, phyisical chmistry under Friedrich Wilhelm Muthmann, and inorganic chemistry under Karl Andreas Hofmann. In 1901, Hahn received his doctorate in Marburg for a dissertation entitled On Bromine Derivates of Isoeugenol, a topic in classical organic chemistry. He completed his one-year military service in the 81st Infantry Regiment, but unlike his brothers,m did not apply for a commission. He then returned to the University of Marburg, where he worked for two years as assistant to his doctoral supervisor, Geheimrat Professor Theodor Zincke.

Discovery of radio thorium, and other "new elements"

Hahn's intention was still to work in industry. He received an offer of employment from Eugen Fischer, the director of , but a condition of employment was that Hahn had to have lived in another country and have a reasonable command of another language. With this in mind, and to improve his knowledge of English, Hahn took up a post at University College London in 1904, working under Sir William Ramsay, who was known for having discovered the inert gases. Here Hahn worked on radiochemistry, at that time a very new field. In early 1905, in the course of his work with salts of radium, Hahn discovered a new substance he called radiothorium, which at that time was believed to be a new radioactive element..
Ramsay was enthusiastic when yet another new element was found in his institute, and he intended to announce the discovery in a correspondingly suitable way. In accordance with tradition this was done before the committee of the venerable Royal Society. At the session of the Royal Society on 16 March 1905 Ramsay communicated Hahn's discovery of radiothorium. The Daily Telegraph informed its readers:
Hahn published his results in the Proceedings of the Royal Society on 24 March 1905. It was the first of over 250 scientific publications of Otto Hahn in the field of radiochemistry. At the end of his time in London, Ramsay asked Hahn about his plans for the future, and Hahn told him about the job offer from Kalle & Co. Ramsay told him radiochemistry had a bright future, and that someone who had discovered a new radioactive element should go to the University of Berlin. Ramsay wrote to Emil Fischer, the head of the chemistry institute there, who replied that Hahn could work in his laboratory, but could not be a Privatdozent because radiochemistry was not taught there. At this point, Hahn decided that he first needed to know more about the subject, so he wrote to the leading expert on the field, Ernest Rutherford. Rutherford agreed to take Hahn on as an assistant, and Hahn's parents undertook to pay Hahn's expenses.
From September 1905 until mid-1906, Hahn worked with Rutherford's group in the basement of the Macdonald Physics Building at McGill University in Montreal. There was some scepticism about the existence of radiothorium, which Bertram Boltwood memorably described as a compound of thorium X and stupidity. Boltwood was soon convinced that it did exist, although he and Hahn differed on what its half life was. William Henry Bragg and Richard Kleeman had noted that the alpha particles emitted from radioactive substances always had the same energy, providing a second way of identifying them, so Hahn set about measuring the alpha particle emissions of radiothorium. In the process, he found that a precipitation of thorium A and thorium B also contained a short-lived "element", which he named thorium C. Hahn was unable to separate it, and concluded that it had a very short half life. He also identified radioactinium and radium D. Rutherford remarked that: "Hahn has a special nose for discovering new elements."

Discovery of mesothorium I

In 1906, Hahn returned to Germany, where Fischer placed at his disposal a former woodworking shop in the basement of Chemical Institute to use as a laboratory. Hahn equipped it with electroscopes to measure alpha and beta particles and gamma rays. In Montreal these had been made from discarded coffee tins; Hahn made the ones in Berlin from brass, with aluminium strips insulated with amber. These were charged with hard rubber sticks that he rubbed then against the sleeves of his suit. It was not possible to conduct research in the wood shop, but Alfred Stock, the head of the inorganic chemistry department, let Hahn use a space in one of his two private laboratories. Hahn purchased two milligrams of radium from Friedrich Oskar Giesel, the discoverer of emanium, for 100 marks a milligram, and obtained thorium for free from Otto Knöfler, whose Berlin firm was a major producer of thorium products.
In the space of a few months Hahn discovered mesothorium I, mesothorium II, and – independently from Boltwood – the mother substance of radium, ionium. In subsequent years, mesothorium I assumed great importance because, like radium-226, it was ideally suited for use in medical radiation treatment, but cost only half as much to manufacture. Along the way, Hahn determined that just as he was unable to separate thorium from radiothorium, so he could not separate mesothorium from radium.
Hahn completed his habilitation in the spring of 1907, and became a Privatdozent. A thesis was not required; the Chemical Institute accepted one of his publications on radioactivity instead. Most of the organic chemists at the Chemical Institute did not regarded Hahn's work as real chemistry. Fischer objected to Hahn's contention in his habilitation colloquium that many radioactive substances existed in such tiny amounts that they could only be detected by their radioactivity, venturing that he had always been able to detect substances with his keen sense of smell, but soon gave in. One department head remarked: "it is incredible what one gets to be a Privatdozent these days!"
, Albert Einstein, Ingrid Franck, James Franck, Lise Meitner, Fritz Haber, and Otto Hahn. Back row, left to right: Walter Grotrian, Wilhelm Westphal,
, and Gustav Hertz
Physicists were more accepting of Hahn's work, and he began attending a colloquium at the Physics Institute conducted by Heinrich Rubens. It was at one of these where, on 28 September 1907, he made the acquaintance of the Austrian physicist Lise Meitner, who was almost the same age as himself. She was only the second woman to receive a doctorate from the University of Vienna, and had already published two papers on radioactivity. Rubens suggested that they So began the thirty-year collaboration and lifelong close friendship between the two scientists.
In Montreal, Hahn had worked with physicists including at least one woman, Harriet Brooks, but it was difficult for Meitner at first. Women were not yet admitted to universities in Prussia. Meitner was allowed to work in the wood shop, which had its own external entrance, but could not set foot in the rest of the institute, including Hahn's laboratory space upstairs. If she wanted to go to the toilet, she had to use one at the restaurant down the street. The following year, women were admitted to universities, and Fischer lifted the restrictions, and had women's toilets installed in the building. The Institute of Physics was more accepting than chemists, and she became friends with the physicists there, including, James Franck, Gustav Hertz, Robert Pohl, Max Planck, and Wilhelm Westphal.

Discovery of radioactive recoil

Harriet Brooks observed a radioactive recoil in 1904, but interpreted it wrongly. Hahn and Meitner, succeeded in demonstrating the radioactive recoil incident to alpha particle emission and interpreted it correctly. Hahn pursued a report by Stefan Meyer and Egon Schweidler of a decay product of actinium with a half-life of about 11.8 days. Hahn determined that it was actinium X. Moreover, he discovered that at the moment when a radioactinium atom emits an alpha particle, it does so with great force, and the actinium X experiences a recoil. This is enough to free it from chemical bonds, and it has a positive charge, and can be collected at a negative electrode. Hahn was thinking only of actinium, but on reading his paper, Meitner told him that he had found a new way of detecting radioactive substances. They set up some tests, and soon found actinium C and thorium C. The physicist Walther Gerlach described radioactive recoil as "a profoundly significant discovery in physics with far-reaching consequences".
In 1910, Hahn was appointed professor by the Prussian Minister of Culture and Education, August von Trott zu Solz. Two years later, Hahn became head of the Radioactivity Department of the newly founded Kaiser Wilhelm Institute for Chemistry in Berlin-Dahlem. This came with an annual salary of 5,000 marks. In addition, he received 66,000 marks in 1914 from Knöfler for the mesothorium process. The new institute was inaugurated on 23 October 1912 in a ceremony presided over by Kaiser Wilhelm II. The Kaiser was shown glowing radioactive substances in a dark room.
The move to new accommodation was fortuitous, as the wood shop had become thoroughly contaminated by radioactive liquids that had been spilt, and radioactive gases that had vented and the decayed and settled as radioactive dust, making sensitive measurements impossible. To ensure that their clean new laboratories stayed that way, Hahn and Meitner instituted strict procedures. Chemical and physical measurements were conducted in different rooms, people handling radioactive substances had to follow protocols that included not shaking hands, and rolls of toilet paper were hung next to ever telephone and door handle. Strongly radioactive substances were stored in the old wood shop, and later in a purpose-built radium house on the institute grounds.

Marriage to Edith Junghans

With a regular income, Hahn was now able to contemplate marriage. In June 1911, while attending a conference in Stettin, Hahn met the, a student at the Royal School of Art in Berlin. They saw each other again in Berlin, and became engaged in November 1912. On 22 March 1913 the couple married in Edith's native city of Stettin, where her father, Paul Ferdinand Junghans, was a high-ranking law officer and President of the City Parliament until his death in 1915. After a honeymoon at Punta San Vigilio on Lake Garda in Italy, the visited Vienna, and the Budapest, where they stayed with George de Hevesy.
Their only child,, was born on 9 April 1922. During World War II, he enlisted in the army in 1942, and served with distinction on the Eastern Front as a panzer commander. He lost an arm in combat. After the war he became a distinguished art historian and architectural researcher, known for his discoveries in the early Cistercian architecture of the 12th century. In August 1960, while on a study trip in France, Hanno died in a car accident, together with his wife and assistant Ilse Hahn Pletz. They left a fourteen-year-old son, Dietrich Hahn.
In 1990, the for outstanding contributions to Italian art history was established in memory of Hanno and Ilse Hahn to support young and talented art historians. It is awarded biennially by the Bibliotheca Hertziana – Max Planck Institute for Art History in Rome.

World War I

In July 1914—shortly before the outbreak of World War I—Hahn was recalled to active duty with the army in a Landwehr regiment. They marched through Belgium, where the platoon he commanded was armed with captured machine guns. He was awarded the Iron Cross for his part in Ypres, Flanders. He was a joyful participant in the Christmas truce of 1914, and was commissioned as a lieutenant. In mid-January 1915, he was summoned to meet chemist Fritz Haber, who explained his plan to break the trench deadlock with chlorine gas. Hahn raised the issue that the Hague Convention banned the use of projectiles containing poison gases, but Haber explained that the French had already initiated chemical warfare with tear gas grenades, and he planned to get around the letter of the convention by releasing gas from cylinders instead of shells.
Haber's new unit was called Pioneer Regiment 35. After brief training in Berlin, Hahn together with physicists James Franck and Gustav Hertz, was sent to Flanders again to scout for a site for a first gas attack. He did not witness the attack because he and Franck were off selecting a position for the next attack. Transferred to Poland, at the Battle of Bolimów on 12 June 1915, they released a mixture of chlorine and phosgene gas. Some German troops were reluctant to advance when the gas started to blow back, so Hahn led them across No Man's land. He witnessed the death agonies of Russians they had poisoned, and unsuccessfully attempted to revive some with gas masks. He was transferred to Berlin as a human Guinea-pig testing poisonous gases and gas masks. On their next attempt on 7 July, the gas again blew back on German lines, and Hertz was poisoned. This assignment was interrupted by a mission at the front in Flanders and again in 1916 by a mission to Verdun to introduce shells filled with phosgene to the Western Front. Then once again he was hunting along both fronts for sites for gas attacks. In December 1916 he joined the new gas command unit at Imperial Headquarters.
Between operations, Hahn return to Berlin, where he was able to slip back to his old laboratory and assist Meitner with her research. In September 1917 he was one of three officers, disguised in Austrian uniforms, sent to the Isonzo front in Italy to find a suitable location for an attack, utilising newly developed rifled minenwerfers that simultaneously hurled hundreds of containers of poison gas onto enemy targets. They selected a site where the Italian trenches were sheltered in a deep valley so that a gas cloud would persist. The Battle of Caporetto broke through the Italian line and the Central Powers overran much of northern Italy. In 1918 the German offensive in the west smashed through the Allies' lines after a massive release of gas from their mortars. That summer Hahn was accentually poisoned by phosgene while testing a new model gas mask. At the end of the war he was in the field in mufti on a secret mission to test a pot that heated and released a cloud of arsenicals.

Discovery of protactinium

In 1913, chemists Frederick Soddy and Kasimir Fajans independently observed that alpha decay caused atoms to shift down two places on the periodic table, while the loss of two beta particles restored it to its original position. Under the resulting reorganisation of the periodic table, radium was placed in group II, actinium in group III, thorium in group IV and uranium in group VI. This left a gap between thorium and uranium. Soddy predicted that this unknown element, which he referred to as "ekatantalium", would be an alpha emitter with chemical properties similar to tantalium. It was not long before Fajans and Oswald Helmuth Göhring discovered it as a decay product of a beta-emitting product of thorium. Based on the radioactive displacement law of Fajans and Soddy, this was an isotope of the missing element, which they named "brevium" after its short half life. However, it was a beta emitter, and therefore could not be the mother isotope of actinium. This had to be another isotope of the same element.
Hahn and Meitner set out to find the missing mother isotope. They developed a new technique for separating the tantalum group from pitchblende, which they hoped would speed the isolation of the new isotope. The work was interrupted by the First World War. Meitner became an X-ray nurse, working in Australian Army hospitals, but she returned to the Kaiser Wilhelm Institute in October 1916. Not only Hahn but most of the students, laboratory assistants and technicians had been called up, so she had to do everything herself, aided only briefly by Hahn when he came home on leave. By December 1917 she was able to isolate the substance, and after further work was able prove that it was indeed the missing isotope. She submitted their findings for publication in March 1918.
Although Fajans and Göhring had been the first to discover the element, custom required that an element was represented by its longest-lived and most abundant isotope, and brevium did not seem appropriate. Fajans agreed to Meitner naming the element protoactinmium, and assigning it the chemical symbol Pa. In June 1918, Soddy and John Cranston announced that they had extracted a sample of the isotope, but unlike Meitner were unable to describe its characteristics. They acknowledged Meitner's priority, and agreed to the name. The connection to uranium remained a mystery, as neither of the known isotopes of uranium decayed into protactinium. It remained unsolved until the mother isotope, uranium-235, was discovered in 1929.
For their discovery Hahn and Meitner were repeatedly nominated for the Nobel Prize in Chemistry in the 1920s by several scientists, among them Max Planck, Heinrich Goldschmidt, and Fajans himself. In 1949, the International Union of Pure and Applied Chemistry named the new element definitely protactinium, and confirmed Hahn and Meitner as discoverers.

Discovery of nuclear isomerism

With the discovery of protactinium, most of the decay chains of uranium had been mapped. When Hahn returned to his work after the war, he looked back over his 1914 results, and considered some anomalies that had been dismissed or overlooked. He dissolved uranium salts in a hydrofluoric acid solution with tantalic acid. First the tantalum in the ore was precipitated, then the protactinium. In addition to the uranium X1 and uranium X2, Hahn detected traces of a radioactive substance with a half life of between 6 and 7 hours. There was one isotope known to have a half life of 6.2 hours, mesothorium II. This was not in any probable decay chain, but it could have been contamination, as the Kaiser Wilhelm Institute for Chemistry had experimented with it. Hahn and Meitner demonstrated in 1919 that when actinium is treated with hydrofluoric acid, it remains in the insoluble residue. Since mesothorium II was an isotope of actinium, the substance was not mesothorium II; it was protactinium.
Hahn now confident enough that he had found something that he named his new isotope "uranium Z", and in February 1921, he published the first report on his discovery.
Hahn determined that uranium Z had a half life of around 6.7 hours and that when uranium X1 decayed, it became uranium X2 about 99.75 per cent of the time, and uranium Z around 0.25 per cent of the time. He found that the proportion of uranium X to uranium Z extracted from several kilograms of uranyl nitrate remained constant over time, strongly indicating that uranium X was the mother of uranium Z. To prove this, Hahn obtained a hundred kilograms of uranyl nitrate; separating the uranium X from it took weeks. He found that the half life of the parent of uranium Z differed from the known 24 day half life of uranium X1 by no more than two or three days, but was unable to get a more accurate value. Hahn concluded that uranium Z and uranium X2 were both the same isotope of protactinium, and they both decayed into uranium II, but with different half lives.
Uranium Z was the first example of nuclear isomerism. Walter Gerlach later remarked that this was "a discovery that was not understood at the time but later became highly significant for nuclear physics". Not until 1936 was Carl Friedrich von Weizsäcker able to provide a theoretical explanation of the phenomenon. For this discovery, whose full significance was recognised by very few, Hahn was again proposed for the Nobel Prize in Chemistry by Bernhard Naunyn, Goldschmidt and Planck.

''Applied Radiochemistry''

In 1924, Hahn was elected to full membership of the Prussian Academy of Sciences in Berlin, by a vote of thirty white balls to two black. While still remaining the head of his own department, he hecame Deputy Director of the Kaiser Wilhelm Institute for Chemistry in 1924, and succeeded Alfred Stock as the director in 1928. Meitner became the director of the Physical Radioactivity Division, while Hahn headed the Chemical Radioactivity Division. In the early 1920s, he created a new line of research. Using the "emanation method", which he had recently developed, and the "emanation ability", he founded what became known as "applied radiochemistry" for the researching of general chemical and physical-chemical questions. In 1936 Cornell University Press published a book in English titled Applied Radiochemistry, which contained the lectures given by Hahn when he was a visiting professor at Cornell University in Ithaca, New York in 1933. This important publication had a major influence on almost all nuclear chemists and physicists in the United States, the United Kingdom, France, and the Soviet Union during the 1930s and 1940s.
In 1966, Glenn T. Seaborg, co-discoverer of many transuranium elements, wrote about this book as follows:

National socialism

had come to the Kaiser Wilhelm Institute for Chemistry to study under Hahn to improve his employment prospects. After the Nazi Party came to power in Germany in 1933, Strassmann declined a lucrative offer of employment because it required political training and Nazi Party membership, and he resigned from the Society of German Chemists when it became part of the Nazi German Labour Front rather than become a member of Nazi-controlled organisation. As a result, he could neither work in the chemical industry nor receive his habilitation, the prequisite for an academic position. Meitner persuaded Hahn to hire Strassmann as an assistant. Soon he would be credited as a third collaborator on the papers they produced, and would sometimes even be listed first.
Hahn spent February to June 1933 in the United States and Canada as a visiting professor at Cornell University. He gave an interview to the Toronto Star Weekly in whiuch he painted a flattering portrait of Adolf Hitler:
The April 1933 Law for the Restoration of the Professional Civil Service banned Jews and communists from academia. Meitner was exempt from its impact because she was an Austrian rather than a German citizen. Haber was likewise exempt as a veteran of World War I, but chose to resign his directorship of the Kaiser Wilhelm Institute of Physical Chemistry and Electrochemistry in protest on 30 April 1933, but the directors of the other Kaiser Wilhelm Institutes, even the Jewish ones, complied with the new law, which applied to the KWS as a whole and those Kaiser Wilhelm institutes with more than 50% state support, which exempted the KWI for Chemistry. Hahn therefore did not have to fire any of his own full-time staff, but as the interim director of Haber's institute, he dismissed a quarter of its staff, including three department heads. Gerhart Jander was appointed the new director of Haber's old institute, and, ironically, reoriented it towards chemical warfare research.
Like most KWS institute directors, Haber had accrued a large discretionary fund. It was his wish that it be distributed to the dismissed staff to facilitate their emigration, but the Rockefeller Foundation insisted that the funds either be used for scientific research or returned. Hahn brokered a deal whereby 10 per cent of the funds would be allocated to Haber's people. In August 1933 the administrators of the KWS were alerted that several boxes of Rockefeller Foundation-funded equipment was about to be shipped to Herbert Freundlich, one of the department heads that Hahn had dismissed, in England. Hahn complied with an order to halt the shipment, but when Planck, the president of the KWS since 1930, returned from vacation, he ordered Hahn to expedite the shipment.
Haber died on 29 January 1934. A memorial service was held on the first anniversary of his death. University professors were forbidden to attend, so they sent their wives in their place. Hahn, Planck and Joseph Koeth attended, and gave speeches. The aging Planck did not seek re-election, and was succeeded in 1937 as president by Carl Bosch, a Nobel Prize winner in Chemistry and the Chairman of the Board of IG Farben, a company which had bankrolled the Nazi Party since 1932. Ernst Telschow became Secretary of the KWS. Telschow was an enthusiastic supporter of the Nazis, but was also loyal to Hahn, being one of his former students, and Hahn welcomed his appointment. Hahn's chief assistant, Otto Erbacher, became the KWI for Chemistry's party steward.

Rubidium–strontium dating

While Hahn was in North America, his attention was drawn to a mica-like mineral from Manitoba that contained rubidium. Some years before he had studied the radioactive decay of rubidium-87, and had estimated its half life at 2 x 1011 years. It occurred to Hahn that by comparing the quantity of strontium in the mineral with that of the remaining rubidium, he could measure the age of the mineral, assuming that his original calculation of the half life was reasonably accurate. This would be a surperior dating method to studying the decay of uranium, because some of the uranium turns into helium, which then escapes, resulting in rocks appearing to be younger than they really were. Jacob Papish helped Hahn obtain several kilograms of the mineral.
From 1,012 grams of the mineral, Strassmann and Ernst Walling extracted 253.4 milligrams of strontium carbonate, all of which was the strontium-87 isotope, indicating that it had all been produced from radioactive decay of rubidium-87. The age of the mineral had been estimated at 1,975 million years from uranium minerals in the same deposit, which implied that the half life of rubidium-87 was 2.3 x x 1011 years: quite close to hahn's original calculation. Rubidium–strontium dating became a widely-used technique for dating rocks in the 1950s, when mass spectrometry became common.

Discovery of nuclear fission

After James Chadwick discovered the neutron in 1932, Irène Curie and Frédéric Joliot irradiated aluminium foil with alpha particles, they found that this results in a short-lived radioactive isotope of phosphorus. They noted that positron emission continued after the neutron emissions ceased. Not only had they discovered a new form of radioactive decay, they had transmuted an element into a hitherto unknown radioactive isotope of another, thereby inducing radioactivity where there had been none before. Radiochemistry was now no longer confined to certain heavy elements, but extended to the entire periodic table. Chadwick noted that being electrically neutral, neutrons could penetrate the atomic nucleus more easily than protons or alpha particles. Enrico Fermi and his colleagues in Rome picked up on this idea, and began irradiating elements with neutrons.
The radioactive displacement law of Fajans and Soddy said that beta decay causes isotopes to move one element up on the periodic table, and alpha decay causes them to move two down. When Fermi's group bombarded uranium atoms were with neutrons, they found a complex mix of half lives. Fermi therefore concluded that the new elements with atomic numbers greater than 92 had been created. Meitner and Hahn had not collaborated for many years, but Meitner was eager to investigate Fermi's results. Hahn, initially, was not, but he changed his mind when Aristid von Grosse suggested that what Fermi had found was an isotope of protactinium. "The only question", Hahn later wrote, "seemed to be whether Fermi had found isotopes of transuranian elements, or isotopes of the next-lower element, protactinium. At that time Lise Meitner and I decided to repeat Fermi's experiments in order to find out whether the 13-minute isotope was a protactinium isotope or not. It was a logical decision, having been the discoverers of protactinium."
Between 1934 and 1938, Hahn, Meitner, and Strassmann found a great number of radioactive transmutation products, all of which they regarded as transuranic. At that time, the existence of actinides was not yet established, and uranium was wrongly believed to be a group 6 element similar to tungsten. It followed that first transuranic elements would be similar to group 7 to 10 elements, i.e. rhenium and platinoids. They established the presence of multiple isotopes of at least four such elements, and identified them as elements with atomic numbers 93 through 96. They were the first scientists to measure the 23-minute half life of uranium-239 and to establish chemically that it was an isotope of uranium, but were unable to continue this work to its logical conclusion and identify the real element 93. They identified ten different half lives, with varying degrees of certainty. To account for them, Meitner had to hypothesise a new class of reaction and the alpha decay of uranium, neither of which had ever been reported before, and for which physical evidence was lacking. Hahn and Strassmann refined their chemical procedures, while Meitner devised new experiments to shine more light on the reaction processes.
In May 1937, they issued parallel reports, one in Zeitschift für Physik with Meiner as the principal author, and one in Chemische Berichte with Hahn as the principal author. Hahn concluded his by stating emphatically: Vor allem steht ihre chemische Verschiedenheit von allen bisher bekannten Elementen außerhalb jeder Diskussion ; Meitner was increasingly uncertain. She considered the possibility that the reactions were from different isotopes of uranium; three were known: uranium-238, uranium-235 and uranium-234. However, when she calculated the neutron cross section, it was too large to be anything other than the most abundant isotope, uranium-238. She concluded that it must be another case of the nuclear isomerism that Hahn had discovered in protactinium. She therefore ended her report on a very different note to Hahn, reporting that: "The process must be neutron capture by uranium-238, which leads to three isomeric nuclei of uranium-239. This result is very difficult to reconcile with current concepts of the nucleus."
With the Anschluss, Germany's unification with Austria on 12 March 1938, Meitner lost her Austrian citizenship, and fled to Sweden. She carried only a little money, but before she left, Hahn gave her a diamond ring he had inherited from his mother. Meitner continued to correspond with Hahn by mail. In late 1938 Hahn and Strassmann found evidence of isotopes of an alkaline earth metal in their sample. Finding a group 2 alkaline earth metal was problematic, because it did not logically fit with the other elements found thus far. Hahn initially suspected it to be radium, produced by splitting off two alpha-particles from the uranium nucleus, but chipping off two alpha particles via this process was unlikely. The idea of turning uranium into barium was seen as preposterous.
During a visit to Copenhage on 10 November, Hahn discussed these results with Niels Bohr, Meitner, and Otto Robert Frisch. Further refinements of the technique, leading to the decisive experiment on 16–17 December 1938, produced puzzling results: the three isotopes consistently behaved not as radium, but as barium. Hahn, who did not inform the physicists in his Institute, described the results exclusively in a letter to Meitner on 19 December:
In her reply, Meitner concurred. "At the moment, the interpretation of such a thoroughgoing breakup seems very difficult to me, but in nuclear physics we have experienced so many surprises, that one cannot unconditionally say: 'it is impossible'." On 22 December 1938, Hahn sent a manuscript to Naturwissenschaften reporting their radiochemical results, which were published on 6 January 1939. On 27 December, Hahn telephoned the editor of Naturwissenschaften and requested an addition to the article, speculating that some platinum group elements previously observed in irradiated uranium, which were originally interpreted as transuranium elements, could in fact be technetium, mistakenly believing that the atomic masses had to add up rather than the atomic numbers. By January 1939, he was sufficiently convinced that formation of light elements that he published a new revision of the article, retracting former claims of observing transuranic elements and neighbours of uranium.
As a chemist, Hahn was reluctant to propose a revolutionary discovery in physics, but Meitner and Frisch, worked out a theoretical interpretation of nuclear fission, a term appropriated by Frisch from biology. In January and February they published two articles discussing and experimentally confirming their theory. In their second publication on nuclear fission, Hahn and Strassmann used the term Uranspaltung for the first time, and predicted the existence and liberation of additional neutrons during the fission process, opening up the possibility of a nuclear chain reaction. This was proved to be the case by Frédéric Joliot and his team in March 1939. Edwin McMillan and Philip Abelson used the cyclotron at the Berkeley Radiation Laboratory to bombard uranium with neutrons, were able to identify an isotope with a 23-minute half life that was the daughter of uranium-239, and therefore the real element 93, which they named neptunium. "There goes a Nobel Prize", Hahn remarked.
At the Kaiser Wilhelm Institute for Chemistry, Kurt Starke independently produced element 93, using only the weak neutron sources available there. Hahn and Strassmann then began researching its chemical properties. They knew that it should decay into the real element 94, which according to the latest version of the liquid drop model of the nucleus propounded by Bohr and John Archibald Wheeler, would be even more fissile than uranium-235, but were unable to detect its radioactive decay. They concluded that it must have an extremely long half life, perhaps millions of years. Part of the problem was that they still believed that element 94 was a platinoid, which confounded their attempts at chemical separation.

World War II

On 24 April 1939, Paul Harteck and his assistant, Wilhelm Groth, had written to Reich Ministry of War, alerting it to the possibility of the development of an atomic bomb. In response, the Army Weapons Branch had established a physics section under nuclear physicist Kurt Diebner. After World War II broke out on 1 September 1939, the HWA moved to control the German nuclear weapons program. From then on, Hahn was participated in a ceaseless series of meetings related to the project. After the Director of the Kaiser Wilhelm Institute for Physics, Peter Debye, left for the United States in 1940 and never returned, Diebner was installed as its director. Hahn reported to the HWA on the progress of his research. Together with his assistants, Hans-Joachim Born, Siegfried Flügge, Hans Götte, Walter Seelmann-Eggebert and Strassmann, he catalogued about one hundred fission product isotopes. They also investigated means of isotope separation; the chemistry of element 93; and methods for purifying uranium oxides and salts.
On the night of 15 February 1944, the Kaiser Wilhelm Institute for Chemistry building was struck by a bomb. Hahn's office was destroyed, along with his correspondence with Rutherford and other researchers, and many of his personal possessions. The office was the intended target of the raid, which had been ordered by Brigadier General Leslie Groves, the director of the Manhattan Project, in the hope of disrupting the German uranium project. Albert Speer, the Reich Minister of Armaments and War Production, arranged for the institute to move to Tailfingen in southern Germany. All work in Berlin ceased by July. Hahn and his family moved to the house of a textile manufacturer there.
Life became precarious for those married to Jewish women. One was Philipp Hoernes, a chemist working for Auergesellschaft, the firm that mined the uranium ore used by the project. After the firm let him go in 1944, Hoernes faced being conscripted for forced labour. At the age of 60, it was doubtful that he would survive. Hahn and Nikolaus Riehl arranged for Hoernes to work at the Kaiser Wilhelm Institute for Chemistry, claiming that his work was essential to the uranium project and that uranium was highly toxic, making it hard to find people to work with it. Hahn was aware that uranium ore was fairly safe in the laboratory, although not so much for the 2,000 female slave labourers from Sachsenhausen concentration camp who mined it in Oranienburg—for other reasons. Another physicist with a Jewish wife was. Hahn certified that his work was important to the war effort, and that his wife Maria, who had a doctorate in physics, was required as his assistant. After he died on 19 September 1944, Maria faced being sent to a concentration camp. Hahn mounted a lobbying campaign to get her released, but to no avail, and she was sent to the Theresienstadt Ghetto in January 1945. She survived the war, and was reunited with her daughters in England after the war.

Incarceration

On 25 April 1945, an armoured task force from the Alsos Mission arrived in Tailfingen, and surrounded the Kaiser Wilhelm Institute for Chemistry. Hahn was informed that he was under arrest. When asked about reports related to his secret work on uranium, Hahn replied: "I have them all here", and handed over 150 reports. He was taken to Hechingen, where he joined Erich Bagge, Horst Korsching, Max von Laue, Carl Friedrich von Weizsäcker and Karl Wirtz. They were then taken to a dilapadated château in Versailles, where they heard about the signing of the German Instrument of Surrender at Reims on 7 May. Over the following days they were joined by Kurt Diebner, Walther Gerlach, Paul Harteck and Werner Heisenberg. All were physicists except Hahn and Harteck, who were chemists, and all had worked on the German nuclear weapons program except von Laue, although he was well aware of it.
They were relocated to the Chateau de Facqueval in Belgium, where Hahn used the time to work on his memoirs and then, on 3 July, were flown to flown to England. They they arrived at Farm Hall, Godmanchester, near Cambridge, on 3 July. Unbeknown to them, their every conversation, indoors and out, was recorded from hidden microphones. They were given British newspapers, which Hahn was able to read. He was greatly disturbed by their reports of the Potsdam Conference, where German territory was ceded to Poland and Russia. In August 1945, the German scientists were informed of the atomic bombing of Hiroshima. Up to this point the scientists, except Hartech, were completely certain that their project was further advanced than any in other countries, and the Alsos Mission's chief scientist, Samuel Goudsmit, did nothing to correct this impression. Now the reason for their incarceration in Farm Hall suddenly became apparent.
As they recovered from the shock of the announcement, they began to rationalise what had happened. Hahn noted that he was glad that they had not succeeded, and von Weizsäcker suggested that they should claim that they had not wanted to. They drafted a memorandum on the project, noting that fission was discovered by Hahn and Strassmann. The revelation that Nagasaki had been destroyed by a plutonium bomb came as another shock, as it meant that the Allies had not only been able to successfully conduct uranium enrichment, but had mastered nuclear reactor technology as well. The memorandum became the first draft of a postwar apologia. The idea that Germany had lost the war because its scientists were morally superior was as outrageous as it was unbelievable, but struck a chord in postwar German academia. It infuriated Goudsmit, whose parents had died in Auschwitz. On 3 January 1946, exactly six months after they had arrived at Farm Hall, the group was allowed to return to Germany. Hahn, Heisenberg, von Laue and von Weizsäcker were brought to Göttingen, which was controlled by the British occupation authorities.

The Nobel Prize in Chemistry 1944

On 16 November 1945 the Royal Swedish Academy of Sciences announced that Hahn had been awarded the 1944 Nobel Prize in Chemistry "for his discovery of the fission of heavy atomic nuclei." Hahn was still at Farm Hall when the announcement was made; thus, his whereabouts were a secret, and it was impossible for the Nobel committee to send him a congratulatory telegram. Instead, he learned about his award on 18 November through the Daily Telegraph. His fellow interned scientists celebrated his award by giving speeches, making jokes, and composing songs.
Hahn had been nominated for the chemistry and the physics Nobel prizes many times even before the discovery of nuclear fission. Several more followed for the discovery of fission. The Nobel prize nominations were vetted by committees of five, one for each award. Although Hahn and Meitner received nominations for physics, radioactivity and radioactive elements had traditionally been seen as the domain of chemistry, and so the Nobel Committee for Chemistry evaluated the nominations. The committee received reports from Theodor Svedberg and Arne Westgren. These chemists were impressed by Hahn's work, but felt that of Meitner and Frisch was not extraordinary, and did not understand why the physics community regarded their work as seminal. As for Strassmann, although his name was on the papers, there was a long-standing policy of conferring awards on the most senior scientist in a collaboration. The committee therefore recommended that Hahn alone be given the chemistry prize.
Under Nazi rule, Germans had been forbidden to accept Nobel prizes after the Nobel Peace Prize had been awarded to Carl von Ossietzky in 1936. The Nobel Committee for Chemistry's recommendation was therefore rejected by the Royal Swedish Academy of Sciences in 1944, which also decided to defer the award for one year. When the Academy reconsidered the award in September 1945, the war was over and thus the German boycott had ended. Also, the chemistry committee had now become more cautious, as it was apparent that much research had taken place in the United States in secret, and suggested deferring for another year, but the Academy was swayed by Göran Liljestrand, who argued that it was important for the Academy to assert its independence from the Allies of World War II, and award the prize to a German, as it had done after World War I when it had awarded it to Fritz Haber. Hahn therefore became the sole recipient of the 1944 Nobel Prize for Chemistry.
The invitation to attend the Nobel festivities was transmitted via the British Embassey in Stockholm. On 4 December, Hahn was persuaded by two of his Alsos captors, American Lieutenant Colonel Horace K. Calvert and British Lieutenant Commander Eric Welsh, to write a letter to the Nobel committee accepting the prize but stating that he would not be able to attend the award ceremony on 10 December since his captors would not allow him to leave Farm Hall. When Hahn protested, Welsh reminded him that Germany had lost the war. Under the Nobel Foundation statutes, Hahn had six months to deliver the Nobel Prize lecture, and until 1 October 1946 to cash the 150,000 Swedish krona cheque.
Hahn was repatriated from Farm Hall on 3 January 1946, but it soon became apparent that difficulties obtaining permission to travel from the British government meant that he would be unable to travel to Sweden before December 1946. Accordingly, the Academy of Sciences and the Nobel Foundation obtained an extension from the Swedish government. Hahn attended the year after he was awarded the prize. On 10 December 1946, the anniversary of the death of Alfred Nobel, King Gustav V of Sweden presented him with his Nobel Prize medal and diploma. Hahn gave 10,000 krona of his prize to Strassmann, who refused to use it.

Founder and President of the Max Planck Society

The suicide of Albert Vögler on 14 April 1945 left the KWS without a president. The British chemist Bertie Blount was placed in charge of its affairs while the Allies decided what to do with it, and he decided to install Max Planck as an interim president. Now aged 87, Plack was in the small town of Rogätz, in an area that the Americans were preparing to hand over to the Soviet Union. The Dutch astronomer Gerard Kuiper from the Alsos Mission fetched Planck in a jeep and brought him to Göttingen on 16 May. Planck wrote to Hahn, who was still in captivity in England, on 25 July, and informed Hahn that the directors of the KWS had voted to make him the next president, and asked if he would accept the position. Hahn did not receive the letter until September, and did not think he was a good choice, as he regarded himself as a poor negotiator, but his colleagues persuded him to accept. After his return to Germany, he assumed the office on 1 April 1946.
Allied Control Council Law No. 25 on the control of scientific research dated 29 April 1946 restricted German scientists to conducting basic research only, and on 11 July the Allied Control Control dissolved the KWS on the insistence of the Americans, who considered that it had been too close to the national socialist regime, and was a threat to world peace. However, the British, who had voted against the dissolution, were more sympathetic, and offered to let the Kaiser Wilhelm Society continued in the British zone, on one condition: that the name be changed. Hahn and Heisenberg were distraught at this prospect. To them it was an international brand that represented political independence and scientific research of the highest order. Hahn noted that it had been suggested that the name be changed during the Weimar Republic, but the Social Democratic Party of Germany had been persuaded not to. To Hahn, the name represented the good old days of the German Empire, however authoritarian and undemocratic it was, before the hated Wemar Republic. Heisenberg asked Niels Bohr for support, but Bohr recommended that the name be changed. Lise Meitner wrote to Hahn, explaining that:
In September 1946, a new Max Planck Society was established at Bad Driburg in the British Zone. On 26 February 1948, after the US and British zones were fused into Bizonia, it was dissolved to make way for the Max Planck Society, with Hahn as the founding president. It took over the 29 institutes of the former Kaiser Wilhelm Society that were located in the British and American zones. When the Federal Republic of Germany was formed in 1949, the five institutes located in the French zone joined them. The Kaiser Wilhelm Institute for Chemistry, now under Strassmann, built and renovated new accommodation in Mainz, but work proceeded slowly, and it did not relocate from Tailfingen until 1949. Hahn's insistence on retaining Ernst Telschow as the general secretary nearly caused a rebellion against his presidency. In his efforts to rebuild German science, Hahn was generous in issuing persilschein, writing one for Gottfried von Droste, who had joined the Sturmabteilung in 1933 and the NSDAP in 1937, and wore his SA uniform at the Kaiser Wilhelm Institute for Chemistry, and for Heinrich Hörlein and Fritz ter Meer from IG Farben. Hahn served as president of the Max Planck Society until 1960, and succeeded in regaining the renown that had once been enjoyed by the Kaiser Wilhelm Society. New institutes were founded and old ones expanded, the budget rose from 12 million Deutsche Marks in 1949 to 47 million in 1960, and the workforce grew from 1,400 to nearly 3,000.

Spokesman for social responsibility

After the Second World War, Hahn came out strongly against the use of nuclear energy for military purposes. He saw the application of his scientific discoveries to such ends as a misuse, or even a crime. Lawrence Badash wrote: "His wartime recognition of the perversion of science for the construction of weapons and his postwar activity in planning the direction of his country's scientific endeavours now inclined him increasingly toward being a spokesman for social responsibility."
In early 1954, he wrote the article "Cobalt 60 – Danger or Blessing for Mankind?", about the misuse of atomic energy, which was widely reprinted and transmitted in the radio in Germany, Norway, Austria, and Denmark, and in an English version worldwide via the BBC. The international reaction was encouraging. The following year he initiated and organized the Mainau Declaration of 1955, in which he and a number of international Nobel Prize-winners called attention to the dangers of atomic weapons and warned the nations of the world urgently against the use of "force as a final resort", and which was issued a week after the similar Russell-Einstein Manifesto. In 1956, Hahn repeated his appeal with the signature of 52 of his Nobel colleagues from all parts of the world.
Hahn was also instrumental in and one of the authors of the Göttingen Manifesto of 13 April 1957, in which, together with 17 leading German atomic scientists, he protested against a proposed nuclear arming of the West German armed forces. This resulted in Hahn receiving an invatation to meet with the Chancellor of Germany, Konrad Adenauer and other senior officials, including the Defense Minister, Franz Josef Strauss, and Generals Hans Speidel and Adolf Heusinger. The two generals argued that the Bundeswehr needed nuclear weapons, and Adenauer accepted their advice. A communique was drafted that said that the Federal Republic did not manufacture nuiclear weapons, and would not ask its scientists to do so. Instead, the German forces were equpped with US nuclear weapons.
, 1979
On 13 November 1957, in the Konzerthaus in Vienna, Hahn warned of the "dangers of A- and H-bomb-experiments", and declared that "today war is no means of politics anymore – it will only destroy all countries in the world". His highly acclaimed speech was transmitted internationally by the Austrian radio, Österreichischer Rundfunk. On 28 December 1957, Hahn repeated his appeal in an English translation for the Bulgarian Radio in Sofia, which was broadcast in all Warsaw pact states.
In 1959 Hahn co-founded in Berlin the Federation of German Scientists, a non-governmental organization, which has been committed to the ideal of responsible science. The members of the Federation feel committed to taking into consideration the possible military, political, and economical implications and possibilities of atomic misuse when carrying out their scientific research and teaching. With the results of its interdisciplinary work the 'VDW' not only addresses the general public, but also the decision-makers at all levels of politics and society. Right up to his death, Otto Hahn never tired of warning urgently of the dangers of the nuclear arms race between the great powers and of the radioactive contamination of the planet.
The historian Lawrence Badash wrote:

Honors and awards

During his lifetime Hahn was awarded orders, medals, scientific prizes, and fellowships of Academies, Societies, and Institutions from all over the world.
As well as the Nobel Prize in Chemistry, Hahn was awarded:
Hahn became the honorary president of the Max Planck Society in 1962.
He was an honorary fellow of University College London,
Objects named after Hahn include:
Proposals were made at various times, first in 1971 by American chemists, that the newly synthesised element 105 should be named hahnium in Hahn's honour, but in 1997 the IUPAC named it dubnium, after the Russian research centre in Dubna. In 1992 element 108 was discovered by a German research team, and they proposed the name hassium. In spite of the long-standing convention to give the discoverer the right to suggest a name, a 1994 IUPAC committee recommended that it be named hahnium. After protests from the German discoverers, the name hassium was adopted internationally in 1997.

Death

Hahn was shot in the back by a disgruntled inventor in October 1951, injured in a motor vehicle accident in 1952, and had a minor heart attack in 1953. In 1962, he published a book, Vom Radiothor zur Uranspaltung. It was released in English in 1966 in 1966 with the title Otto Hahn: A Scientific Autobiography, with an introduction by Glenn Seaborg. The success of this book may have prompted him to write another, fuller autobiography, Otto Hahn. Mein Leben, but before it could be published, he fractured one of the vertebrae in his neck while getting out of a car. He gradually became weaker and weaker, and died in Göttingen on 28 July 1968. His wife Edith only survived him by a fortnight. He was buried in the Stadtfriedhof in Göttingen.
The day after his death, the Max Planck Society published the following obituary notice in all the major newspapers in Germany, Austria, and Switzerland:
Fritz Strassmann wrote:
Otto Robert Frisch recalled:
The Royal Society in London wrote in an obituary:

Legacy

At the end of 1999, the German news magazine Focus published an inquiry of 500 leading natural scientists, engineers, and physicians about the most important scientists of the 20th century. In this poll Hahn was elected third, after the theoretical physicists Albert Einstein and Max Planck, and thus the most significant chemist of his time.

Publications in English