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{{Short description|German nuclear chemist and Nobel laureate (1879–1968)}}
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{{Infobox scientist
| honorific_suffix  = {{post-nominals|country=GBR|size=100%|ForMemRS}}
| image             = Otto Hahn 1970.jpg
| birth_date        = {{birth date|1879|03|08|df=yes}}
| birth_place       = [[Frankfurt]], Kingdom of Prussia, German Empire
| death_date        = {{death date and age|df=y|1968|7|28|1879|3|8}}
| death_place       = [[Göttingen]], Lower Saxony, West Germany
| education         = [[University of Marburg]]<br />[[University of Munich]]
| known_for         = {{plainlist|
* Discovery of radioactive elements (1905–1921)
* [[Radiothorium]] (<sup>228</sup>Th, 1905)
* [[Radioactinium]] (<sup>227</sup>Th, 1906)
* [[Mesothorium]] (<sup>228</sup>Ra, 1907)
* [[Ionium]] (<sup>230</sup>Th, 1907)
* [[Atomic recoil|Radioactive recoil]] (1909)
* [[Fajans–Paneth–Hahn Law]]
* [[Protactinium]] (Pa, 1917)
* [[Nuclear isomerism]] (1921)
* {{nowrap|''[[Applied Radiochemistry]]'' (1936)}}
* [[Rubidium-strontium dating]] (1938)
* [[Discovery of nuclear fission]] (1938)
}}
| spouse            = {{marriage|Edith Junghans|1913}}
| children          = [[Hanno Hahn]]
| awards            = {{ubl|[[Nobel Prize in Chemistry]] (1944)|[[Max Planck Medal]] (1949)|''[[Pour le Mérite#Civil class|Pour le Mérite]]'' (1952)|[[Faraday Lectureship Prize]] (1956)|[[Wilhelm Exner Medal]] (1958)|[[Legion of Honour]] (1959)|[[Enrico Fermi Award]] (1966)}}
| fields            = [[Nuclear chemistry]]<br>[[Radiochemistry]]
| work_institutions = {{ubl|[[University College London]]|[[McGill University]]|[[University of Berlin]]| [[Kaiser Wilhelm Institute for Chemistry]]|[[Max Planck Society]]}}
| doctoral_advisor  = [[Theodor Zincke]]
| academic_advisors = {{ubl|[[Adolf von Baeyer]]|[[Emil Fischer]]|[[William Ramsay]]|[[Ernest Rutherford]]}}
| doctoral_students = {{ubl|[[Hans-Joachim Born]]|[[Walter Seelmann-Eggebert]]|[[Siegfried Flügge]]|[[Aristid von Grosse]]|[[Nikolaus Riehl]]|[[Salomon Rosenblum]]|[[Fritz Strassmann]]}}
| signature         = Otto Hahn signature.svg
}}

'''Otto Hahn''' ({{IPA|de|ˈɔtoː ˈhaːn|lang|De-Otto Hahn.ogg}}; 8 March 1879{{snd}}28 July 1968) was a German [[chemist]] who was a pioneer in the field of [[radiochemistry]]. He is referred to as the father of [[nuclear chemistry]] and discoverer of [[nuclear fission]], the science behind [[nuclear reactor]]s and [[nuclear weapon]]s. Hahn and [[Lise Meitner]] discovered [[isotopes]] of the radioactive elements [[isotopes of radium|radium]], [[Isotopes of thorium|thorium]], [[isotopes of protactinium|protactinium]] and [[isotopes of uranium|uranium]]. He also discovered the phenomena of [[atomic recoil]] and [[nuclear isomerism]], and pioneered [[rubidium–strontium dating]]. In 1938, Hahn, Meitner and [[Fritz Strassmann]] [[Discovery of nuclear fission|discovered nuclear fission]], for which Hahn alone was awarded the 1944 [[Nobel Prize in Chemistry]].

A graduate of the [[University of Marburg]], which awarded him a doctorate in 1901, Hahn studied under Sir [[William Ramsay]] at [[University College London]] and at [[McGill University]] in Montreal, Canada, under [[Ernest Rutherford]], where he discovered several new radioactive isotopes. He returned to Germany in 1906; [[Emil Fischer]] let him use a former woodworking shop in the basement of the Chemical Institute at the [[University of Berlin]] as a laboratory. Hahn completed his [[habilitation]] in early 1907 and became a ''[[Privatdozent]]''. In 1912, he became head of the Radioactivity Department of the newly founded [[Kaiser Wilhelm Institute for Chemistry]] (KWIC). Working with Austrian physicist Lise Meitner in the building that now bears their names, they made a series of groundbreaking discoveries, culminating with her isolation of the longest-lived isotope of protactinium in 1918.

During [[World War I]] Hahn served with a ''[[Landwehr]]'' regiment on the [[Western Front (World War I)|Western Front]], and with the [[chemical warfare]] unit headed by [[Fritz Haber]] on the Western, [[Eastern Front (World War I)|Eastern]] and [[Italian front (World War I)|Italian]] fronts, earning the [[Iron Cross]] (2nd Class) for his part in the [[First Battle of Ypres]]. After the war he became the head of the KWIC, while remaining in charge of his own department. Between 1934 and 1938, he worked with Strassmann and Meitner on the study of isotopes created by neutron bombardment of uranium and thorium, which led to the discovery of nuclear fission. He was an opponent of [[Nazism]] and the [[The Holocaust|persecution of Jews]] by the [[Nazi Party]] that caused the removal of many of his colleagues, including Meitner, who was forced to flee Germany in 1938. Nonetheless, during [[World War II]], he worked on the [[German nuclear weapons program]], cataloguing the [[fission product]]s of uranium. At the end of the war he was arrested by the Allied forces and detained in [[Farm Hall]] with nine other German scientists, from July 1945 to January 1946.

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. In 1959, he co-founded the [[Federation of German Scientists]], a non-governmental organisation committed to the ideal of responsible science. As he worked to rebuild German science, he became one of the most influential and respected citizens of post-war [[West Germany]].

==Early life and education==
Otto Hahn was born in [[Frankfurt am Main]] on 8 March 1879, the youngest son of Heinrich Hahn, a prosperous [[glazier]] and founder of the Glasbau Hahn company, and Charlotte Hahn (née 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 the ''Klinger Oberrealschule'' in Frankfurt. At the age of 15, Otto began to take a special interest in chemistry, and carried out simple experiments in the laundry room of the family home. His father wanted him 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]].{{sfn|Hahn|1966|pp=2–6}}

In 1897, after passing his ''[[Abitur]]'', 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'' ([[Coburger Convent der akademischen Landsmannschaften und Turnerschaften]]). He spent his third and fourth semesters  at the [[University of Munich]], studying organic chemistry under [[Adolf von Baeyer]], physical chemistry under {{ill|Wilhelm Muthmann|de|Wilhelm Muthmann (Chemiker)}}, 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 (instead of the usual two because he had a doctorate) in the 81st Infantry Regiment, but unlike his brothers, 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]].{{sfn|Hahn|1966|pp=7–11}}{{sfn|Spence|1970|pp=281–282}}

==Early career in London and Canada==

===Discovery of radiothorium and other "new elements"===
[[File:William Ramsay.jpg|thumb|left|upright|[[William Ramsay]], London 1905]]
Hahn's intention was still to work in industry. He received an offer of employment from Eugen Fischer, the director of {{ill|Kalle & Co.|de|Chemische Fabrik Kalle}} (and the father of organic chemist [[Hans Fischer]]), 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 [[noble gas]]es. 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]] (thorium-228), which at that time was believed to be a new [[radioactive]] element.{{sfn|Hahn|1966|pp=7–11}} In fact, it was an [[isotope]] of the known element [[thorium]]; the concept of an isotope, along with the term, was coined in 1913 by the British chemist [[Frederick Soddy]].{{sfn|Hughes|2009|p=135}}

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.{{sfn|Hoffmann|2001|p=35}} The ''[[Daily Telegraph]]'' informed its readers:
{{blockquote|Very soon the scientific papers will be agog with a new discovery which has been added to the many brilliant triumphs of Gower Street. Dr. Otto Hahn, who is working at University College, has discovered a new radioactive element, extracted from a mineral from Ceylon, named Thorianite, and possibly, it is conjectured, the substance which renders thorium radioactive. Its activity is at least 250,000 times as great as that of thorium, weight for weight. It gives off a gas (generally called an emanation), identical with the radioactive emanation from thorium. Another theory of deep interest is that it is the possible source of a radioactive element possibly stronger in radioactivity than radium itself, and capable of producing all the curious effects which are known of radium up to the present. – The discoverer read a paper on the subject to the Royal Society last week, and this should rank, when published, among the most original of recent contributions to scientific literature.<ref>{{cite news |newspaper=[[The Daily Telegraph]] |title=A New Element |location=London |date=18 March 1905}}</ref>}}

[[File:Ernest Rutherford 1905.jpg|thumb|right|[[Ernest Rutherford]] at McGill University, Montreal 1905]]
Hahn published his results in the ''[[Proceedings of the Royal Society]]'' on 24 May 1905.<ref>{{cite journal |title=A New Radio-active Element, Which Evolves Thorium Emanation. Preliminary Communication |first=Otto |last=Hahn |journal=Proceedings of the Royal Society of London. Series A, Containing Papers of a Mathematical and Physical Character |issn=0080-4630 |volume=76 |issue=508 |date=24 May 1905 |pages=115–117 |doi=10.1098/rspa.1905.0009 |bibcode=1905RSPSA..76..115H |doi-access=free }}</ref> It was the first of more than 250 scientific publications in the field of radiochemistry.<ref>{{harvnb|Spence|1970|pp=303–313}} for a full list</ref> 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.{{sfn|Hahn|1966|pp=15–18}}

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 ([[polonium]]-216) and thorium B (lead-212) also contained a short-lived "element", which he named thorium C (which was later identified as polonium-212). Hahn was unable to separate it, and concluded that it had a very short half-life (it is about 300 ns). He also identified radioactinium (thorium-227) and radium D (later identified as lead-210).{{sfn|Spence|1970|pp=282–283}}{{sfn|Hahn|1966|pp=24–25}} Rutherford remarked that: "Hahn has a special nose for discovering new elements."{{sfn|Hahn|1988|p=59}}

==Chemical Institute in Berlin==

===Discovery of mesothorium I===
[[File:Otto Hahn und Lise Meitner.jpg|thumb|left|upright|Hahn and Meitner, 1913, in the chemical laboratory of the [[Kaiser Wilhelm Institute for Chemistry]]. When a colleague she did not recognise said that they had met before, Meitner replied: "You probably mistake me for Professor Hahn."{{sfn|Hahn|1966|p=66}}]]
In 1906, Hahn returned to Germany, where Fischer placed at his disposal a former woodworking shop (''Holzwerkstatt'') in the basement of the Chemical Institute to use as a laboratory. Hahn equipped it with [[electroscope]]s to measure alpha and [[beta particle]]s 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 against the sleeves of his suit.{{sfn|Hahn|1966|pp=37–38}} 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.{{sfn|Hahn|1966|p=52}} Hahn purchased two milligrams of radium from [[Friedrich Oskar Giesel]], the discoverer of [[emanium]] (radon), for 100 marks a milligram ({{Inflation|DE|100|1906|fmt=eq|cursign=€|r=-2}}),{{sfn|Hahn|1966|pp=37–38}} and obtained thorium for free from Otto Knöfler, whose Berlin firm was a major producer of thorium products.{{sfn|Hahn|1966|pp=39–40}}

In the space of a few months Hahn discovered [[mesothorium]] I (radium-228), mesothorium II (actinium-228), and{{snd}}independently from Boltwood{{snd}}the mother substance of radium, ionium (later identified as [[thorium-230]]). In subsequent years, mesothorium I  assumed great importance because, like radium-226 (discovered by [[Pierre Curie|Pierre]] and [[Marie Curie]]), 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 I from radium.{{sfn|Hahn|1966|pp=40–50}}<ref>{{Cite journal |date=December 1945 |title=Nobel Prize for Chemistry for 1944: Prof. Otto Hahn |journal=Nature |volume=156 |issue=3970 |pages=657 |doi=10.1038/156657b0 |bibcode=1945Natur.156R.657. |issn=0028-0836 |doi-access=free}}</ref>

In Canada there had been no requirement to be circumspect when addressing the egalitarian New Zealander Rutherford, but many people in Germany found his manner off-putting, and characterised him as an "Anglicised Berliner".{{sfn|Hahn|1966|p=68}} Hahn completed his [[Habilitation#Germany|habilitation]] in early 1907, and became a ''Privatdozent''. A thesis was not required; the Chemical Institute accepted one of his publications on radioactivity instead.{{sfn|Stolz|1989|p=20}} Most of the organic chemists at the Chemical Institute did not regard Hahn's work as real chemistry.{{sfn|Hahn|1966|p=50}} Fischer objected to Hahn's contention in his habilitation [[seminar|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.{{sfn|Hahn|1966|p=52}} One department head remarked: "it is incredible what one gets to be a ''Privatdozent'' these days!"{{sfn|Hahn|1966|p=50}}

[[File:Berliner Physiker u Chemiker 1920.jpg|thumb|right|Physicists and chemists in Berlin in 1920. Front row, left to right: [[Hertha Sponer]], [[Albert Einstein]], Ingrid Franck, [[James Franck]], [[Lise Meitner]], [[Fritz Haber]], and Otto Hahn. Back row, left to right: [[Walter Grotrian]], [[Wilhelm Westphal]],
{{ill|Otto von Baeyer|de}}, {{ill|Peter Pringsheim|de}} 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 colloquia where, on 28 September 1907, he made the acquaintance of the Austrian physicist [[Lise Meitner]]. 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 her as a possible collaborator. So began the thirty-year collaboration and lifelong close friendship between the two scientists.{{sfn|Hahn|1966|p=50}}{{sfn|Hahn|1966|p=65}}

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 enter 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.{{sfn|Sime|1996|pp=28–29}}

===Discovery of radioactive recoil===
Harriet Brooks observed a [[atomic recoil|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 (physicist)|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 ([[radium-223]]). He also discovered that at the moment when a radioactinium (thorium-227) 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.{{sfn|Hahn|1966|pp=58–64}}

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{{'}}{{'}} (thallium-207) and thorium C{{'}}{{'}} (thallium-208).{{sfn|Hahn|1966|pp=58–64}} The physicist [[Walther Gerlach]] described radioactive recoil as "a profoundly significant discovery in physics with far-reaching consequences".{{sfn|Gerlach|Hahn|1984|p=39}}

==Kaiser Wilhelm Institute for Chemistry==
[[File:Dahlem Thielallee Hahn-Meitner-Bau.JPG|thumb|right|Former Kaiser Wilhelm Institute for Chemistry building in Berlin. Heavily damaged by bombing during the Second World War, it was restored and became part of the [[Free University of Berlin]]. It was renamed the Otto Hahn Building in 1956, and the Hahn-Meitner Building in 2010.{{sfn|Sime|1996|p=368}}<ref>{{cite web |title=Ehrung der Physikerin Lise Meitner Aus dem Otto-Hahn-Bau wird der Hahn-Meitner-Bau |publisher=Free University of Berlin |language=de |trans-title=Honouring physicist Lise Meitner as the Otto Hahn building becomes the Hahn-Meitner building  |url=https://www.fu-berlin.de/campusleben/campus/2010/101028_hahn-meitner/index.html |date=28 October 2010 |access-date=10 June 2020}}</ref> ]]

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]] (KWIC) in Berlin-Dahlem (in what is today the Hahn-Meitner-Building of the [[Free University of Berlin]]). This came with an annual salary of 5,000 marks ({{Inflation|DE|5,000|1912|fmt=eq|cursign=€|r=-3}}). In addition, he received 66,000 marks in 1914 ({{Inflation|DE|66,000|1914|fmt=eq|cursign=€|r=-3}}) from Knöfler for the mesothorium process, of which he gave 10 per cent to Meitner. The new institute was inaugurated on 23 October 1912 in a ceremony presided over by [[Kaiser Wilhelm II]].{{sfn|Sime|1996|pp=44–47}} The Kaiser was shown glowing radioactive substances in a dark room.{{sfn|Hahn|1966|pp=70–72}}

The move to new accommodation was fortuitous, as the wood shop had become heavily contaminated by radioactive liquids that had been spilt, and radioactive gases that had vented and then 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 every 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.{{sfn|Sime|1996|p=48}}

===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]] (2nd Class) for his part in the [[First Battle of Ypres]]. He was a joyful participant in the [[Christmas truce]] of 1914, and was commissioned as a lieutenant.{{sfn|Spence|1970|pp=286–287}} In mid-January 1915, he was summoned to meet chemist [[Fritz Haber]], who explained his plan to break the trench deadlock with [[Chlorine#World War I|chlorine gas]]. Hahn raised the issue that the [[Hague Conventions of 1899 and 1907|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.<ref name="Van der Kloot-2004">{{cite journal|last1=Van der Kloot|first1=W.|title=April 1918: Five Future Nobel Prize-winners Inaugurate Weapons of Mass Destruction and the Academic-industrial-military Complex |journal=Notes and Records of the Royal Society of London |issn=0035-9149 |date=2004 |volume=58 |issue=2 |pages=149–160 |doi=10.1098/rsnr.2004.0053|s2cid=145243958}}</ref>

[[File:Ottohahn1915.jpg|thumb|left|upright|Hahn in uniform in 1915]]
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 [[Second Battle of Ypres|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. 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 [[Battle of Verdun|Verdun]] to introduce shells filled with phosgene to the [[Western Front (World War I)|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.<ref name="Van der Kloot-2004" />{{sfn|Sime|1996|pp=57–61}}

Between operations, Hahn returned to Berlin, where he was able to slip back to his old laboratory and work with Meitner, continuing with their 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, using newly developed rifled ''[[minenwerfer]]s'' 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 following [[Battle of Caporetto]] broke the Italian lines, and the Central Powers overran much of northern Italy. That summer Hahn was accidentally poisoned by phosgene while testing a new model of gas mask. At the end of the war he was in the field in [[mufti (dress)|mufti]] on a secret mission to test a pot that heated and released a cloud of [[arsenical]]s.{{sfn|Spence|1970|pp=287–288}}<ref name="Van der Kloot-2004" />

===Discovery of protactinium===
[[File:Decay Chain of Actinium.svg|thumb|right|upright|The decay chain of actinium. [[Alpha decay]] shifts two elements down; [[beta decay]] shifts one element up.]]
In 1913, chemists Frederick Soddy and [[Kasimir Fajans]] independently observed that [[alpha decay]] caused atoms to move 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 (after [[Dmitri Mendeleev]]) as "ekatantalium", would be an alpha emitter with chemical properties similar to [[tantalum]]. 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.<ref name="Sime-1986">{{cite journal |first=Ruth Lewin |last=Sime |author-link=Ruth Lewin Sime |title=The Discovery of Protactinium |journal=Journal of Chemical Education |issn=0021-9584 |volume=63 |issue=8 |pages=653–657 |date=August 1986 |doi=10.1021/ed063p653 |bibcode=1986JChEd..63..653S }}</ref>

Hahn and Meitner set out to find the missing mother isotope. They developed a new technique for separating the [[refractory metals|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 Austrian Army hospitals, but she returned to the Kaiser Wilhelm Institute in October 1916. Hahn joined the new gas command unit at Imperial Headquarters in Berlin in December 1916 after travelling between the western and eastern front, Berlin and [[Leverkusen]] between mid-1914 and late 1916.{{sfn|Sime|1996|pp=57–61}}

Most of the students, laboratory assistants and technicians had been called up, so Hahn, who was stationed in Berlin between January and September 1917,{{sfn|Hahn|1988|pp=117–132}} and Meitner had to do everything themselves. By December 1917 she was able to isolate the substance, and after further work were able to prove that it was indeed the missing isotope. Meitner submitted her and Hahn's findings for publication in March 1918 to the scientific paper ''[[Physikalische Zeitschrift|Physikalischen Zeitschrift]]'' under the title {{lang|de|Die Muttersubstanz des Actiniums; Ein Neues Radioaktives Element von Langer Lebensdauer}} ("The Mother Substance of Actinium; A New Radioactive Element with a Long Lifetime").<ref name="Sime-1986" /><ref name=":0">{{cite journal |journal=Zeitschrift für Elektrochemie und Angewandte Physikalische Chemie |issn=0372-8323 |title=Die Muttersubstanz des Actiniums, Ein Neues Radioaktives Element von Langer Lebensdauer |trans-title=The Parent Substance of Actinium; A New Radioactive Element with a Long Lifetime |first=Lise |last=Meitner |author-link=Lise Meitner |date=1 June 1918 |language=de |doi=10.1002/bbpc.19180241107 |volume=24 |issue=11–12 |pages=169–173 |s2cid=94448132}}</ref> 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 while brevium had a half-life of 1.7 minutes, Hahn and Meitner's isotope had one of 32,500 years. The name brevium no longer seemed appropriate. Fajans agreed to Meitner and Hahn naming the element "[[protoactinium]]".<ref>{{cite journal |url=https://www.nature.com/articles/244137a0 |doi=10.1038/244137a0 |title=Discovery and Naming of the Isotopes of Element 91 |year=1973 |last1=Fajans |first1=Kasimir |last2=Morris |first2=Donald F. C. |journal=Nature |issn=0028-0836 |volume=244 |issue=5412 |pages=137–138 |bibcode=1973Natur.244..137F |hdl=2027.42/62921 |s2cid=4224336 |hdl-access=free }}</ref>{{sfn|Scerri|2020|pp=302–306}}

In June 1918, Soddy and [[John Arnold Cranston|John Cranston]] announced that they had extracted a sample of the isotope, but unlike Hahn and Meitner were unable to describe its characteristics. They acknowledged Hahn´s and Meitner's priority, and agreed to the name.{{sfn|Scerri|2020|pp=302–306}} 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.<ref name="Sime-1986" /><ref name=":0" /> 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 Jacob Goldschmidt|Heinrich Goldschmidt]], and Fajans himself.<ref name="Nobel Media AB-2020">{{cite web |title=Nomination Archive: Otto Hahn |url=https://www.nobelprize.org/nomination/archive/show_people.php?id=3787 |publisher=Nobel Foundation |access-date=9 June 2020}}</ref><ref>{{cite web |title=Nomination Archive: Lise Meitner |url=https://www.nobelprize.org/nomination/archive/show_people.php?id=6097 |publisher=Nobel Foundation |access-date =9 June 2020}}</ref> In 1949, the International Union of Pure and Applied Chemistry ([[IUPAC]]) named the new element definitively protactinium, and confirmed Hahn and Meitner as discoverers.<ref>{{cite web |title=Protactinium &#124; Pa (Element)  |publisher=National Library of Medicine |website=PubChem |url=https://pubchem.ncbi.nlm.nih.gov/element/Protactinium#section=History |access-date=18 June 2020}}</ref>

===Discovery of nuclear isomerism===
[[File:Decay Chain of Uranium-238corrected.svg|thumb|left|upright|Decay chain of uranium-238]]
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 [[tantalum pentoxide|tantalic acid]]. First the tantalum in the ore was precipitated, then the protactinium. In addition to the uranium X1 (thorium-234) and uranium X2 (protactinium-234), 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 (actinium-228). This was not in any probable decay chain, but it could have been contamination, as the KWIC 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.{{sfn|Hahn|1966|pp=95–103}}{{sfn|Berninger|1983|pp=213–220}} Hahn was now confident enough he had found something that he named his new isotope "uranium Z". In February 1921, he published the first report on his discovery.<ref>{{cite journal |last1= Hahn |first1= O. |title=Über ein neues radioaktives Zerfallsprodukt im Uran |trans-title=On a New Radioactive Decay Product in Uranium |lang=de |doi=10.1007/BF01491321 |journal= Die Naturwissenschaften |issn=0028-1042 |volume=9 |issue=5 |page=84 |year=1921 |bibcode=1921NW......9...84H |s2cid= 28599831 |url= https://zenodo.org/record/2482506 }}</ref>

Hahn determined that uranium Z had a half-life of around 6.7 hours (with a two per cent margin of error) 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 ([[protactinium-234]]), and they both decayed into uranium II ([[uranium-234]]), but with different half-lives.{{sfn|Hahn|1966|pp=95–103}}{{sfn|Berninger|1983|pp=213–220}}<ref>{{cite journal |first=Otto |last=Hahn |title=Uber das Uran Z und seine Muttersubstanz |trans-title=About Uranium Z and its Parent Substance |lang=de |journal=Zeitschrift für Physikalische Chemie  |issn=0942-9352  |volume=103 |issue=1 |pages=461–480 |year=1923 |doi=10.1515/zpch-1922-10325  |s2cid=99021215 }}</ref>

Uranium Z was the first example of [[nuclear isomer]]ism. Walther Gerlach later remarked that this was "a discovery that was not understood at the time but later became highly significant for nuclear physics".{{sfn|Gerlach|Hahn|1984|p=39}} Not until 1936 was [[Carl Friedrich von Weizsäcker]] able to provide a theoretical explanation of the phenomenon.{{sfn|Hoffmann|2001|p=93}}{{sfn|Feather|Bretscher|Appleton|1938|pages=530–535}} 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.<ref name="Nobel Media AB-2020" />

===''Applied Radiochemistry''===
{{quote box|As a young graduate student at the University of California at Berkeley in the mid-1930s and in connection with our work with plutonium a few years later, I used his book ''Applied Radiochemistry'' as my bible. This book was based on a series of lectures which Professor Hahn had given at Cornell in 1933; it set forth the "laws" for the [[co-precipitation]] of minute quantities of radioactive materials when insoluble substances were precipitated from aqueous solutions. I recall reading and rereading every word in these laws of co-precipitation many times, attempting to derive every possible bit of guidance for our work, and perhaps in my zealousness reading into them more than the master himself had intended. I doubt that I have read sections in any other book more carefully or more frequently than those in Hahn's ''Applied Radiochemistry''. In fact, I read the entire volume repeatedly and I recall that my chief disappointment with it was its length. It was too short.|author=Glenn Seaborg |source={{sfn|Hahn|1966|pp=ix–x}} |align=right |width=450px}}

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.{{sfn|Hoffmann|2001|p=94}} While still remaining the head of his own department, he became Deputy Director of the KWIC in 1924, and succeeded Alfred Stock as the director in 1928.<ref name="Max-Planck-Gesellschaft">{{cite web |title=Otto Hahn |publisher=Max-Planck-Gesellschaft |url=https://www.mpg.de/8241484/otto-hahn |access-date=24 June 2020}}</ref> Meitner became the director of the Physical Radioactivity Division, while Hahn headed the Chemical Radioactivity Division.{{sfn|Hoffmann|2001|p=95}}

In the early 1920s, Hahn 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 (and later in Russian) 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 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.{{sfn|Hahn|1966|pp=ix–x}} Hahn is referred to as the father of nuclear chemistry, which emerged from applied radiochemistry.{{sfn|Hahn|1966|p=ix}}<ref>{{cite web |last1=Tietz |first1=Tabea |title=Otto Hahn – the Father of Nuclear Chemistry |url=http://scihi.org/otto-hahn-nuclear-chemistry/ |website=SciHi Blog |date=8 March 2018}}</ref><ref>{{cite web |title=Otto Hahn |url=https://www.atomicheritage.org/profile/otto-hahn |website=Atomic Heritage Foundation |access-date=12 October 2024}}</ref>

==Nazi Germany==

===Impact of Nazism===
[[Fritz Strassmann]] had come to the KWIC to study under Hahn to improve his employment prospects. After the [[Nazi Party]] (NSDAP) [[Adolf Hitler's rise to power|came to power]] in Germany in 1933, Strassmann declined a lucrative offer of employment because it required political training and Nazi Party membership. Later, rather than become a member of a Nazi-controlled organisation, Strassmann resigned from the [[Society of German Chemists]] when it became part of the Nazi [[German Labour Front]]. As a result, he could neither work in the chemical industry nor receive his habilitation, the prerequisite 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.{{sfn|Sime|1996|pp=156–157, 169}}{{sfn|Walker|2006|p=122}}

Hahn spent February to June 1933 in the United States and Canada as a visiting professor at Cornell University.{{sfn|Hahn|1966|p=283}} He gave an interview to the [[Toronto]] ''[[Star Weekly]]'' in which he painted a flattering portrait of [[Adolf Hitler]]: {{blockquote|I am not a Nazi. But Hitler is the hope, the powerful hope, of German youth... At least 20 million people revere him. He began as a nobody, and you see what he has become in ten years.... In any case for the youth, for the nation of the future, Hitler is a hero, a Führer, a saint... In his daily life he is almost a saint. No alcohol, not even tobacco, no meat, no women. In a word: Hitler is an unequivocal Christ.{{sfn|Sime|2006|p=6}} }}

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.{{sfn|Sime|1996|pp=138–139}} 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. The directors of the other Kaiser Wilhelm Institutes, even the Jewish ones, complied with the new law,{{sfn|Sime|1996|pp=8–9}} 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.{{sfn|Sime|2006|p=7}} 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 reoriented it towards chemical warfare research.{{sfn|Sime|2006|p=10}}
[[File:München-2025-Deutsches_Museum-Hahn.jpg|thumb|left|upright|Otto Hahn's marble bust at the [[Deutsches Museum]] in Munich]]
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. Hahn brokered a deal whereby 10 per cent of the funds would be allocated to Haber's people and the rest to KWS, but the [[Rockefeller Foundation]] insisted that the funds be used for their original scientific research or else be returned. In August 1933 the administrators of the KWS were alerted that several boxes of Rockefeller Foundation-funded equipment were about to be shipped to [[Herbert Freundlich]], one of the department heads that Hahn had dismissed, who was now working in England. {{ill|Ernst Telschow|de}}, a Nazi Party member, was in charge while Planck, the president of the KWS since 1930, was on vacation, and he ordered the shipment halted. Hahn complied, but he disagreed with the decision on the grounds that funds from abroad should not be diverted to military research, which the KWS was increasingly undertaking. When Planck returned from vacation, he ordered Hahn to expedite the shipment.{{sfn|Sime|2006|pp=10–12}}<ref>{{cite web |title=Max Planck Becomes President of the KWS |publisher=Max-Planck Gesellschaft |url=https://www.mpg.de/947146/28_person6-1930 |access-date=23 June 2020}}</ref>

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.{{sfn|Sime|2006|p=10}}{{sfn|Walker|2006|pp=122–123}} The ageing Planck did not seek re-election, and was succeeded in 1937 as president by [[Carl Bosch]], a winner of the Nobel Prize in Chemistry and the chairman of the board of [[IG Farben]], a company which had bankrolled the Nazi Party since 1932. Telschow became Secretary of the KWS. He was an enthusiastic supporter of the Nazis, but was also loyal to Hahn, being one of his former students, and Hahn welcomed his appointment.<ref>{{cite web |title=The KWS Introduces the 'Führerprinzip' |publisher=Max-Planck Gesellschaft |url=https://www.mpg.de/946951/35_event24-1937 |access-date=23 June 2020}}</ref>{{sfn|Sime|2006|p=10}} Hahn's chief assistant, Otto Erbacher, became the KWI for Chemistry's party steward (''Vertrauensmann'').{{sfn|Sime|1996|p=143}}

===Rubidium–strontium dating===
While Hahn was in North America in 1905–1906, his attention had been drawn to a mica-like mineral from [[Manitoba]] that contained [[rubidium]]. He had studied the radioactive decay of [[rubidium-87]], and had estimated its half-life at 2 x 10<sup>11</sup> years. It occurred to him that by comparing the quantity of strontium in the mineral (which had once been rubidium) 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 superior 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.{{sfn|Hahn|1966|pp=85–88}}

In 1937, Strassmann and Ernst Walling extracted 253.4 milligrams of strontium carbonate from 1,012 grams of the mineral, 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 10<sup>11</sup> years: quite close to Hahn's original calculation.<ref>{{cite journal |last1=Hahn |first1=O. |last2=Strassman |first2=F. |last3=Walling |first3=E. |title=Herstellung wägbaren Mengen des Strontiumisotops 87 als Umwandlungsprodukt des Rubidiums aus einem kanadischen Glimmer |trans-title=Production of Weighable Amounts of the Strontium Isotope 87 as a Conversion Product of Rubidium from Canadian Mica |language=de |journal=Naturwissenschaften |issn=0028-1042 |volume=25 |issue=12 |date=19 March 1937 |page=189 |doi=10.1007/BF01492269 |bibcode=1937NW.....25..189H |doi-access=free }}</ref><ref>{{cite journal |last1=Hahn |first1=O. |last2=Walling |first2=E. |title=Über die Möglichkeit geologischer Alterbestimmung rubidiumhaltiger Mineralen und Gesteine |trans-title=On the Possibility of Geological Age Determination of Minerals and Rocks Containing Rubidium |language=de  |journal=Zeitschrift für anorganische und allgemeine Chemie |issn=0044-2313 |volume=236 |issue=1 |date=12 March 1938 |pages=78–82 |doi=10.1002/zaac.19382360109 }}</ref> [[Rubidium–strontium dating]] became a widely used technique for dating rocks in the 1950s, when [[mass spectrometry]] became common.{{sfn|Bowen|1994|pp=162–163}}

===Discovery of nuclear fission===
{{Main|Discovery of nuclear fission}}
[[File:Nuclear Fission Experimental Apparatus 1938 - Deutsches Museum - Munich.jpg|thumb|right| This set up is on display in the [[Deutsches Museum]] in [[Munich]]. The table and instruments are original, but the instruments would not have been together on the one table in the same room.<ref>{{cite web |url=https://digital.deutsches-museum.de/de/digital-catalogue/collection-object/71930/ |title=Originalgeräte zur Entdeckung der Kernspaltung, 'Hahn-Meitner-Straßmann-Tisch' |trans-title=Original equipment for the discovery of nuclear fission, 'Hahn-Meitner-Straßmann table' |lang=de |publisher=Deutsches Museum |access-date=8 October 2024 }}</ref> Pressure from historians, scientists and feminists caused the museum to alter the display in 1988 to acknowledge [[Lise Meitner]], [[Otto Frisch]] and [[Fritz Strassmann]].{{sfn|Sime|2010|pages=206–210}} ]]
After [[James Chadwick]] discovered the [[neutron]] in 1932,{{sfn|Rhodes|1986|pp=39, 160–167, 793}} [[Irène Curie]] and [[Frédéric Joliot]] irradiated aluminium foil with alpha particles. They found that this results in a short-lived radioactive [[isotopes of phosphorus|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.{{sfn|Rhodes|1986|pp=200–201}}{{sfn|Sime|1996|pp=161–162}} Chadwick noted that being electrically neutral, neutrons could penetrate the [[atomic nucleus]] more easily than protons or alpha particles.{{sfn|Fergusson|2011|pp=150–151}} [[Enrico Fermi]] and his colleagues in Rome picked up on this idea,{{sfn|Rhodes|1986|pp=210–211}} and began irradiating elements with neutrons.<ref name="Segrè-1989" />

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 with neutrons, they found a complex mix of half-lives. Fermi therefore concluded that the new elements with atomic numbers greater than 92 (known as [[transuranium elements]]) had been created.<ref name="Segrè-1989">{{cite journal |title=Discovery of Nuclear Fission |first=Emilio G. |last=Segrè |author-link=Emilio Segrè |journal=Physics Today |issn=0031-9228 |volume=42 |issue=7 |date=July 1989 |pages=38–43 |doi=10.1063/1.881174 |bibcode=1989PhT....42g..38S }}</ref> 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.{{sfn|Sime|1996|pp=164–165}} They set out to determine whether or not the 13-minute isotope was indeed an isotope of protactinium.{{sfn|Hahn|1966|pp=140–141}}

Between 1934 and 1938, Hahn, Meitner and Strassmann found a great number of radioactive transmutation products, all of which they regarded as transuranic.<ref>{{cite magazine|last1=Hahn |first1=O. |title=The Discovery of Fission |doi= 10.1038/scientificamerican0258-76 |magazine=Scientific American |volume=198 |issue=2 |pages=76–84 |year=1958 |bibcode=1958SciAm.198b..76H}}</ref> At that time, the existence of [[actinide]]s was not yet established, and uranium was wrongly believed to be a [[group 6 element]] similar to [[tungsten]]. It followed that the first transuranic elements would be similar to group 7 to 10 elements, i.e. [[rhenium]] and [[platinoid]]s. They established the presence of multiple isotopes of at least four such elements, and (mistakenly) 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.{{sfn|Sime|1996|pp=170–172}} 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.{{sfn|Sime|1996|pp=170–172}}
[[File:Otto Hahn's notebook 1938 - Deutsches Museum - Munich.jpg|thumb|left|Otto Hahn's notebook]]
In May 1937, they issued parallel reports, one in the ''[[Zeitschrift für Physik]]'' with Meitner as the principal author, and one in the ''[[Chemische Berichte]]'' with Hahn as the principal author.{{sfn|Sime|1996|pp=170–172}}<ref name="L.-1937">{{cite journal |first1=Meitner |last1=L. |author-link1=Lise Meitner |first2=Hahn |last2=O. |first3=F. |last3=Strassmann |author-link3=Fritz Strassmann |title=Über die Umwandlungsreihen des Urans, die durch Neutronenbestrahlung erzeugt werden |trans-title=On the series of transformations of uranium that are generated by neutron radiation |language=de |journal=Zeitschrift für Physik |issn=0939-7922 |volume=106 |issue=3–4 |pages=249–270 |date=May 1937 |doi=10.1007/BF01340321 |bibcode=1937ZPhy..106..249M |s2cid=122830315 }}</ref><ref name="O.-1937">{{cite journal |first1=Hahn |last1=O.  |first2=Meitner |last2=L. |author-link2=Lise Meitner |first3=F. |last3=Strassmann |author-link3=Fritz Strassmann |title=Über die Trans-Urane und ihr chemisches Verhalten |trans-title=On the transuranes and their chemical behaviour |lang=de |journal=Berichte der Deutschen Chemischen Gesellschaft |issn=0365-9496 |date=9 June 1937 |volume=70 |issue=6 |pages=1374–1392 |doi=10.1002/cber.19370700634 }}</ref> Hahn concluded his by stating emphatically: {{lang|de|Vor allem steht ihre chemische Verschiedenheit von allen bisher bekannten Elementen außerhalb jeder Diskussion}} ("Above all, their chemical distinction from all previously known elements needs no further discussion").<ref name="O.-1937"/> Meitner, however, 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,{{sfn|Sime|1996|p=177}} reporting that: {{lang|de|Also müssen die Prozesse Einfangprozesse des Uran 238 sein, was zu drei isomeren Kernen Uran 239 führt. Dieses Ergebnis ist mit den bisherigen Kernvorstellungen sehr schwer in Übereinstimmung zu bringen}} ("The processes 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.")<ref name="L.-1937" />

With the ''[[Anschluss]]'', Germany's annexation of Austria on 12 March 1938, Meitner lost her Austrian citizenship,{{sfn|Sime|1996|pp=184–185}} 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.{{sfn|Sime|1996|pp=200–207}} 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 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]] (by removing around 100 nucleons) was seen as preposterous.{{sfn|Sime|1996|pp=227–230}}

During a visit to Copenhagen on 10 November, Hahn discussed these results with [[Niels Bohr]], Meitner, and [[Otto Robert Frisch]].{{sfn|Sime|1996|pp=227–230}} 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:
{{blockquote|We are more and more coming to the awful conclusion that our Ra isotopes behave not like Ra, but like Ba... Perhaps you can come up with some fantastic explanation. We ourselves realize that it ''can't'' actually burst apart into Ba. Now we want to test whether the Ac-isotopes derived from the "Ra" behave not like Ac but like La.{{sfn|Sime|1996|p=233}}}}

[[File:Dahlem Thielallee Hahn-Meitner-Bau-2.JPG|thumb|right|upright|Plaque commemorating Hahn and Strassmann's discovery of fission in Berlin (unveiled in 1956)]]
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.<ref>{{cite journal |last1= Hahn |first1= O. |author-link= Otto Hahn |last2= Strassmann |first2= F. |author-link2=Fritz Strassmann |doi= 10.1007/BF01488241 |title=Über den Nachweis und das Verhalten der bei der Bestrahlung des Urans mittels Neutronen entstehenden Erdalkalimetalle |language=de |journal= Die Naturwissenschaften |issn=0028-1042 |volume=27 |issue=1 |pages=11–15 |year=1939 |bibcode=1939NW.....27...11H |s2cid= 5920336 |trans-title=On the detection and characteristics of the alkaline earth metals formed by irradiation of uranium with neutrons}}</ref> On 27 December, Hahn telephoned the editor of the ''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]] (then called "masurium"), mistakenly believing that the [[atomic mass]]es had to add up rather than the [[atomic number]]s. By January 1939, he was sufficiently convinced of the formation of light elements that he published a new revision of the article, retracting former claims of observing transuranic elements and neighbours of uranium.{{sfn|Sime|1996|pp=248–249}}

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.{{sfn|Frisch|1979|pp=115–116}}<ref>{{cite journal |last1= Meitner |first1= L. |author-link1= Lise Meitner |last2= Frisch |first2= O. R. |author-link2= Otto Robert Frisch |doi= 10.1038/143239a0 |title= Disintegration of Uranium by Neutrons: A New Type of Nuclear Reaction |journal= Nature |issn=0028-0836 |volume=143 |issue=3615 |page=239 |date=January 1939 |bibcode= 1939Natur.143..239M|s2cid= 4113262 }}</ref><ref>{{cite journal |last1=Frisch |first1=O. R. |author-link1=Otto Robert Frisch |title=Physical Evidence for the Division of Heavy Nuclei Under Neutron Bombardment |doi=10.1038/143276a0 |journal=Nature |issn=0028-0836 |volume=143 |issue=3616 |page=276 |date=February 1939 |bibcode=1939Natur.143..276F|s2cid= 4076376 |doi-access= free }}</ref> In their second publication on nuclear fission, Hahn and Strassmann used the term ''Uranspaltung'' (uranium fission) 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]].<ref>{{cite journal |last1=Hahn |first1=O. |last2=Strassmann |first2=F. |author-link2=Fritz Strassmann |title=Nachweis der Entstehung aktiver Bariumisotope aus Uran und Thorium durch Neutronenbestrahlung; Nachweis weiterer aktiver Bruchstücke bei der Uranspaltung |trans-title=Evidence of the Formation of Active Barium Isotopes From Uranium and Thorium by Neutron Irradiation; Evidence of Further Active Fragments During Uranium Fission |lang=de |journal=Naturwissenschaften |issn=0028-1042 |volume=27 |issue=6 |pages=89–95 |date=February 1939 |doi=10.1007/BF01488988 |bibcode=1939NW.....27...89H |s2cid=33512939 }}</ref> This was shown to be the case by Frédéric Joliot and his team in March 1939.<ref>{{cite journal|last1=Von Halban |first1=H. |author-link=Hans von Halban |last2=Joliot |first2=F. |author-link2=Frédéric Joliot |last3=Kowarski |first3=L. |author-link3=Lew Kowarski |title=Number of Neutrons Liberated in the Nuclear Fission of Uranium|journal=Nature  |issn=0028-0836 |volume=143|issue=3625|date=22 April 1939|pages=680 |doi=10.1038/143680a0|bibcode=1939Natur.143..680V |s2cid=4089039 |doi-access=free }}</ref> [[Edwin McMillan]] and [[Philip Abelson]] used the [[cyclotron]] at the [[Berkeley Radiation Laboratory]] to bombard uranium with neutrons, and 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]].{{sfn|Walker|1993|pp=22–23}} "There goes a Nobel Prize", Hahn remarked.{{sfn|Hoffmann|2001|p=150}}

At the KWIC, [[Kurt Starke]] independently produced element 93, using only the weak neutron sources available there. Hahn and Strassmann then began researching its chemical properties.{{sfn|Hahn|1966|pp=175–177}} They knew that it should decay into the [[plutonium|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.{{sfn|Walker|1993|pp=22–23}} Part of the problem was that they still believed that element 94 was a platinoid, which confounded their attempts at chemical separation.{{sfn|Hahn|1966|pp=175–177}}

===World War II===
{{Main|German nuclear weapons program}}
On 24 April 1939, [[Paul Harteck]] and his assistant, [[Wilhelm Groth]], had written to the [[Oberkommando der Wehrmacht|Armed Forces High Command]] (OKW), alerting it to the possibility of the development of an [[atomic bomb]]. In response, the Army Weapons Branch (HWA) had established a physics section under the 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 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.{{sfn|Hoffmann|2001|pp=156–161}} 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.{{sfn|Walker|2006|p=132}}

On the night of 15 February 1944, the KWIC building was struck by a bomb.{{sfn|Walker|2006|p=132}} Hahn's office was destroyed, along with his correspondence with Rutherford and other researchers, and many of his personal possessions.{{sfn|Walker|2006|p=137}}{{sfn|Hoffmann|2001|p=188}} The office was the intended target of the raid, which had been ordered by [[Brigadier general (United States)|Brigadier General]] [[Leslie Groves]], the director of the [[Manhattan Project]], in the hope of disrupting the German uranium project.{{sfn|Norris|2002|pp=294–295}} [[Albert Speer]], the [[Reich Minister of Armaments and War Production]], arranged for the institute to move to Tailfingen (today part of [[Albstadt]]) in southern Germany. All work in Berlin ceased by July. Hahn and his family moved to the house of a textile manufacturer there.{{sfn|Walker|2006|p=137}}{{sfn|Hoffmann|2001|p=188}}

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 [[Forced labour under German rule during World War II|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 KWIC, 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 the [[Sachsenhausen concentration camp]] who mined it in [[Oranienburg]]. Another physicist with a Jewish wife was {{ill|Heinrich Rausch von Traubenberg|de|Heinrich Rausch von Traubenberg}}. 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.{{sfn|Walker|1993|pp=132–133}}{{sfn|Sime|2006|pp=19–21}}

==Post-war==

===Incarceration in Farm Hall===
{{Main|Operation Epsilon}}
On 25 April 1945, an armoured task force from the British−American [[Alsos Mission]] arrived in Tailfingen, and surrounded the KWIC. 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 dilapidated 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]].{{sfn|Hahn|1966|p=179}}{{sfn|Walker|1993|pp=158–159}}{{sfn|Hoffmann|2001|p=195}} 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.{{sfn|Sime|2006|pp=24–25}}

[[File:Farm Hall.jpg|thumb|right|Farm Hall (seen here in 2015)]]
They were moved to the Château de Facqueval in [[Modave]], Belgium, where Hahn used the time to work on his memoirs and then, on 3 July, were flown to England. They arrived at [[Farm Hall]], [[Godmanchester]], near [[Cambridge]], on 3 July. While they were there, all their conversations, indoors and out, were covertly recorded with 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 the USSR. In August 1945, the German scientists were informed of the [[atomic bombing of Hiroshima]]. Up to this point the scientists, except Harteck, 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.{{sfn|Sime|2006|pp=24–25}}{{sfn|Walker|1993|pp=159–160}}{{sfn|Hoffmann|2001|pp=196–199}}{{sfn|Walker|2006|p=139}}

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 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 academe.{{sfn|Sime|2006|pp=26–28}} It infuriated Goudsmit, whose parents had been murdered in [[Auschwitz]].{{sfn|Sime|1996|p=319}} On 3 January 1946, six months after they had arrived at Farm Hall, the group was allowed to return to Germany.{{sfn|Hoffmann|2001|p=201}} Hahn, Heisenberg, von Laue and von Weizsäcker were brought to [[Göttingen]], which was controlled by the British occupation authorities.{{sfn|Hoffmann|2001|pp=205–206}}

===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."<ref>{{cite web |title=The Nobel Prize in Chemistry 1944 |publisher=Nobel Foundation |url=http://nobelprize.org/nobel_prizes/chemistry/laureates/1944/index.html |access-date=17 December 2007}}</ref><ref name="Nobel Foundation">{{cite web |title=The Nobel Prize in Chemistry 1944: Presentation Speech |publisher=Nobel Foundation |url=http://nobelprize.org/nobel_prizes/chemistry/laureates/1944/press.html |access-date=3 January 2008}}</ref> 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''.{{sfn|Bernstein|2001|pp=282–283}} His fellow interned  scientists celebrated his award by giving speeches, making jokes, and composing songs.{{sfn|Bernstein|2001|pp=286–288, 323–324}}

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.<ref name="Nobel Media AB-2020"/> 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 {{ill|Arne Westgren|de||sv}}. 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.{{sfn|Crawford|Sime|Walker|1997|pp=27–32}}

[[File:Otto Hahn Nobelpreis 1945-a.jpg|thumb|left|upright=1.3|Hahn's Nobel Prize for Chemistry]]
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.{{sfn|Crawford|2000|pp=38–40}} 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.{{sfn|Crawford|Sime|Walker|1997|pp=27–32}}

The invitation to attend the Nobel festivities was transmitted via the British Embassy in Stockholm.{{sfn|Crawford|2000|p=49}} On 4 December, Hahn was persuaded by two of his Alsos captors, American [[Lieutenant colonel (United States)|Lieutenant Colonel]] Horace K. Calvert and British [[Lieutenant commander (Royal Navy)|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.{{sfn|Bernstein|2001|pp=311, 325}} 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.<ref>{{cite web |title=Statutes of the Nobel Foundation  |date= 9 August 2018 |publisher= Nobel Foundation |url=https://www.nobelprize.org/about/statutes-of-the-nobel-foundation/ |access-date=25 June 2020}}</ref>{{sfn|Crawford|2000|pp=49–50}}

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.{{sfn|Crawford|2000|pp=49–50}} 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.<ref name="Nobel Foundation"/>{{sfn|Crawford|2000|pp=49–50}}{{sfn|Hoffmann|2001|p=209}} Hahn gave 10,000 krona of his prize to Strassmann, who refused to use it.{{sfn|Hoffmann|2001|p=209}}{{sfn|Sime|1996|p=343}}

===Founder and President of the Max Planck Society===
[[File:Gedenktafel Otto Hahn.jpg|thumb|upright|Monument in Berlin-Dahlem, in front of the Otto-Hahn-Platz]]
The suicide of [[Albert Vögler]] on 14 April 1945 left the KWS without a president.<ref name="Max-Planck-Gesellschaft" /> 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, Planck 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.<ref>{{cite web |title=Gerard Kuiper's Daring Rescue of Max Planck at the End of World War II |first=Brandon R. |last=Brown |date=16 May 2015 |publisher=Scientific American Blog Network |url=https://blogs.scientificamerican.com/guest-blog/gerard-kuiper-s-daring-rescue-of-max-planck-at-the-end-of-world-war-ii/ |access-date=27 June 2020}}</ref><ref>{{cite web |title=The End of the War and Transition. Max Planck is Interim President of the KWS  |publisher=Max-Planck-Gesellschaft |url=https://www.mpg.de/955542/3_event1-1945 |access-date=27 June 2020}}</ref> 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.<ref name="Max-Planck-Gesellschaft" /> 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 persuaded him to accept. After his return to Germany, he assumed the office on 1 April 1946.{{sfn|Hoffmann|2001|p=199}}{{sfn|Macrakis|1993|pp=189–190}}

[[Allied Control Council]] Law No. 25 on the control of scientific research dated 29 April 1946 restricted German scientists to conducting basic research only,<ref name="Max-Planck-Gesellschaft" /> and on 11 July the Allied Control Council dissolved the KWS on the insistence of the Americans,{{sfn|Macrakis|1993|pp=190–191}} who considered that it had been too close to the national socialist regime, and was a threat to world peace.<ref name="Max-Planck-Gesellschaft-2">{{cite web |title=The Birth of the Max Planck Society |publisher=Max-Planck-Gesellschaft |url=https://www.mpg.de/955581/5_event2-1946 |access-date=27 June 2020}}</ref> However, the British, who had voted against the dissolution, were more sympathetic, and offered to let the Kaiser Wilhelm Society continue 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.{{sfn|Walker|2006|pp=145–147}} To Hahn, the name represented the good old days of the [[German Empire]], however authoritarian and undemocratic it was, before the hated Weimar Republic.{{sfn|Walker|2006|p=152}} Heisenberg asked Niels Bohr for support, but Bohr recommended that the name be changed.{{sfn|Walker|2006|pp=145–147}} Lise Meitner wrote to Hahn, explaining that:{{blockquote|Outside of Germany it is considered so obvious that the tradition from the period of Kaiser Wilhelm has been disastrous and that changing the name of the KWS is desirable, that no one understands the resistance against it. For the idea, that the Germans are the chosen people and have the right to use any and all means to subordinate the "inferior" people, has been expressed over and over again by historians, philosophers, and politicians and finally the Nazis tried to translate it into fact... The best people among the English and Americans wish that the best Germans would understand that there should be a definitive break with this tradition, which has brought the entire world and Germany itself the greatest misfortune. And as a small sign of German understanding the name of the KWS should be changed. What's in a name, if it is a matter of the existence of Germany and thereby Europe?{{sfn|Walker|2006|p=147}} }}

In September 1946, a new Max Planck Society was established at [[Bad Driburg]] in the British Zone.<ref name="Max-Planck-Gesellschaft-2"/> 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]] (or West-Germany) was formed in 1949, the five institutes located in the French zone joined them.<ref>{{cite web |title=The Founding of Today's Max Planck Society |publisher=Max-Planck-Gesellschaft |url=https://www.mpg.de/955620/7_event3-1948 |access-date=27 June 2020}}</ref> The KWIC, now under Strassmann, built and renovated new accommodation in [[Mainz]], but work proceeded slowly, and it did not relocate from Tailfingen until 1949.<ref>{{cite web |title=Overview  |publisher= Max Planck Institute for Chemistry |url=https://www.mpic.de/3537786/Overview |access-date=27 June 2020}}</ref> Hahn's insistence on retaining Telschow as the general secretary nearly caused a rebellion against his presidency.{{sfn|Sime|2006|p=12}} In his efforts to rebuild German science, Hahn was generous in issuing  ''[[persilschein]]'' (whitewash certificates), writing one for [[Gottfried von Droste]], who had joined the ''[[Sturmabteilung]]'' (SA) in 1933 and the NSDAP in 1937, and wore his SA uniform at the KWIC,{{sfn|Walker|2006|p=124}} and for [[Heinrich Hörlein]] and [[Fritz ter Meer]] from IG Farben.{{sfn|Sime|2004|p=48}} 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 Mark]]s in 1949 (equivalent to €{{Inflation|DE|12|1949}} million in {{Inflation/year|DE}}) to 47 million in 1960 (equivalent to €{{Inflation|DE|47|1960}} million in {{Inflation/year|DE}}), and the workforce grew from 1,400 to nearly 3,000.<ref name="Max-Planck-Gesellschaft" />

===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. The historian [[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."{{sfn|Badash|1983|p=176}}

[[File:Forschungsschiff OTTO HAHN (Kiel 33.305).jpg|thumb|left|Hahn (fourth from the left) at the [[Howaldtswerke-Deutsche Werft]] plant in June 1964]]

In early 1954, he wrote the article "Cobalt 60{{snd}}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.{{sfn|Hoffmann|2001|pp=218–221}} The following year he initiated and organised the [[Mainau Declaration]] of 1955, in which he and other international Nobel Prize-winners called attention to the dangers of atomic weapons and urgently warned the nations of the world 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.{{sfn|Hoffmann|2001|pp=221–222}}

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 (''[[Bundeswehr]]'').{{sfn|Hoffmann|2001|pp=231–232}} This resulted in Hahn receiving an invitation to meet the [[Chancellor of Germany]], [[Konrad Adenauer]] and other senior officials, including the Defense Minister, [[Franz Josef Strauss]], and  Generals [[Hans Speidel]] and [[Adolf Heusinger]] (who had both been generals in the Nazi era). The two generals argued that the ''Bundeswehr'' needed nuclear weapons, and Adenauer accepted their advice. A communiqué was drafted that said that the Federal Republic did not manufacture nuclear weapons, and would not ask its scientists to do so.{{sfn|Hoffmann|2001|pp=235–238}} Instead, the German forces were equipped with US nuclear weapons.<ref>{{cite magazine |title=NATO Chief Backs Germany's Vow to Keep War-Ready US Nukes |magazine=Defence News |first=Sebastian |last=Sprenger  |date=11 May 2020 |url=https://www.defensenews.com/global/europe/2020/05/11/nato-chief-backs-german-vow-to-keep-war-ready-us-nukes/ |access-date=28 June 2020}}</ref>

On 13 November 1957, in the ''Konzerthaus'' (Concert Hall) in [[Vienna]], Hahn warned of the "dangers of A- and H-bomb-experiments", and declared that "today war is no means of politics anymore{{snd}}it will only destroy all countries in the world". His highly acclaimed speech was transmitted internationally by the Austrian radio, [[Österreichischer Rundfunk]] (ÖR). 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.{{sfn|Hahn|1988|p=288}}{{sfn|Hoffmann|2001|p=242}}
[[File:12. Tagung 1962 Physik; Bayrisches Frühstück Otto Hahn, L. Meitner - W134Nr.069602d - Willy Pragher.jpg|thumb|right|With Meitner in 1962]]
In 1959 Hahn co-founded in Berlin the [[Federation of German Scientists]] (VDW), a non-governmental organisation, 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 economic 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.<ref>{{cite web|title=FGS Brochure|url=https://www.cbd.int/doc/external/mop-04/fgs-1-en.pdf |archive-url=https://web.archive.org/web/20080511012724/http://www.cbd.int/doc/external/mop-04/fgs-1-en.pdf |archive-date=11 May 2008 |url-status=live|website=Convention on Biological Diversity|publisher=Federation of German Scientists|access-date=28 June 2020}}</ref> Right up to his death, Otto Hahn never tired of warning of the dangers of the [[nuclear arms race]] between the great powers and of the [[radioactive contamination]] of the planet.{{sfn|Hoffmann|2001|p=248}}

Lawrence Badash wrote:

{{blockquote|The important thing is not that scientists may disagree on where their responsibility to society lies, but that they are conscious that a responsibility exists, are vocal about it, and when they speak out they expect to affect policy. Otto Hahn, it would seem, was even more than just an example of this twentieth-century conceptual evolution; he was a leader in the process.{{sfn|Badash|1983|p=178}} }}
He was one of the signatories of the agreement to convene a convention for drafting a [[world constitution]].<ref>{{Cite web |title=Letters from Thane Read asking Helen Keller to sign the World Constitution for world peace. 1961 |url=https://www.afb.org/HelenKellerArchive?a=d&d=A-HK01-07-B149-F04-022.1.8 |access-date=1 July 2023 |department=Helen Keller Archive |publisher=American Foundation for the Blind}}</ref><ref>{{Cite web |title=Letter from World Constitution Coordinating Committee to Helen, enclosing current materials |url=https://www.afb.org/HelenKellerArchive?a=d&d=A-HK01-07-B154-F05-028.1.6 |access-date=3 July 2023 |department=Helen Keller Archive |publisher=American Foundation for the Blind}}</ref> As a result, for the first time in human history, a [[World Constituent Assembly]] convened to draft and adopt a [[Constitution for the Federation of Earth]].<ref>{{Cite encyclopedia |title=Preparing Earth Constitution |url=http://encyclopedia.uia.org/en/strategy/193465 |access-date=15 July 2023 |encyclopedia=The Encyclopedia of World Problems |publisher=Union of International Associations |archive-date=19 July 2023 |archive-url=https://web.archive.org/web/20230719215501/http://encyclopedia.uia.org/en/strategy/193465 |url-status=dead }}</ref>

==Private life==
[[File:Otto Hahn Punta BMK.jpg|thumb|right|Marble plaque in Latin by Professor Massimo Ragnolini, commemorating the honeymoon of Otto Hahn and his wife Edith at Punta San Vigilio, [[Lake Garda]], Italy, in March and April 1913]]
In June 1911, while attending a conference in [[Stettin]], Hahn met {{ill|Edith Junghans|de}} (1887–1968), 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 were married in Stettin, where Edith's 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, they visited Vienna, and then Budapest, where they stayed with [[George de Hevesy]].{{sfn|Spence|1970|p=286}}

They had one child, [[Hanno Hahn]], who was born on 9 April 1922.<ref name="Hanno" /> Hanno enlisted in the army in 1942, and served on the [[Eastern Front (World War II)|Eastern Front]] in World War II as a panzer commander. He lost an arm in combat. After the war he became an art historian and architectural researcher (at the Hertziana in Rome), 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 née Pletz. They left a fourteen-year-old son, {{ill|Dietrich Hahn|de}}.<ref name="Hanno">{{cite web |first=Lee |last=Sorensen |title=Hahn, Hanno  |date= 21 February 2018 |publisher= Dictionary of Art Historians |url=https://arthistorians.info/hahnh |access-date=18 June 2020}}</ref>

In 1990, the {{ill|Hanno and Ilse Hahn Prize|de|Hanno-und-Ilse-Hahn-Preis}} 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.<ref>{{cite web |archive-url=https://web.archive.org/web/20110109132200/http://www.mpg.de/ueberDieGesellschaft/profil/auszeichnungen/auszeichnungDerMPG/hannoIlseHahnPreis/index.html |archive-date=9 January 2011 |language=de |title=Hanno-und-Ilse-Hahn-Preis |url=http://www.mpg.de/ueberDieGesellschaft/profil/auszeichnungen/auszeichnungDerMPG/hannoIlseHahnPreis/index.html |publisher=Max-Planck-Gesellschaft }}</ref>

==Death and legacy==

===Death===
[[File:Grave of Otto Hahn at Stadtfriedhof Göttingen 2017 01.jpg|thumb|Hahn's grave in Göttingen. The inscription refers to his discovery of nuclear fission.]]
Hahn was shot in the back in October 1951 by a disgruntled inventor who wished to highlight the neglect of his ideas by mainstream scientists. Hahn was injured in a motor vehicle accident in 1952, and had a minor heart attack the following year. In 1962, he published a book, {{Lang|de|Vom Radiothor zur Uranspaltung}} ({{lit|''From Radiothorium to Uranium Fission''}}). It was released in English 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 died in Göttingen on 28 July 1968. His wife Edith survived him by only a fortnight.{{sfn|Spence|1970|pp=300–301}} He was buried in the [[Stadtfriedhof]] in Göttingen.<ref>{{cite web |title=Grab von Otto Hahn aus Göttingen |website=www.friedhofguide.de |url=https://www.friedhofguide.de/grab/deutschland/G%C3%B6ttingen/Stadtfriedhof+G%C3%B6ttingen/Otto+Hahn+08.03.1879_28.07.1968 |access-date=28 June 2020 |archive-url=https://web.archive.org/web/20200629233454/https://www.friedhofguide.de/grab/deutschland/G%C3%B6ttingen/Stadtfriedhof+G%C3%B6ttingen/Otto+Hahn+08.03.1879_28.07.1968 |archive-date=29 June 2020}}</ref><ref>{{cite web |title=Nobelpreisträger auf dem Stadtfriedhof |trans-title=Nobel Prize winners at the City Cemetery |publisher=City of Göttingen |lang=de |url=http://www.goettingen.de/pics/medien/1_1229606299/Nobel-Rondell.pdf |archive-url=https://web.archive.org/web/20150924022846/http://www.goettingen.de/pics/medien/1_1229606299/Nobel-Rondell.pdf |archive-date=24 September 2015 |url-status=dead }}</ref>
The day after his death, the Max Planck Society published the following obituary notice:
{{blockquote|On 28 July, in his 90th year, our Honorary President Otto Hahn passed away. His name will be recorded in the history of humanity as [[atomic age|the founder of the atomic age]]. In him Germany and the world have lost a scholar who was distinguished in equal measure by his integrity and personal humility. The Max Planck Society mourns its founder, who continued the tasks and traditions of the Kaiser Wilhelm Society after the war, and mourns also a good and much loved human being, who will live in the memories of all who had the chance to meet him. His work will continue. We remember him with deep gratitude and admiration.<ref>Quoted in {{cite journal |first1=Markos |last1=Sgantzos |first2=George |last2=Kyrgias |first3=Panagiotis |last3=Georgoulias |first4=Marianna |last4=Karamanou |first5=Gregory |last5=Tsoucalas |first6=George |last6=Androutsos |title=Otto Hahn (1879–1968): Pioneer in Radiochemistry and Discoverer of Radiotherapeutic Mesothorium |journal= Journal of BUON |date=2014 |volume=19 |issue=3 |pages=866–869 |pmid=25261683 |issn=1107-0625 }}</ref> }}
Fritz Strassmann wrote:
{{blockquote|The number of those who had been able to be near Otto Hahn is small. His behaviour was completely natural for him, but for the next generations he will serve as a model, regardless of whether one admires in the attitude of Otto Hahn his humane and scientific sense of responsibility or his personal courage.<ref>{{cite news |last=Strassmann |first=Fritz |date=29 July 1968 |title=Zum Tode von Otto Hahn |trans-title=On the Death of Otto Hahn |lang=de |newspaper=Die Welt |location=Hamburg}}</ref>}}
Otto Robert Frisch recalled:
{{blockquote|Hahn remained modest and informal all his life. His disarming frankness, unfailing kindness, good common sense, and impish humour will be remembered by his many friends all over the world.<ref>{{cite journal |last=Frisch |first=Otto R. |author-link=Otto Robert Frisch |title=Otto Hahn |journal=Physics Bulletin |issn=0031-9112 |volume=19 |issue=10 |year=1968 |page=354 |doi=10.1088/0031-9112/19/10/010}}</ref>}}
The Royal Society in London wrote in an obituary:
{{blockquote|It was remarkable, how, after the war, this rather unassuming scientist who had spent a lifetime in the laboratory, became an effective administrator and an important public figure in Germany. Hahn, famous as the discoverer of nuclear fission, was respected and trusted for his human qualities, simplicity of manner, transparent honesty, common sense and loyalty.{{sfn|Spence|1970|pp=301–302}} }}

===Legacy===
Hahn is considered the father of radiochemistry and nuclear chemistry.{{sfn|Hahn|1966|p=ix}} He is chiefly remembered for the discovery of nuclear fission, the basis of nuclear power and nuclear weapons.{{sfn|Hahn|1966|pp=x–xi}} Glenn Seaborg wrote that "it has been given to very few men to make contributions to science and to humanity of the magnitude of those made by Otto Hahn".{{sfn|Hahn|1966|p=ix}} His award of the 1944 Nobel Prize for Chemistry was in recognition for this discovery. However later commentators have argued that Lise Meitner's exclusion reflected sexism and antisemitism within the Nobel Committee.<ref name="Antisemitism">{{cite magazine|title=How Antisemitism and Professional Betrayal Marred Lise Meitner's Scientific Legacy |first1=Katie |last1=Hafner |first2=Ashraya |last2=Gupta |magazine=Scientific American |date=14 September 2023 |url=https://www.scientificamerican.com/article/how-antisemitism-and-professional-betrayal-marred-lise-meitners-scientific-legacy/ |access-date=4 November 2024}}</ref> Conflict between chemists and physicists and the theorists and experimentalists also played a role.{{sfn|Crawford|Sime|Walker|1997|pp=27–32}}{{vague|date=October 2025}} Hahn's efforts to rehabilitate the image of Germany after the war have also been viewed as problematic. Hahn has been described as politically passive during the Nazi era, suggesting that while he was not a party member, he tolerated colleagues who were and thus shared moral complicity.<ref name="Antisemitism" />{{sfn|Walker|2006|p=124}}{{sfn|Sime|2004|p=48}} In a letter to [[James Franck]] dated 22 February 1946, Meitner wrote:{{blockquote|Hahn is without doubt a decent man with many good traits. He only lacks thoughtfulness and perhaps also a certain strength of character, things that in normal times are minor flaws, but in the complicated times of today have deeper implications.<ref name="Antisemitism" />}}

===Honours and awards===
During his lifetime Hahn was awarded orders, medals, scientific prizes, and fellowships of Academies, Societies, and Institutions from all over the world. At the end of 1999, the German news magazine ''[[Focus (German 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 (with 81 points), after the theoretical physicists [[Albert Einstein]] and Max Planck, and thus the most significant chemist of his time.<ref>{{cite magazine |magazine=Focus |title=Die Allmacht Der Unschärfe |trans-title=The Omnipotence of Blurring |language=de |first=Ernst Peter |last=Fischer |author-link=Ernst Peter Fischer |issue=52 |date=27 December 1999 |pages=103–108 |url=https://www.focus.de/wissen/mensch/focus-liste-die-50-top-forscher-die-allmacht-der-unschaerfe_aid_179705.html |access-date=28 June 2020 |archive-date=24 October 2020 |archive-url=https://web.archive.org/web/20201024100559/https://www.focus.de/wissen/mensch/focus-liste-die-50-top-forscher-die-allmacht-der-unschaerfe_aid_179705.html |url-status=dead }}</ref>

As well as the Nobel Prize in Chemistry ([[List of Nobel laureates in Chemistry#1944|1944]]), Hahn was awarded:
* the [[Emil Fischer Medal]] of the Society of German Chemists (1922),{{sfn|Spence|1970|p=302}}
* the Cannizaro Prize of the Royal Academy of Science in Rome (1938),{{sfn|Spence|1970|p=302}}
* the Copernicus Prize of the [[University of Konigsberg]] (1941),{{sfn|Spence|1970|p=302}}
* the Gothenius Medal of the [[Akademie der Naturforscher]] (1943),{{sfn|Spence|1970|p=302}}
* the [[Max Planck Medal]] of the [[German Physical Society]], with Lise Meitner (1949),{{sfn|Spence|1970|p=302}}
* the [[Goethe Medal]] of the city of Frankfurt-on-the-Main (1949),{{sfn|Spence|1970|p=302}}
* the Golden Paracelsus Medal of the Swiss Chemical Society (1953),{{sfn|Spence|1970|p=302}}
* the [[Faraday Lectureship Prize]] with Medal from the [[Royal Society of Chemistry]] (1956),{{sfn|Spence|1970|p=302}}
* the Grotius Medal of the Hugo Grotius Foundation (1956),{{sfn|Spence|1970|p=302}}
* the [[Wilhelm Exner Medal]] of the Austrian Industry Association (1958),<ref>{{cite web |title=Alle Medaillenträger |publisher=Wilhelm Exner Medaillen Stiftung |url=https://wilhelmexner.wordpress.com/die-medaillentraeger-seit-1921/ |access-date=28 June 2020 |archive-date=22 March 2019 |archive-url=https://web.archive.org/web/20190322095103/https://wilhelmexner.wordpress.com/die-medaillentraeger-seit-1921/ |url-status=dead }}</ref>
* the [[Helmholtz Medal]] of the Berlin-Brandenburg Academy of Sciences and Humanities (1959),
* and the [[Harnack medal]] in Gold from the Max Planck Society (1959).<ref>{{cite web |title=Harnack Medal |publisher=Max-Planck-Gesellschaft |url=https://www.mpg.de/188634/Harnack_Medal |access-date=28 June 2020}}</ref>{{sfn|Hoffmann|2001|pp=243–244}}

[[File:Otto Hahn (Frankfurt).jpg|thumb|right|upright|Bust by Knud Knudsen]]
Hahn became the honorary president of the Max Planck Society in 1962.{{sfn|Spence|1970|p=300}}
* He was elected a [[List of Fellows of the Royal Society elected in 1957|Foreign Member of the Royal Society]] (1957).{{sfn|Spence|1970|p=279}}
* His honorary memberships of foreign academies and scientific societies included:
** the Romanian Physical Society in Bucharest,{{sfn|Spence|1970|pp=302–303}}
** the Royal Spanish Society for Chemistry and Physics and the [[Spanish National Research Council]],{{sfn|Spence|1970|pp=302–303}}
** and the Academies in [[Allahabad]], [[Bangalore]], Berlin, [[Boston]], [[Bucharest]], Copenhagen, Göttingen, Halle, Helsinki, Lisbon, Madrid, Mainz, Munich, Rome, Stockholm, the Vatican, and Vienna.{{sfn|Spence|1970|pp=302–303}}

He was an honorary fellow of University College London,{{sfn|Spence|1970|pp=302–303}}
* and an honorary citizen of the cities of Frankfurt am Main and Göttingen in 1959, and of [[Berlin]] (1968).{{sfn|Spence|1970|p=302}}
* Hahn was made an Officer of the ''Ordre National de la [[Légion d'Honneur]]'' of France (1959),{{sfn|Spence|1970|p=302}}
* and was awarded the Grand Cross First Class of the [[Order of Merit of the Federal Republic of Germany]] (1959).{{sfn|Spence|1970|p=302}}
* In 1966, US President [[Lyndon B. Johnson]] and the [[United States Atomic Energy Commission]] (AEC) awarded Hahn, Lise Meitner and Fritz Strassmann the [[Enrico Fermi Award]]. The diploma for Hahn bore the words: "For pioneering research in the naturally occurring radioactivities and extensive experimental studies culminating in the discovery of fission."<ref>{{cite web |title=Otto Hahn, 1966 Citation |url=https://science.osti.gov/fermi/Award-Laureates/1960s/hahn |website=U.S. Department of Energy |date=28 December 2010 |access-date=14 December 2019}}</ref>
* He received honorary doctorates from
** the [[University of Gottingen]],{{sfn|Spence|1970|p=302}}
** the [[Technische Universität Darmstadt]],{{sfn|Spence|1970|p=302}}
** the [[Goethe University Frankfurt]] in 1949,{{sfn|Spence|1970|p=302}}
** and the [[University of Cambridge]] in 1957.{{sfn|Spence|1970|p=302}}

Objects named after Hahn include:
* [[Otto Hahn (ship)|NS ''Otto Hahn'']], the only European nuclear-powered civilian ship (1964);<ref>{{cite web |title=Schornstein des Kernernergie Forschungsschiffes ''Otto Hahn'' |publisher=Deutsches Schiffahrtsmuseum |lang=de |url=https://www.dsm.museum/museum/museumshafen/schornstein-des-kernernergie-forschungsschiffes-otto-hahn |access-date=8 October 2024}}</ref><ref>{{cite web |title= NS ''Otto Hahn'' – The First German Nuclear Ship |date=1981 |publisher=International Atomic Energy Agency |url=https://inis.iaea.org/search/search.aspx?orig_q=RN:12629968 |access-date=8 October 2024}}</ref>
* a [[Hahn (crater)|crater on the Moon]] (shared with his namesake [[Friedrich von Hahn]]);<ref>{{cite web |title=Moon – Hahn  |publisher=U.S. Geological Survey |url=https://planetarynames.wr.usgs.gov/Feature/2314 |access-date=28 June 2020}}</ref>
* and the asteroid ''[[19126 Ottohahn]]'';<ref>{{cite web |title=IAU Minor Planet Center |website=minorplanetcenter.net |url=https://minorplanetcenter.net/db_search/show_object?object_id=19126 |access-date=28 June 2020}}</ref>
* the [[Otto Hahn Prize]] of both the German Chemical and Physical Societies and the city of Frankfurt/Main;<ref>{{cite web |title=GDCh-Preise  |publisher= Gesellschaft Deutscher Chemiker |url=https://www.gdch.de/gdch/preise-und-auszeichnungen/gdch-preise.html |access-date=28 June 2020}}</ref>
* the [[Otto Hahn Medal]]{{snd}}An Incentive for Young Scientists{{snd}}and the Otto Hahn Award of the Max Planck Society;<ref>{{cite web |title=Otto Hahn Medal |publisher=Max-Planck -Gesellschaft |url=https://www.mpg.de/prizes/otto-hahn-medal |access-date=28 June 2020}}</ref><ref>{{cite web |title=Otto Hahn Award |publisher=Max-Planck-Gesellschaft |url=https://www.mpg.de/prizes/otto-hahn-award |access-date=28 June 2020}}</ref>
* and the [[Otto Hahn Peace Medal]] in Gold of the United Nations Association of Germany (DGVN) in Berlin (1988).<ref>{{cite web |title=Verleihung der Otto-Hahn-Friedensmedaille |trans-title=Award of the Otto Hahn Peace Medal |lang=de |publisher=Deutsche Gesellschaft für die Vereinten Nationen e.V. |url=https://dgvn.de/aktivitaeten/einzelansicht/verleihung-der-otto-hahn-friedensmedaille/ |access-date=28 June 2020 |archive-date=2 July 2020 |archive-url=https://web.archive.org/web/20200702000534/https://dgvn.de/aktivitaeten/einzelansicht/verleihung-der-otto-hahn-friedensmedaille/ |url-status=dead  }}</ref>

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]] (after [[Hesse]]). 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''.<ref>{{cite web |url=http://iupac.org/publications/pac/1994/pdf/6612x2419.pdf |archive-url=https://web.archive.org/web/20080228023225/http://www.iupac.org/publications/pac/1994/pdf/6612x2419.pdf |archive-date=28 February 2008 |url-status=live |title=Names and Symbols of Transfermium Elements (IUPAC Recommendations 1994)|publisher=IUPAC |access-date=23 June 2020}}</ref> After protests from the German discoverers, the name hassium (Hs) was adopted internationally in 1997.<ref>{{cite web |url=http://iupac.org/publications/pac/1997/pdf/6912x2471.pdf |archive-url=https://web.archive.org/web/20060302044100/http://www.iupac.org/publications/pac/1997/pdf/6912x2471.pdf |archive-date=2 March 2006 |url-status=live |title=Names and Symbols of Transfermium Elements (IUPAC Recommendations 1997)|publisher=IUPAC |access-date=23 June 2020}}</ref>

==See also==
* [[List of peace activists]]

==Publications in English==
* {{cite book |last=Hahn |first=Otto |year=1936 |title=Applied Radiochemistry |publisher=Cornell University Press |location=Ithaca, New York |ref=none}}
* {{cite book |last=Hahn |first=Otto |year=1950 |title=New Atoms: Progress and Some Memories |publisher=Elsevier Inc. |location=New York-Amsterdam-London-Brussels |ref=none}}
* {{cite book |last=Hahn |first=Otto |year=1966 |title=Otto Hahn: A Scientific Autobiography |translator-first=Willy |translator-last=Ley |publisher=Charles Scribner's Sons |location=New York |ref=none}}
* {{cite book |last=Hahn |first=Otto |year=1970 |title=My Life |translator-first=Ernst |translator-last=Kaiser |translator-first2=Eithne  |translator-last2=Wilkins |publisher=Herder and Herder |location=New York |ref=none}}

==Notes==
{{Reflist}}

==References==
{{refbegin|30em}}
* {{cite book |last=Badash |first=Lawrence |author-link=Lawrence Badash |contribution=Otto Hahn, Science, and Social Responsibility |pages=167–180 |editor-last=Shea |editor-first=William R. |title=Otto Hahn and the Rise of Nuclear Physics |year=1983 |volume=22 |series=The University of Western Ontario Series in the Philosophy of Science |publisher=D. Reidel Publishing Company |location=Dordrecht / Boston / Lancaster |isbn=90-277-1584-X |oclc=797094010}}
* {{cite book |last=Berninger |first=Ernst |contribution=The Discovery of Uranium Z by Otto Hahn: The First Example of Nuclear Isomerism |pages=213–220 |editor-last=Shea |editor-first=William R. |title=Otto Hahn and the Rise of Nuclear Physics |year=1983 |volume=22 |series=The University of Western Ontario Series in the Philosophy of Science |publisher=D. Reidel Publishing Company |location=Dordrecht / Boston / Lancaster |isbn=90-277-1584-X |oclc=797094010}}
* {{cite book |last=Bernstein |first= Jeremy |author-link=Jeremy Bernstein |year= 2001 |title= Hitler's Uranium Club: The Secret Recordings at Farm Hall |edition= 2nd |place= New York |publisher= Springer-Verlag |isbn= 978-0-387-95089-1 |oclc=7324621011 |url= https://archive.org/details/hitlersuraniumcl00bern}}
* {{cite book |last=Bowen |first=Robert |title=Isotopes in the Earth Sciences |publisher=Chapman and Hall |location=London |year=1994 |isbn=978-0-412-53710-3}}
* {{cite journal |last1=Crawford |first1=Elisabeth |first2=Ruth Lewin |last2=Sime |author-link2=Ruth Lewin Sime |first3=Mark |last3=Walker |title=A Nobel Tale of Postwar Injustice |journal=Physics Today |issn=0031-9228 |volume=50 |issue=9 |pages=26–32 |date=1997 |doi=10.1063/1.881933 |bibcode=1997PhT....50i..26C}}
* {{cite journal |last=Crawford |first=Elisabeth |title=German Scientists and Hitler's Vendetta Against the Nobel Prizes |journal=Historical Studies in the Physical and Biological Sciences |issn=0890-9997 |year=2000 |volume=31 |issue=1 |pages=37–53 |doi=10.2307/27757845 |jstor=27757845}}
* {{cite journal |last1=Feather |first1=Norman |author-link=Norman Feather |first2=E. |last2=Bretscher |author-link2=Egon Bretscher |first3=Edward Victor |last3=Appleton |author-link3=Edward Victor Appleton |title=Uranium Z and the problem of nuclear isomerism |journal=Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences |issn=1364-5021 |volume=165 |issue=923 |year=1938 |pages=530–551 |doi=10.1098/rspa.1938.0075 |bibcode=1938RSPSA.165..530F |doi-access=free }}
* {{cite journal |last=Fergusson |first=Jack E.  |title=The History of the Discovery of Nuclear Fission |journal=Foundations of Chemistry |issn=1386-4238 |volume=13 |issue=2 |date=July 2011 |pages=145–166 |doi=10.1007/s10698-011-9112-2 |s2cid=93361285 }}
* {{cite book |last=Frisch |first=Otto |author-link=Otto Robert Frisch |title=What Little I Remember |publisher=Cambridge University Press |location=Cambridge |year=1979 |isbn=0-521-40583-1 |oclc=861058137}}
* {{cite book |last1=Gerlach |first1=Walther |author-link=Walther  Gerlach |last2=Hahn |first2=Dietrich |title=Otto Hahn – Ein Forscherleben unserer Zeit |trans-title=Otto Hahn – A Researcher's Life of Our Time |language=de  |publisher=Wissenschaftliche Verlagsgesellschaft (WVG) |location=Stuttgart |year=1984 |isbn=978-3-8047-0757-3 |oclc=473315990}}
* {{cite book |editor-last=Hahn |editor-first=Dietrich |editor-link=:de:Dietrich Hahn |title=Otto Hahn – Leben und Werk in Texten und Bildern |trans-title=Otto Hahn – His Life and Work in Texts and Images |language=de |publisher=Suhrkamp-Insel Publishers |location=Frankfurt am Main |year=1988 |isbn=3-458-32789-4 |oclc=42847178}}
* {{cite book |last=Hahn |first=Otto |translator-first=Willy |translator-last=Ley |title=Otto Hahn: A Scientific Autobiography |year=1966 |publisher=Charles Scribner's Sons |location=New York |oclc=646422716}}
* {{cite book |last1=Hoffmann |first1=Klaus |translator-last=Cole |translator-first=J. Michael |title=Otto Hahn: Achievement and Responsibility |date=2001 |publisher=Springer |location=New York |isbn=0-387-95057-5 |oclc=468996162}}
* {{cite journal |last=Hughes |first= Jeff |author-link=Jeff Hughes (historian) |date=2009 |volume=29 |pages=131–165 |title=Making Isotopes Matter: Francis Aston and the Mass-Spectrograph |journal=Dynamis |issn=0211-9536 |doi=10.4321/S0211-95362009000100007 |doi-access=free |hdl=10481/77556 |hdl-access=free }}
* {{cite book |last=Macrakis |first=Kristie |author-link=Kristie Macrakis |title=Surviving the Swastika: Scientific Research in Nazi Germany |publisher=Oxford University Press |location=New York  |year=1993 |oclc=538154456 |isbn=978-0-19-507010-1}}
* {{cite book |last=Norris |first=Robert S. |title=Racing for the Bomb: General Leslie R. Groves, the Manhattan Project's Indispensable Man |year=2002 |publisher=Steerforth Press |location=South Royalton, Vermont |isbn=1-58642-039-9 |oclc=48544060|url-access=registration |url=https://archive.org/details/racingforbombgen00norr }}
* {{cite book |last=Rhodes |first=Richard |author-link=Richard Rhodes |title=The Making of the Atomic Bomb |title-link=The Making of the Atomic Bomb |date=1986 |publisher=Simon and Schuster |location=New York |isbn=0-671-65719-4 |oclc=224864936}}
* {{cite book |last=Scerri |first=Eric |author-link=Eric Scerri |title=The Periodic Table: Its Story and Its Significance |chapter=The Seven Last Infra-Uranium Elements to be Discovered |pages=299–346 |year=2020 |publisher=Oxford University Press |location=New York |url=https://academic.oup.com/book/40719/chapter-abstract/348476454?redirectedFrom=fulltext |url-access=subscription |doi=10.1093/oso/9780190914363.003.0016 |isbn=978-0-19-091436-3 |oclc=1096234740}}
* {{cite book |last=Sime |first=Ruth Lewin |author-link=Ruth Lewin Sime |year=1996 |title=Lise Meitner: A Life in Physics |publisher=University of California Press |location=Berkeley |isbn=978-0-520-08906-8 |oclc=32893857|url=https://archive.org/details/lisemeitnerlifei00sime}}
* {{cite book |last=Sime |first=Ruth Lewin |author-link=Ruth Lewin Sime |date=2004 |title=Otto Hahn und die Max-Planck-Gesellschaft Zwischen Vergangenheit und Erinnerung |trans-title=Otto Hahn and the Max Planck Society Between Past and Memory |language=de |publisher=Max-Planck-Gesellschaft |location=Berlin |url=https://www.mpiwg-berlin.mpg.de/KWG/Ergebnisse/Ergebnisse14.pdf |archive-url=https://web.archive.org/web/20050330155722/http://www.mpiwg-berlin.mpg.de/KWG/Ergebnisse/Ergebnisse14.pdf |archive-date=30 March 2005 |url-status=live |access-date=28 June 2020}}
* {{cite journal |last=Sime |first=Ruth Lewin |author-link=Ruth Lewin Sime |date=March 2006 |title=The Politics of Memory: Otto Hahn and the Third Reich |journal=Physics in Perspective |volume=8 |issue=1 |pages=3–51 |issn=1422-6944 |doi=10.1007/s00016-004-0248-5 |bibcode=2006PhP.....8....3S |s2cid=119479637}}
* {{cite journal |last=Sime |first=Ruth Lewin |author-link=Ruth Lewin Sime |journal=Physics in Perspective |issn=1422-6944 |date=15 June 2010 |title=An Inconvenient History: The Nuclear-Fission Display in the Deutsches Museum |volume=12 |issue=2 |pages=190–218 |doi=10.1007/s00016-009-0013-x |bibcode=2010PhP....12..190S |s2cid=120584702 }}
* {{cite journal |last=Spence |first=Robert |year=1970 |title=Otto Hahn 1879–1968 |journal=Biographical Memoirs of Fellows of the Royal Society |volume=16 |pages=279–313 |doi=10.1098/rsbm.1970.0010 |doi-access=free}}
* {{cite book |last=Stolz|first=Werner |title=Otto Hahn/Lise Meitner |chapter=Die Arbeitsgemeinschaft Hahn – Meitner |trans-chapter=The Hahn – Meitner working group |date=1989 |doi=10.1007/978-3-322-82223-9_3 |series=Biographien hervorragender Naturwissenschaftler, Techniker und Mediziner |pages=19–41 |language=de |publisher=Vieweg+Teubner Verlag |location=Berlin |isbn=978-3-322-00685-1 |oclc=263971970}}
* {{cite journal |last=Walker |first=Mark  |title=Otto Hahn: Responsibility and Repression |date=May 2006 |journal=Physics in Perspective |volume=8 |issue=2 |pages=116–163 |issn=1422-6944 |doi=10.1007/s00016-006-0277-3 |bibcode=2006PhP.....8..116W |s2cid=120992662}}
* {{cite book |last=Walker |first=Mark  |title=German National Socialism and the Quest for Nuclear Power |year=1993 |location=Cambridge |publisher=Cambridge University Press |isbn=0-521-36413-2 |oclc=722061969}}
{{refend}}

==Further reading==
{{refbegin}}
* {{cite book |last=Berninger |first= Ernst H. |title= Otto Hahn 1879–1968 |publisher= Inter Nationes |location= Bonn |year= 1970 |oclc=168069 |ref=none}}
* {{cite book |last=Beyerchen |first= Alan D. |title= Scientists under Hitler |url= https://archive.org/details/scientistsunderh00alan |url-access= registration |publisher= Yale University Press |location= New Haven and London |year= 1977 |isbn= 9780300018301 |oclc=970896098 |ref=none}}
* {{cite book |last1=Graetzer |first1=Hans D. |first2=David L. |last2=Anderson |year=1971 |title=The Discovery of Nuclear Fission: A Documentary History |publisher=Van Nostrand-Reinhold |location=New York |oclc=1130319295 |ref=none}}
* {{cite book |last=Kant |first=Horst |year=2002 |title=Werner Heisenberg and the German Uranium Project. Otto Hahn and the declarations of Mainau and Göttingen |publisher=Max-Planck-Insitut für Wissenschaftsgeschichte |location=Berlin |oclc=5894621006 |ref=none}}
* {{cite book |last=Reid |first=Robert William  |year=1969 |title=Tongues of Conscience: War and the Scientist's Dilemma |publisher=Constable & Co. |location=London |oclc=638683343 |ref=none}}
* {{cite book |last=Whiting |first=Jim |year=2004 |title=Otto Hahn and the Discovery of Nuclear Fission |series=Unlocking the Secrets of Science |publisher=Mitchell Lane |location=Bear, Delaware |isbn=978-1-58415-204-0 |oclc=52312062 |ref=none}}
* {{cite thesis |last=Yruma |first=Jeris Stueland |title=How Experiments Are Remembered: The Discovery of Nuclear Fission, 1938–1968 |publisher=Princeton University |degree=PhD |date=November 2008 |id=275679276 |oclc=297148928 |ref=none }}
{{refend}}

==External links==
{{Commons}}
{{Wikiquote|Otto Hahn}}
* [http://science.energy.gov/fermi/award-laureates/1960s/hahn/ Otto Hahn – winner of the Enrico Fermi Award 1966] U.S Government, Department of Energy
* {{Nobelprize}} including the Nobel Lecture on 13 December 1946 ''From the Natural Transmutations of Uranium to Its Artificial Fission''
* [http://nobelprize.org/nobel_prizes/chemistry/laureates/1944/press.html Award Ceremony Speech honoring Otto Hahn] by Professor Arne Westgren, Stockholm.
* [http://br.de/fernsehen/br-alpha/sendungen/schulfernsehen/meilensteine-otto-hahn-kernspaltung100.html Otto Hahn and the Discovery of Nuclear Fission] {{Webarchive|url=https://web.archive.org/web/20140201200151/http://br.de/fernsehen/br-alpha/sendungen/schulfernsehen/meilensteine-otto-hahn-kernspaltung100.html |date=1 February 2014 }} BR, 2008
* [https://web.archive.org/web/20130615064023/http://www.pro-physik.de/details/news/1123417/Otto_Hahn_-_Entdecker_der_Kernspaltung.html Otto Hahn – Discoverer of Nuclear Fission] Author: Dr. Anne Hardy (Pro-Physik, 2004)
* [https://web.archive.org/web/20120727060649/http://www.visitberlin.de/en/article/otto-hahn Otto Hahn (1879–1968) – The discovery of fission] Visit Berlin, 2011.
* [https://web.archive.org/web/20130606173121/http://www.humantouchofchemistry.com/otto-hahn.htm Otto Hahn – Discoverer of nuclear fission]
* [https://web.archive.org/web/20140219163724/http://www.ohg-marktredwitz.de/schule/ottohahn.html Otto Hahn – Founder of the Atomic Age] Author: Dr Edmund Neubauer (Translation: Brigitte Hippmann) – Website of the Otto Hahn Gymnasium (OHG), 2007.
* [http://www.mpg.de/295774/Otto_Hahn_Groups Otto Hahn Award]
* [https://web.archive.org/web/20141027095222/http://www.dgvn-berlin.de/index.php/veranstaltungen/otto-hahn-friedensmedaille Otto Hahn Peace Medal in Gold] Website of the United Nations Association of Germany (DGVN) in Berlin
* [http://www.mpg.de/290879/Otto_Hahn_Medal Otto Hahn Medal]
* [https://web.archive.org/web/20130417140002/http://www.helmholtz-berlin.de/zentrum/historie/lise-meitner-campus/index_en.html The history of the Hahn Meitner Institute (HMI)] Helmholtz-Zentrum, Berlin 2011.
* [http://www.mpg.de/957419/person4 Otto Hahn heads a delegation to Israel 1959] Website of the Max Planck Society, 2011.
* [http://www.biographyonline.net/scientists/otto-hahn.html Biography Otto Hahn 1879–1968]
* [http://www.hiroshima-u.ac.jp/news/show/lang/en/id/1271/dir_id/0 Otto Hahn – A Life for Science, Humanity and Peace] {{Webarchive|url=https://web.archive.org/web/20150924030316/http://www.hiroshima-u.ac.jp/news/show/lang/en/id/1271/dir_id/0 |date=24 September 2015 }} Hiroshima University Peace Lecture, held by Dietrich Hahn, 2 October 2013.
* [https://web.archive.org/web/20140122074347/http://www.gmx.ch/themen/wissen/mensch/26az93y-otto-hahn-entdecker-kernspaltung-grossvater-atombombe Otto Hahn – Discoverer of nuclear fission, grandfather of the Atombomb] GMX, Switzerland, 17 December 2013. Author: Marinus Brandl.
* {{PM20}}

{{Nobel Prize in Chemistry}}
{{1944 Nobel Prize winners}}
{{World Constitutional Convention call signatories}}
{{Portal bar|Biography|Chemistry|History of science|Germany}}
{{Authority control}}

{{DEFAULTSORT:Hahn, Otto}}
[[Category:Otto Hahn|Otto Hahn]]
[[Category:1879 births]]
[[Category:1968 deaths]]
[[Category:20th-century German chemists]]
[[Category:Cornell University faculty]]
[[Category:Discoverers of chemical elements]]
[[Category:Enrico Fermi Award recipients]]
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[[Category:Humboldt University of Berlin alumni]]
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[[Category:Max Planck Institute directors]]
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[[Category:Recipients of the Cothenius Medal]]