Otto Hahn

Otto Hahn (German: [ˈɔtoː ˈhaːn] ; 8 March 1879 – 28 July 1968) was a German chemist and a pioneer in the field of radiochemistry. He is widely recognized as the father of nuclear chemistry and the discoverer of nuclear fission, the fundamental scientific principle underpinning nuclear reactors and nuclear weapons. Hahn, in collaboration with Lise Meitner, identified isotopes of the radioactive elements radium, thorium, protactinium, and uranium. His contributions also include the discovery of atomic recoil and nuclear isomerism, as well as pioneering rubidium–strontium dating. In 1938, Hahn, Meitner, and Fritz Strassmann jointly discovered nuclear fission, an achievement for which Hahn solely received the 1944 Nobel Prize in Chemistry.

Otto Hahn (German: [ˈɔtoːˈhaːn] ; 8 March 1879 – 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 reactors and nuclear weapons. Hahn and Lise Meitner discovered isotopes of the radioactive elements radium, thorium, protactinium and uranium. He also discovered the phenomena of atomic recoil and nuclear isomerism, and pioneered rubidium–strontium dating. In 1938, Hahn, Meitner and Fritz Strassmann discovered nuclear fission, for which Hahn alone was awarded the 1944 Nobel Prize in Chemistry.

Hahn graduated from the University of Marburg, earning his doctorate in 1901. He subsequently pursued studies under Sir William Ramsay at University College London and with Ernest Rutherford at McGill University in Montreal, Canada, where he identified several novel radioactive isotopes. Returning to Germany in 1906, Hahn was granted access to a former woodworking shop in the basement of the Chemical Institute at the University of Berlin by Emil Fischer, which he converted into a laboratory. He completed his habilitation in early 1907, subsequently becoming a Privatdozent. In 1912, he assumed leadership of the Radioactivity Department at the newly established Kaiser Wilhelm Institute for Chemistry (KWIC). Collaborating with Austrian physicist Lise Meitner in the facility that now bears their names, they made a series of significant discoveries, culminating in Meitner's isolation of the longest-lived isotope of protactinium in 1918.

During World War I, Hahn served with a Landwehr regiment on the Western Front and later with the chemical warfare unit led by Fritz Haber across the Western, Eastern, and Italian fronts. His involvement in the First Battle of Ypres earned him the Iron Cross (2nd Class). Following the war, he became the head of the KWIC while concurrently managing his own department. Between 1934 and 1938, he collaborated with Strassmann and Meitner on investigating isotopes produced by neutron bombardment of uranium and thorium, a research path that ultimately led to the discovery of nuclear fission. Hahn opposed Nazism and the persecution of Jews by the Nazi Party, which resulted in the dismissal of many colleagues, including Meitner, who was compelled to flee Germany in 1938. Nevertheless, during World War II, he participated in the German nuclear weapons program, cataloging the fission products of uranium. At the conclusion of the war, Allied forces arrested him, and he was detained at Farm Hall with nine other German scientists from July 1945 to January 1946.

Hahn served as the final president of the Kaiser Wilhelm Society for the Advancement of Science in 1946 and subsequently 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 organization dedicated to promoting responsible scientific practices. As he actively contributed to the reconstruction of German science, he emerged as one of the most influential and respected figures in post-war West Germany.

Early Life and Education

Otto Hahn was born in Frankfurt am Main on March 8, 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, from his mother's previous marriage, and two older full brothers, Heiner and Julius. The family resided above his father's workshop. The three younger boys received their education at the Klinger Oberrealschule in Frankfurt. At the age of 15, Otto developed a particular interest in chemistry, conducting rudimentary experiments in the family home's laundry room. Although his father, who had built or acquired several residential and business properties, desired him to study architecture, Otto successfully persuaded him of his ambition to become an industrial chemist.

After successfully completing his Abitur in 1897, Hahn commenced his chemistry studies at the University of Marburg. His minor subjects included mathematics, physics, mineralogy, and philosophy. Hahn became a member of the Students' Association of Natural Sciences and Medicine, a student fraternity that served as a precursor to the contemporary Landsmannschaft Nibelungi (Coburger Convent der akademischen Landsmannschaften und Turnerschaften). During his third and fourth semesters, he pursued studies at the University of Munich, focusing on organic chemistry with Adolf von Baeyer, physical chemistry with Wilhelm Muthmann, and inorganic chemistry with Karl Andreas Hofmann. In 1901, Hahn was awarded his doctorate in Marburg for his dissertation, "On Bromine Derivatives of Isoeugenol," which addressed a subject within classical organic chemistry. He fulfilled a one-year military service obligation, reduced from the standard two years due to his doctoral degree, with the 81st Infantry Regiment; however, unlike his siblings, he did not seek a commission. Subsequently, he returned to the University of Marburg, serving for two years as an assistant to his doctoral supervisor, Geheimrat Professor Theodor Zincke.

Early Career in London and Canada

Discovery of Radiothorium and Other "New Elements"

Hahn's primary career aspiration remained employment within the industrial sector. Eugen Fischer, director of Kalle & Co. and father of organic chemist Hans Fischer, extended a job offer to Hahn; however, a prerequisite for this position was that Hahn must have resided in a foreign country and possess proficiency in another language. Consequently, and with the aim of enhancing his English language skills, Hahn accepted a position at University College London in 1904, where he worked under Sir William Ramsay, renowned for his discovery of the noble gases. In this role, Hahn engaged in radiochemistry, a nascent scientific discipline at the time. During early 1905, while conducting research with radium salts, Hahn identified a novel substance he designated radiothorium (thorium-228), which was initially presumed to be a distinct radioactive element. Subsequent findings revealed it to be an isotope of the already identified element thorium; the concept and term "isotope" were later introduced in 1913 by British chemist Frederick Soddy.

Ramsay expressed considerable enthusiasm upon the discovery of another new element within his institution and planned to formally announce this finding through an appropriate channel. Traditionally, such announcements were made before the esteemed committee of the Royal Society. During the Royal Society session on March 16, 1905, Ramsay formally presented Hahn's discovery of radiothorium. The Daily Telegraph subsequently reported to its readership:

Scientific publications are anticipated to soon feature a new discovery, adding to the numerous notable achievements from Gower Street. Dr. Otto Hahn, affiliated with University College, has identified a novel radioactive element. This element, extracted from a Ceylonese mineral called Thorianite, is hypothesized to be the substance responsible for thorium's radioactivity. Its activity is estimated to be at least 250,000 times greater than that of thorium, on a weight-for-weight basis. It emits a gas, commonly referred to as an emanation, which is identical to the radioactive emanation produced by thorium. A further intriguing hypothesis suggests it may be the origin of a radioactive element potentially more potent than radium itself, capable of generating all the remarkable effects currently associated with radium. The discoverer presented a paper on this topic to the Royal Society last week, and its eventual publication is expected to be recognized as one of the most original recent contributions to scientific literature.

Hahn's findings were published in the Proceedings of the Royal Society on May 24, 1905. This marked the initial entry among over 250 scientific publications he would author in the domain of radiochemistry. As his tenure in London concluded, Ramsay inquired about Hahn's future plans, to which Hahn disclosed a job offer from Kalle & Co. Ramsay emphasized the promising prospects of radiochemistry, suggesting that an individual who had identified a novel radioactive element ought to pursue opportunities at the University of Berlin. Consequently, Ramsay corresponded with Emil Fischer, the director of the chemistry institute at Berlin, who agreed to allow Hahn to work in his laboratory but stated that Hahn could not hold the position of a Privatdozent, as radiochemistry was not an established subject within the curriculum. Given this situation, Hahn determined that further expertise in the field was necessary and subsequently contacted Ernest Rutherford, a preeminent authority in radiochemistry. Rutherford consented to accept Hahn as an assistant, with Hahn's parents agreeing to cover his associated expenses.

Between September 1905 and mid-1906, Hahn collaborated with Rutherford's research group, operating from the basement of the Macdonald Physics Building at McGill University in Montreal. The existence of radiothorium initially met with skepticism, famously characterized by Bertram Boltwood as "a compound of thorium X and stupidity." Boltwood was subsequently persuaded of its existence, though he and Hahn held differing views regarding its half-life. William Henry Bragg and Richard Kleeman had previously observed that alpha particles emitted from radioactive substances consistently possessed uniform energy, offering an alternative method for their identification; consequently, Hahn proceeded to measure the alpha particle emissions of radiothorium. During this investigation, he discovered that a precipitate containing thorium A (polonium-216) and thorium B (lead-212) also included a short-lived "element," which he designated thorium C (subsequently identified as polonium-212). Hahn's attempts to isolate it were unsuccessful, leading him to conclude that it possessed an extremely brief half-life (approximately 300 nanoseconds). Furthermore, he identified radioactinium (thorium-227) and radium D (later recognized as lead-210). Rutherford notably commented, "Hahn has a special nose for discovering new elements."

The Chemical Institute in Berlin

The Discovery of Mesothorium I

Upon his return to Germany in 1906, Hahn was provided by Fischer with a former woodworking shop (Holzwerkstatt) located in the basement of the Chemical Institute, which was designated for his laboratory use. Hahn outfitted this space with electroscopes designed for measuring alpha and beta particles, as well as gamma rays. While in Montreal, these instruments had been constructed from discarded coffee tins; in Berlin, Hahn fabricated them from brass, incorporating aluminum strips insulated with amber. Charging was achieved using hard rubber sticks, which he rubbed against the sleeves of his suit. Although the woodworking shop proved unsuitable for research, Alfred Stock, who headed the inorganic chemistry department, granted Hahn access to a section of one of his two private laboratories. Hahn acquired two milligrams of radium from Friedrich Oskar Giesel, the discoverer of emanium (radon), at a cost of 100 marks per milligram (equivalent to €700 in 2021); additionally, he procured thorium without charge from Otto Knöfler, whose Berlin-based company was a prominent manufacturer of thorium products.

Within a few months, Hahn successfully identified mesothorium I (radium-228), mesothorium II (actinium-228), and, independently of Boltwood, ionium (subsequently identified as thorium-230), which is the parent substance of radium. In subsequent years, mesothorium I gained significant importance because, similar to radium-226 (discovered by Pierre and Marie Curie), it proved highly effective for medical radiation therapy while being only half as expensive to produce. Concurrently, Hahn ascertained that, analogous to his inability to separate thorium from radiothorium, he was likewise unable to isolate mesothorium I from radium.

Rutherford's direct manner, acceptable in Canada, was often perceived negatively in Germany, leading to his characterization as an "Anglicised Berliner." Hahn successfully completed his habilitation in early 1907, subsequently attaining the position of Privatdozent. His habilitation did not necessitate a traditional thesis; instead, the Chemical Institute accepted one of his published works on radioactivity. However, the majority of organic chemists at the institute did not consider Hahn's research to be legitimate chemistry. Fischer, for instance, initially challenged Hahn's assertion during his habilitation colloquium that numerous radioactive substances existed in quantities so minute they were detectable solely by their radioactivity, claiming his own acute sense of smell sufficed for detection, though he eventually conceded. A department head critically observed, "It is remarkable what qualifications now suffice for a Privatdozent appointment!"

In contrast, physicists demonstrated greater acceptance of Hahn's research, leading him to participate in a colloquium at the Physics Institute led by Heinrich Rubens. During one such colloquium on September 28, 1907, Hahn met the Austrian physicist Lise Meitner. Meitner, who was approximately Hahn's age, held the distinction of being only the second woman to earn a doctorate from the University of Vienna and had already authored two publications on radioactivity. Rubens proposed her as a potential collaborator. This encounter marked the commencement of a thirty-year professional collaboration and an enduring personal friendship between the two distinguished scientists.

While Hahn had previously collaborated with female physicists, including Harriet Brooks, in Montreal, Meitner initially encountered significant challenges. At that time, women were not permitted to attend universities in Prussia. Meitner was granted access only to the wood shop, which possessed an independent external entrance, but was prohibited from entering other areas of the institute, including Hahn's upstairs laboratory. For restroom facilities, she was compelled to use those located at a nearby restaurant. However, in the subsequent year, universities began admitting women, prompting Fischer to remove the existing restrictions and arrange for the installation of women's restrooms within the institute building.

Discovery of Radioactive Recoil

Although Harriet Brooks observed radioactive recoil in 1904, her interpretation was erroneous. Hahn and Meitner, however, successfully demonstrated and accurately interpreted the radioactive recoil associated with alpha particle emission. Hahn investigated a report by Stefan Meyer and Egon Schweidler concerning an actinium decay product with an approximate half-life of 11.8 days. He identified this product as actinium X (radium-223). Furthermore, Hahn discovered that the emission of an alpha particle by a radioactinium (thorium-227) atom occurs with substantial force, causing actinium X to undergo recoil. This recoil is sufficient to sever chemical bonds, impart a positive charge to the atom, and enable its collection at a negative electrode.

Initially, Hahn's focus was exclusively on actinium; however, upon reviewing his paper, Meitner informed him that he had inadvertently identified a novel method for detecting radioactive substances. Subsequent experiments led to the discovery of actinium C'' (thallium-207) and thorium C'' (thallium-208). Physicist Walther Gerlach characterized radioactive recoil as "a profoundly significant discovery in physics with far-reaching consequences."

Kaiser Wilhelm Institute for Chemistry

In 1910, August von Trott zu Solz, the Prussian Minister of Culture and Education, appointed Hahn as a professor. Two years later, Hahn assumed leadership of the Radioactivity Department at the newly established Kaiser Wilhelm Institute for Chemistry (KWIC) in Berlin-Dahlem, an area now home to the Hahn-Meitner-Building of the Free University of Berlin. This position included an annual salary of 5,000 marks, which is equivalent to €29,000 in 2021. Additionally, in 1914, Hahn received 66,000 marks (equivalent to €369,000 in 2021) from Knöfler for the mesothorium process, allocating 10 percent of this sum to Meitner. The new institute was formally inaugurated on October 23, 1912, during a ceremony presided over by Kaiser Wilhelm II, who was presented with luminous radioactive substances in a darkened chamber.

The relocation to new facilities proved advantageous, given that the previous wood shop had become severely contaminated. This contamination resulted from spilled radioactive liquids and vented radioactive gases, which subsequently decayed and settled as radioactive dust, thereby precluding the execution of precise measurements. To maintain the pristine condition of their new laboratories, Hahn and Meitner implemented rigorous protocols. These procedures mandated the separation of chemical and physical measurements into distinct rooms. Furthermore, individuals working with radioactive materials were required to adhere to specific guidelines, such as refraining from handshakes, and toilet paper rolls were strategically placed beside all telephones and door handles. Highly radioactive materials were initially stored in the former wood shop and subsequently transferred to a dedicated radium facility constructed on the institute's premises.

World War I

In July 1914, immediately preceding the commencement of World War I, Hahn was recalled to active military service within a Landwehr regiment. His unit advanced through Belgium, where the platoon under his command was equipped with captured machine guns. For his contributions during the First Battle of Ypres, he received the Iron Cross (2nd Class). He actively and enthusiastically participated in the Christmas truce of 1914, subsequently receiving a commission as a lieutenant. By mid-January 1915, Hahn was summoned to a meeting with chemist Fritz Haber, who outlined his strategy for overcoming the trench stalemate through the deployment of chlorine gas. Hahn expressed concern regarding the Hague Convention's prohibition on projectiles containing poison gases. However, Haber clarified that the French had already commenced chemical warfare using tear gas grenades, and he intended to circumvent the convention's explicit wording by releasing gas from cylinders rather than shells.

Haber's newly formed unit was designated Pioneer Regiment 35. Following a concise training period in Berlin, Hahn, alongside physicists James Franck and Gustav Hertz, was redeployed to Flanders to identify a suitable location for an initial gas attack. He did not observe this particular assault, as he and Franck were engaged in selecting a position for the subsequent attack. Subsequently transferred to Poland, they deployed a mixture of chlorine and phosgene gas during the Battle of Bolimów on June 12, 1915. When the gas began to drift back towards German lines, some troops hesitated to advance, prompting Hahn to lead them across No Man's Land. He observed the fatal suffering of the poisoned Russian soldiers and made unsuccessful attempts to resuscitate some using gas masks. During their subsequent attempt on July 7, the gas once more drifted back onto German positions, resulting in Hertz's poisoning. This deployment was punctuated by a mission to the Flanders front and, in 1916, by an assignment to Verdun to introduce phosgene-filled shells to the Western Front. Following these interruptions, he resumed scouting for gas attack locations across both fronts. In December 1916, he became a member of the newly established gas command unit at Imperial Headquarters.

During intervals between military operations, Hahn returned to Berlin, where he discreetly resumed work with Meitner in their former laboratory, advancing their research. In September 1917, he was among three officers, disguised in Austrian uniforms, dispatched to the Isonzo front in Italy. Their objective was to identify an optimal site for an assault utilizing newly developed rifled minenwerfers, capable of simultaneously launching hundreds of poison gas containers at enemy positions. They chose a location where Italian trenches were situated within a deep valley, a topography conducive to the persistence of a gas cloud. The subsequent Battle of Caporetto resulted in the collapse of Italian lines, leading to the Central Powers' extensive occupation of northern Italy. During that summer, Hahn suffered accidental phosgene poisoning while evaluating a novel gas mask design. Towards the conclusion of the war, he was engaged in a clandestine field mission, dressed in civilian clothes, to test a device designed to heat and disperse a cloud of arsenical compounds.

Discovery of Protactinium

In 1913, Frederick Soddy and Kasimir Fajans, both chemists, independently observed that alpha decay resulted in atoms shifting two positions lower on the periodic table, whereas the emission of two beta particles restored them to their initial placement. This subsequent reorganization of the periodic table positioned radium in group II, actinium in group III, thorium in group IV, and uranium in group VI. Consequently, a vacant position remained between thorium and uranium. Soddy hypothesized that this unidentified element, which he termed "ekatantalium" in homage to Dmitri Mendeleev, would exhibit alpha emission and possess chemical characteristics analogous to tantalum. Shortly thereafter, Fajans and Oswald Helmuth Göhring identified this element as a decay product originating from a beta-emitting thorium derivative. Applying the radioactive displacement law formulated by Fajans and Soddy, this substance was determined to be an isotope of the previously missing element, which they designated "brevium" due to its brief half-life. Nevertheless, its nature as a beta emitter precluded it from being the parent isotope of actinium. This implied the existence of a different isotope of the identical element.

Hahn and Meitner subsequently endeavored to locate this elusive parent isotope. They devised a novel methodology for isolating the tantalum group from pitchblende, anticipating that this would accelerate the purification of the new isotope. Their research was disrupted by the outbreak of the First World War. Meitner served as an X-ray nurse in Austrian Army hospitals before resuming her work at the Kaiser Wilhelm Institute in October 1916. Hahn joined the newly established gas command unit at Imperial Headquarters in Berlin in December 1916, following extensive travel between the Western and Eastern Fronts, Berlin, and Leverkusen from mid-1914 to late 1916.

With the majority of students, laboratory assistants, and technicians conscripted, Hahn, stationed in Berlin from January to September 1917, and Meitner were compelled to perform all tasks independently. By December 1917, Meitner successfully isolated the substance, and subsequent investigations confirmed its identity as the sought-after isotope. In March 1918, Meitner submitted her and Hahn's discoveries for publication in the scientific journal Physikalischen Zeitschrift, under the title Die Muttersubstanz des Actiniums; Ein Neues Radioaktives Element von Langer Lebensdauer ("The Mother Substance of Actinium; A New Radioactive Element with a Long Lifetime"). Despite Fajans and Göhring's prior discovery of the element, established scientific convention dictated that an element be characterized by its longest-lived and most prevalent isotope. Brevium possessed a half-life of 1.7 minutes, whereas Hahn and Meitner's isotope exhibited a half-life of 32,500 years. Consequently, the designation "brevium" was deemed unsuitable. Fajans consented to Meitner and Hahn naming the element "protoactinium."

In June 1918, Soddy and John Cranston reported the extraction of an isotope sample; however, unlike Hahn and Meitner, they could not delineate its properties. They recognized Hahn and Meitner's precedence and accepted the proposed name. The relationship to uranium persisted as an enigma, given that none of the then-known uranium isotopes decayed into protactinium. This mystery was not resolved until the discovery of the parent isotope, uranium-235, in 1929. For their groundbreaking discovery, Hahn and Meitner received multiple nominations for the Nobel Prize in Chemistry throughout the 1920s from various scientists, including Max Planck, Heinrich Goldschmidt, and Fajans. In 1949, the International Union of Pure and Applied Chemistry (IUPAC) officially designated the new element as protactinium and formally acknowledged Hahn and Meitner as its discoverers.

Discovery of Nuclear Isomerism

After protactinium's discovery, the majority of uranium's decay chains had been delineated. Upon resuming his research post-war, Hahn re-examined his 1914 findings, focusing on previously overlooked or dismissed anomalies. His experimental procedure involved dissolving uranium salts in a hydrofluoric acid solution containing tantalic acid, which facilitated the sequential precipitation of tantalum from the ore, followed by protactinium. Beyond the known uranium X1 (thorium-234) and uranium X2 (protactinium-234), Hahn identified minute quantities of a radioactive substance exhibiting a half-life between 6 and 7 hours. Although mesothorium II (actinium-228), an isotope with a known half-life of 6.2 hours, existed, it did not fit within any plausible decay chain and was initially considered potential contamination due to prior experimentation at the KWIC. However, Hahn and Meitner's 1919 research established that actinium, when treated with hydrofluoric acid, persists in the insoluble residue. Consequently, given that mesothorium II is an actinium isotope, the detected substance could not be mesothorium II; it was definitively identified as protactinium. With this confirmation, Hahn confidently designated his newly identified isotope "uranium Z" and subsequently published the initial report of his discovery in February 1921.

Hahn precisely determined that uranium Z possessed a half-life of approximately 6.7 hours, with a two percent margin of error. His investigations revealed that uranium X1 decayed into uranium X2 in approximately 99.75 percent of instances, while transforming into uranium Z in about 0.25 percent of cases. He observed that the ratio of uranium X to uranium Z, isolated from multiple kilograms of uranyl nitrate, remained constant over time, strongly suggesting a parent-daughter relationship where uranium X was the precursor to uranium Z. To substantiate this, Hahn acquired one hundred kilograms of uranyl nitrate, a process that required several weeks for the separation of uranium X. He ascertained that the half-life of uranium Z's parent diverged from the established 24-day half-life of uranium X1 by a maximum of two or three days, though a more precise measurement proved elusive. Ultimately, Hahn concluded that both uranium Z and uranium X2 represented the same isotope of protactinium (protactinium-234), decaying into uranium II (uranium-234) but exhibiting distinct half-lives.

Uranium Z constituted the inaugural instance of nuclear isomerism. Walther Gerlach subsequently noted that this represented "a discovery that was not understood at the time but later became highly significant for nuclear physics." A comprehensive theoretical explanation for this phenomenon was not advanced until 1936 by Carl Friedrich von Weizsäcker. Despite its profound implications being initially recognized by only a select few, this discovery led to Hahn's renewed nomination for the Nobel Prize in Chemistry by Bernhard Naunyn, Goldschmidt, and Planck.

Applied Radiochemistry

In 1924, Hahn attained full membership in the Prussian Academy of Sciences in Berlin, securing election with a vote of thirty in favor and two against. Concurrently, while retaining leadership of his own department, he assumed the role of Deputy Director at the KWIC in 1924, subsequently succeeding Alfred Stock as Director in 1928. During this period, Meitner directed the Physical Radioactivity Division, while Hahn presided over the Chemical Radiochemistry Division.

During the early 1920s, Hahn pioneered a novel research domain. Leveraging his recently developed "emanation method" and the concept of "emanation ability," he established what became known as "applied radiochemistry," dedicated to investigating fundamental chemical and physical-chemical inquiries. In 1936, Cornell University Press published a book, later translated into Russian, titled Applied Radiochemistry. This volume compiled lectures Hahn delivered as a visiting professor at Cornell University in Ithaca, New York, in 1933. The publication significantly influenced nearly all nuclear chemists and physicists across the United States, the United Kingdom, France, and the Soviet Union throughout the 1930s and 1940s. Hahn is widely recognized as the progenitor of nuclear chemistry, a field that originated from applied radiochemistry.

Nazi Germany

Impact of Nazism

Fritz Strassmann initially joined the Kaiser Wilhelm Institute for Chemistry (KWIC) to pursue studies under Hahn, aiming to enhance his career opportunities. Following the Nazi Party's (NSDAP) ascent to power in Germany in 1933, Strassmann rejected a financially advantageous job offer due to its mandatory political training and Nazi Party membership. Subsequently, he resigned from the Society of German Chemists when it integrated into the Nazi German Labour Front, choosing to avoid affiliation with a Nazi-controlled entity. This decision precluded him from working in the chemical industry or obtaining his habilitation, a necessary qualification for an academic role. Meitner successfully convinced Hahn to employ Strassmann as an assistant. He soon gained recognition as a third collaborator on their publications, occasionally even receiving primary authorship.

From February to June 1933, Hahn served as a visiting professor at Cornell University in the United States and Canada. During this period, he granted an interview to the Toronto Star Weekly, presenting a highly favorable depiction of Adolf Hitler:

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.

The Law for the Restoration of the Professional Civil Service, enacted in April 1933, prohibited Jews and communists from holding academic positions. Meitner was unaffected by this legislation due to her Austrian citizenship, distinguishing her from German nationals. Similarly, Haber was exempt as a World War I veteran; however, he chose to resign his directorship of the Kaiser Wilhelm Institute of Physical Chemistry and Electrochemistry on April 30, 1933, in protest. Directors of other Kaiser Wilhelm Institutes, including Jewish individuals, adhered to the new law. This statute applied broadly to the Kaiser Wilhelm Society (KWS) and to institutes receiving over 50% state funding, which notably exempted the KWI for Chemistry. Consequently, Hahn was not compelled to dismiss any of his permanent staff. Nevertheless, in his capacity as interim director of Haber's institute, he terminated the employment of a quarter of its personnel, including three department heads. Gerhart Jander was subsequently appointed as the new director of Haber's former institute, redirecting its research focus towards chemical warfare.

Haber, like many directors within the KWS, had accumulated a substantial discretionary fund. His explicit desire was for these funds to be distributed among the dismissed staff to aid their emigration. Hahn mediated an agreement proposing that 10 percent of the funds be allocated to Haber's former employees, with the remainder going to the KWS. However, the Rockefeller Foundation stipulated that the funds must be utilized for their original scientific research purposes or be returned. In August 1933, KWS administrators were informed that several crates of Rockefeller Foundation-funded equipment were slated for shipment to Herbert Freundlich, one of the department heads dismissed by Hahn, who was then employed in England. Ernst Telschow, a Nazi Party member, acting president during Planck's vacation (Planck had been KWS president since 1930), ordered the shipment to be stopped. Hahn complied with the directive but expressed his dissent, arguing that foreign funds should not be redirected towards military research, an area the KWS was increasingly pursuing. Upon Planck's return from vacation, he instructed Hahn to expedite the shipment.

Haber passed away on January 29, 1934. A memorial service commemorating the first anniversary of his death was held, which university professors were prohibited from attending, leading them to send their wives as representatives. Hahn, Planck, and Joseph Koeth were present and delivered eulogies. The aging Planck did not seek re-election and was succeeded in 1937 as president by Carl Bosch, a Nobel Prize in Chemistry laureate and chairman of the board of IG Farben, a corporation that had financially supported the Nazi Party since 1932. Telschow assumed the position of Secretary of the KWS. Despite being an ardent Nazi supporter, Telschow maintained loyalty to Hahn, having been one of his former students, an appointment Hahn endorsed. Otto Erbacher, Hahn's chief assistant, was appointed as the KWI for Chemistry's party steward (Vertrauensmann).

Rubidium–strontium dating

During his 1905–1906 Having previously investigated the radioactive decay of rubidium-87 and estimated its half-life at 2 x 10¹¹ years, he conceived a method for determining the mineral's age. This involved comparing the concentration of strontium (a decay product of rubidium) with the remaining rubidium content, contingent on the accuracy of his initial half-life calculation. This technique presented an advantage over uranium-based dating, as uranium decay produces helium, which can escape, leading to an underestimation of rock ages. Jacob Papish facilitated Hahn's acquisition of several kilograms of this mineral.

In 1937, Strassmann and Ernst Walling successfully isolated 253.4 milligrams of strontium carbonate from 1,012 grams of the mineral. The entirety of this strontium was identified as the strontium-87 isotope, confirming its exclusive origin from the radioactive decay of rubidium-87. Concurrently, uranium minerals within the same geological formation had yielded an estimated age of 1,975 million years for the mineral, which in turn suggested a rubidium-87 half-life of 2.3 x 10¹¹ years—a value remarkably consistent with Hahn's initial calculation. The rubidium–strontium dating method subsequently gained widespread adoption for geological dating in the 1950s, coinciding with advancements in mass spectrometry.

Discovery of Nuclear Fission

Following James Chadwick's discovery of the neutron in 1932, Irène Curie and Frédéric Joliot conducted experiments involving the irradiation of aluminum foil with alpha particles. Their observations revealed the formation of a short-lived radioactive phosphorus isotope, noting that positron emission persisted even after neutron emissions had ceased. This groundbreaking work not only unveiled a novel mode of radioactive decay but also demonstrated the transmutation of an element into a previously unknown radioactive isotope, thereby artificially inducing radioactivity. Consequently, the scope of radiochemistry expanded beyond its traditional focus on heavy elements to encompass the entire periodic table. Chadwick further posited that the electrical neutrality of neutrons enabled them to penetrate atomic nuclei with greater facility than protons or alpha particles. This concept was subsequently adopted by Enrico Fermi and his research team in Rome, who initiated experiments involving the neutron irradiation of various elements.

According to the radioactive displacement law formulated by Fajans and Soddy, beta decay results in an isotope shifting one position higher on the periodic table, while alpha decay causes a shift of two positions lower. When Fermi's research group subjected uranium atoms to neutron bombardment, they observed a intricate spectrum of half-lives. This led Fermi to postulate the formation of novel elements with atomic numbers exceeding 92, termed transuranium elements. Although Meitner and Hahn had not collaborated for an extended period, Meitner expressed keen interest in scrutinizing Fermi's findings. Hahn, initially hesitant, reconsidered his stance after Aristid von Grosse proposed that Fermi's discovery might be an isotope of protactinium. Consequently, they embarked on an investigation to ascertain whether the observed 13-minute isotope was indeed protactinium.

From 1934 to 1938, Hahn, Meitner, and Strassmann identified numerous radioactive transmutation products, which they initially classified as transuranic elements. During this period, the actinide series was not yet recognized, and uranium was erroneously categorized as a Group 6 element, analogous to tungsten. Consequently, it was presumed that the initial transuranic elements would exhibit chemical properties similar to those of Group 7 to 10 elements, such as rhenium and the platinoids. The team confirmed the existence of multiple isotopes for at least four such elements, which they incorrectly assigned atomic numbers 93 through 96. Notably, they were the first to determine the 23-minute half-life of uranium-239 and chemically verify its identity as a uranium isotope. However, they were unable to extend this research to definitively identify the true element 93. Their investigations yielded the identification of ten distinct half-lives, albeit with varying degrees of certainty. To explain these observations, Meitner proposed a novel class of nuclear reaction and hypothesized the alpha decay of uranium, both concepts previously undocumented and lacking empirical physical evidence. While Hahn and Strassmann focused on refining their chemical methodologies, Meitner concurrently designed innovative experiments to further elucidate the underlying reaction processes.

In May 1937, parallel reports were published: one in the Zeitschrift für Physik, primarily authored by Meitner, and another in the Chemische Berichte, with Hahn as the lead author. Hahn's report concluded with an emphatic assertion: Vor allem steht ihre chemische Verschiedenheit von allen bisher bekannten Elementen außerhalb jeder Diskussion (Their chemical distinction from all previously identified elements is beyond dispute). In contrast, Meitner expressed growing reservations. She explored the hypothesis that the reactions originated from various uranium isotopes, specifically the three known forms: uranium-238, uranium-235, and uranium-234. Nevertheless, her calculation of the neutron cross-section indicated a value too substantial to correspond to any isotope other than the most prevalent, uranium-238. Consequently, she posited that this phenomenon represented another instance of nuclear isomerism, a concept Hahn had previously identified in protactinium. Her report, therefore, concluded with a markedly different perspective from Hahn's, stating: 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 (These processes must involve neutron capture by uranium-238, resulting in three isomeric uranium-239 nuclei. This outcome is exceedingly difficult to reconcile with existing nuclear theories).

Following the Anschluss, Germany's annexation of Austria on March 12, 1938, Meitner's Austrian citizenship was revoked, prompting her emigration to Sweden. She departed with minimal funds, though Hahn provided her with a diamond ring inherited from his mother prior to her departure. Meitner maintained correspondence with Hahn via mail. In late 1938, Hahn and Strassmann identified evidence of alkaline earth metal isotopes within their experimental samples. The presence of a Group 2 metal posed a significant challenge, as it did not align logically with previously observed elements. Hahn initially hypothesized the substance was radium, formed by the emission of two alpha particles from the uranium nucleus; however, this mechanism for alpha particle removal was considered improbable. The concept of transmuting uranium into barium, which would necessitate the removal of approximately 100 nucleons, was widely regarded as implausible.

On November 10, during a Subsequent methodological refinements culminated in a pivotal experiment conducted on December 16–17, 1938, which yielded perplexing observations: the three isotopes consistently exhibited characteristics of barium rather than radium. Hahn, withholding this information from the physicists at his own institute, communicated these results exclusively to Meitner in a letter dated December 19:

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.

In her response, Meitner expressed agreement, stating: "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 December 22, 1938, Hahn submitted a manuscript detailing their radiochemical findings to Naturwissenschaften, which was subsequently published on January 6, 1939. Five days later, on December 27, Hahn contacted the editor of Naturwissenschaften to request an addendum to the article. He speculated that certain platinum group elements previously detected in irradiated uranium, initially identified as transuranium elements, might actually be technetium (then known as "masurium"). This speculation was based on a misconception that atomic masses, rather than atomic numbers, should sum up. By January 1939, Hahn had become sufficiently convinced of the production of lighter elements to publish a revised version of the article, thereby retracting his earlier assertions regarding the observation of transuranic elements and uranium neighbors.

Hahn, a chemist, initially hesitated to propose a groundbreaking physics discovery; however, Meitner and Frisch developed a theoretical framework for nuclear fission, a term Frisch adapted from biology. During January and February, they published two articles that both discussed and experimentally validated their theory. In their subsequent publication concerning nuclear fission, Hahn and Strassmann introduced the term Uranspaltung (uranium fission) and hypothesized the generation and release of supplementary neutrons during the fission process, thereby suggesting the potential for a nuclear chain reaction. Frédéric Joliot and his research team subsequently confirmed this hypothesis in March 1939. Edwin McMillan and Philip Abelson utilized the cyclotron at the Berkeley Radiation Laboratory to irradiate uranium with neutrons, successfully identifying an isotope possessing a 23-minute half-life. This isotope was determined to be the decay product of uranium-239, thus representing the authentic element 93, which they designated neptunium. Hahn reportedly commented, "There goes a Nobel Prize."

Concurrently at the KWIC, Kurt Starke independently synthesized element 93, employing only the limited neutron sources accessible at that facility. Subsequently, Hahn and Strassmann initiated investigations into its chemical characteristics. They understood that it was expected to decay into the true element 94, which, based on the most recent iteration of the nuclear liquid drop model proposed by Bohr and John Archibald Wheeler, would exhibit greater fissility than uranium-235. However, they could not detect its radioactive decay. Consequently, they inferred that it possessed an exceptionally long half-life, potentially spanning millions of years. A contributing factor to this difficulty was their persistent belief that element 94 belonged to the platinoid group, which complicated their efforts in chemical separation.

The Second World War

On April 24, 1939, Paul Harteck and his assistant, Wilhelm Groth, formally notified the Armed Forces High Command (OKW) regarding the potential for developing an atomic bomb. In response, the Army Weapons Branch (HWA) established a dedicated physics section, led by nuclear physicist Kurt Diebner. Following the outbreak of World War II on September 1, 1939, the HWA assumed control over the German nuclear weapons program. Subsequently, Hahn engaged in a continuous series of meetings pertinent to the project. When Peter Debye, the Director of the Kaiser Wilhelm Institute for Physics, departed for the United States in 1940 and did not return, Diebner was appointed as his successor. Hahn regularly reported his research advancements to the HWA. Collaborating with his assistants—Hans-Joachim Born, Siegfried Flügge, Hans Götte, Walter Seelmann-Eggebert, and Strassmann—he cataloged approximately one hundred fission product isotopes. Their investigations further encompassed methods for isotope separation, the chemical properties of element 93, and techniques for purifying uranium oxides and salts.

During the night of February 15, 1944, the KWIC building sustained a direct bomb strike. Hahn's office was obliterated, resulting in the loss of his correspondence with Rutherford and other researchers, as well as numerous personal effects. This office was the specific target of the aerial assault, which Brigadier General Leslie Groves, director of the Manhattan Project, had authorized with the objective of impeding the German uranium project. Albert Speer, the Reich Minister of Armaments and War Production, subsequently arranged for the institute's relocation to Tailfingen (currently part of Albstadt) in southern Germany. All research activities in Berlin concluded by July. Hahn and his family subsequently relocated to the residence of a local textile manufacturer.

The circumstances for individuals married to Jewish women became increasingly perilous. Philipp Hoernes, a chemist employed by Auergesellschaft, the company responsible for mining the uranium ore utilized in the project, exemplifies this situation. After his employment was terminated in 1944, Hoernes faced conscription for forced labor. At 60 years old, his survival was highly improbable. Hahn and Nikolaus Riehl intervened, arranging for Hoernes to work at the KWIC. They asserted that his contributions were indispensable to the uranium project and that uranium's extreme toxicity made it challenging to recruit personnel. Hahn was cognizant that uranium ore posed minimal risk in a laboratory setting, a stark contrast to the severe danger faced by the 2,000 female slave laborers from the Sachsenhausen concentration camp who mined it in Oranienburg. Heinrich Rausch von Traubenberg, another physicist with a Jewish wife, also experienced such difficulties. Hahn attested to the critical importance of von Traubenberg's work for the war effort, further certifying that his wife, Maria, a physics doctorate holder, was essential as his assistant. Following Heinrich's death on September 19, 1944, Maria was threatened with deportation to a concentration camp. Hahn initiated a lobbying effort to secure her release, but it proved unsuccessful, and she was subsequently sent to the Theresienstadt Ghetto in January 1945. Maria ultimately survived the war and was reunited with her daughters in England.

Post-war

Incarceration at Farm Hall

On April 25, 1945, an armored task force from the British-American Alsos Mission arrived in Tailfingen and encircled the KWIC. Hahn was informed of his arrest. When questioned about reports concerning his confidential uranium research, Hahn responded, "I have them all here," and surrendered 150 documents. He was then transported to Hechingen, where he joined Erich Bagge, Horst Korsching, Max von Laue, Carl Friedrich von Weizsäcker, and Karl Wirtz. Subsequently, they were moved to a dilapidated château in Versailles, where they learned of the signing of the German Instrument of Surrender at Reims on May 7. In the ensuing days, Kurt Diebner, Walther Gerlach, Paul Harteck, and Werner Heisenberg joined the group. All were physicists, with the exception of Hahn and Harteck, who were chemists. Furthermore, all had participated in the German nuclear weapons program, except for von Laue, who was nonetheless fully informed about it.

The group was relocated to the Château de Facqueval in Modave, Belgium, where Hahn dedicated his time to writing his memoirs. On July 3, they were flown to England, arriving at Farm Hall, Godmanchester, near Cambridge, on the same day. During their stay, all conversations, both indoors and outdoors, were surreptitiously recorded using hidden microphones. They were provided with British newspapers, which Hahn was able to read. He expressed significant distress over reports concerning the Potsdam Conference, which detailed the cession of German territory to Poland and the USSR. In August 1945, the German scientists were informed of the atomic bombing of Hiroshima. Prior to this revelation, the scientists, with the exception of Harteck, were entirely convinced that their project was more advanced than any in other nations, an impression that Samuel Goudsmit, the Alsos Mission's chief scientist, did not correct. At this juncture, the rationale for their incarceration at Farm Hall suddenly became evident.

As they recovered from the initial shock of the announcement, the scientists began to rationalize the events. Hahn remarked that he was relieved they had not succeeded, while von Weizsäcker proposed that they should assert they had never intended to. They subsequently drafted a memorandum regarding the project, emphasizing that nuclear fission had been discovered by Hahn and Strassmann. The disclosure that Nagasaki had been devastated by a plutonium bomb presented another profound shock, as it indicated that the Allies had not only achieved uranium enrichment but had also mastered nuclear reactor technology. This memorandum evolved into the initial draft of a postwar apologia. The notion that Germany's defeat in the war stemmed from the moral superiority of its scientists was both outrageous and implausible, yet it resonated within postwar German academia. This narrative deeply infuriated Goudsmit, whose parents had been murdered in Auschwitz. On January 3, 1946, six months after their arrival at Farm Hall, the group was permitted to return to Germany. Hahn, Heisenberg, von Laue, and von Weizsäcker were transported to Göttingen, which was under the control of the British occupation authorities.

The Nobel Prize in Chemistry 1944

The Royal Swedish Academy of Sciences announced on November 16, 1945, that Otto Hahn had been awarded the 1944 Nobel Prize in Chemistry for his "discovery of the fission of heavy atomic nuclei." As Hahn was still detained at Farm Hall, his location remained confidential, preventing the Nobel committee from dispatching a congratulatory telegram. Consequently, he became aware of his accolade on November 18, via the Daily Telegraph. His fellow interned scientists commemorated his achievement with speeches, humor, and musical compositions.

Prior to the discovery of nuclear fission, Hahn had received numerous nominations for both the Nobel Prizes in Chemistry and Physics. Subsequent nominations followed specifically for his fission discovery. Nobel Prize nominations were rigorously reviewed by five-member committees, each dedicated to a specific award category. Despite Hahn and Meitner's nominations in physics, the fields of radioactivity and radioactive elements were historically considered within the purview of chemistry; thus, the Nobel Committee for Chemistry undertook the evaluation of these nominations. The committee reviewed reports submitted by Theodor Svedberg and Arne Westgren. While these chemists acknowledged the significance of Hahn's contributions, they deemed the work of Meitner and Frisch to be less exceptional and did not comprehend its perceived seminal importance within the physics community. Regarding Strassmann, despite his co-authorship on relevant publications, a long-established policy favored awarding the prize to the most senior scientist involved in a collaborative effort. Consequently, the committee advocated for Hahn to be the sole recipient of the chemistry prize.

During the Nazi regime, Germans were prohibited from accepting Nobel Prizes, a policy instituted after Carl von Ossietzky received the Nobel Peace Prize in 1936. Consequently, the Royal Swedish Academy of Sciences rejected the Nobel Committee for Chemistry's recommendation in 1944, opting instead to postpone the award for one year. By September 1945, when the Academy re-evaluated the award, the war had concluded, thereby lifting the German boycott. Furthermore, the chemistry committee had adopted a more circumspect approach, recognizing that substantial clandestine research had occurred in the United States, and proposed an additional year's deferral. However, the Academy was influenced by Göran Liljestrand, who contended that it was crucial for the institution to affirm its autonomy from the Allies of World War II by bestowing the prize upon a German, mirroring its action after World War I with Fritz Haber. Thus, Hahn was ultimately designated the sole recipient of the 1944 Nobel Prize in Chemistry.

The invitation for the Nobel festivities was conveyed through the British Embassy in Stockholm. On December 4, Hahn was convinced by two of his Alsos captors, American Lieutenant Colonel Horace K. Calvert and British Lieutenant Commander Eric Welsh, to draft a letter to the Nobel committee. This letter accepted the prize but indicated his inability to attend the award ceremony on December 10, citing his captors' refusal to permit his departure from Farm Hall. When Hahn expressed his objections, Welsh underscored Germany's defeat in the war. According to the Nobel Foundation statutes, Hahn was allotted six months to deliver his Nobel Prize lecture and until October 1, 1946, to redeem the 150,000 Swedish krona cheque.

Hahn was repatriated from Farm Hall on January 3, 1946; however, it quickly became evident that securing travel authorization from the British government would prevent his journey to Sweden before December 1946. Consequently, the Academy of Sciences and the Nobel Foundation successfully obtained an extension from the Swedish government. Hahn ultimately attended the ceremony one year after the prize was awarded. On December 10, 1946, commemorating the anniversary of Alfred Nobel's death, King Gustav V of Sweden formally presented him with his Nobel Prize medal and diploma. Hahn subsequently allocated 10,000 krona of his prize to Strassmann, who declined to accept the funds.

Hahn served as the Founder and President of the Max Planck Society.

The suicide of Albert Vögler on April 14, 1945, created a vacancy in the KWS presidency. Bertie Blount, a British chemist, was subsequently appointed to manage the organization's affairs while the Allied powers deliberated its future. Blount ultimately decided to install Max Planck as an interim president. At 87 years old, Planck was residing in Rogätz, a small town situated in an area that American forces were preparing to transfer to Soviet control. Gerard Kuiper, a Dutch astronomer associated with the Alsos Mission, retrieved Planck via Jeep and transported him to Göttingen on May 16. On July 25, Planck communicated with Hahn, who remained in captivity in England, informing him that the KWS directors had voted to appoint him as the next president and inquiring about his acceptance of the role. Hahn did not receive this correspondence until September and initially expressed reservations, considering himself an ineffective negotiator; however, his colleagues ultimately persuaded him to accept the position. Following his return to Germany, Hahn formally assumed the presidency on April 1, 1946.

Allied Control Council Law No. 25, enacted on April 29, 1946, imposed restrictions on German scientists, limiting their activities exclusively to basic research. Subsequently, on July 11, the Allied Control Council officially dissolved the KWS, primarily at the insistence of the Americans, who perceived the institution as having been excessively aligned with the National Socialist regime and thus posing a threat to global peace. In contrast, the British, who had opposed the dissolution, demonstrated greater leniency, proposing that the Kaiser Wilhelm Society could continue operations within the British Zone, provided its name was altered. This proposition deeply distressed Hahn and Heisenberg, who regarded the KWS name as an internationally recognized symbol of political autonomy and scientific excellence. Hahn recalled that a name change had been suggested during the Weimar Republic era, but the Social Democratic Party of Germany had been convinced to retain the original designation. For Hahn, the name evoked a nostalgic image of the German Empire's past, despite its authoritarian and undemocratic nature, predating the widely disfavored Weimar Republic. Heisenberg sought support from Niels Bohr, who nonetheless advised in favor of the name change. Lise Meitner subsequently wrote to Hahn, articulating her perspective:

Outside of Germany, the prevailing view is that the traditions stemming from the Kaiser Wilhelm period were catastrophic, rendering a change to the KWS name highly desirable. Consequently, the resistance to this alteration is widely incomprehensible. The notion that Germans constitute a chosen people, entitled to employ any means to subjugate "inferior" populations, has been repeatedly articulated by historians, philosophers, and politicians, culminating in the Nazis' attempts to implement this ideology. The most respected individuals among the English and Americans hope that leading Germans will recognize the imperative for a definitive break from this tradition, which has inflicted immense misfortune upon both the world and Germany itself. As a modest gesture of German understanding, the KWS name ought to be changed. What significance does a name hold when the very existence of Germany, and by extension Europe, is at stake?

A new Max Planck Society was founded in September 1946 at Bad Driburg, located within the British Zone. This organization was subsequently dissolved on February 26, 1948, following the merger of the US and British zones into Bizonia, to facilitate the establishment of the Max Planck Society, with Hahn serving as its inaugural president. The new society assumed control over the 29 institutes of the former Kaiser Wilhelm Society situated in the British and American zones. Upon the formation of the Federal Republic of Germany (West Germany) in 1949, the five institutes located in the French zone also integrated into the society. The Kaiser Wilhelm Institute for Chemistry (KWIC), then directed by Strassmann, undertook the construction and renovation of new facilities in Mainz; however, progress was slow, and its relocation from Tailfingen was not completed until 1949. Hahn's firm stance on retaining Telschow as general secretary almost provoked a significant challenge to his presidency. As part of his endeavors to reconstruct German science, Hahn liberally issued persilschein (whitewash certificates), including one for Gottfried von Droste, who had joined the Sturmabteilung (SA) in 1933 and the NSDAP in 1937, and had worn his SA uniform at the KWIC. He also provided certificates for Heinrich Hörlein and Fritz ter Meer of IG Farben. Hahn presided over the Max Planck Society until 1960, successfully restoring the prestige previously associated with the Kaiser Wilhelm Society. During his tenure, new institutes were established and existing ones expanded; the budget increased from 12 million Deutsche Marks in 1949 (equivalent to €32 million in 2021) to 47 million in 1960 (equivalent to €115 million in 2021), and the workforce expanded from 1,400 to almost 3,000 employees.

Advocate for Social Responsibility

Following the Second World War, Hahn became a vocal opponent of employing nuclear energy for military applications. He regarded the utilization of his scientific discoveries for such objectives as a misuse, or even a criminal act. Historian Lawrence Badash observed: "His wartime recognition of the perversion of science for the construction of weapons, and his postwar activity in planning the direction of his country's scientific endeavours now inclined him increasingly toward being a spokesman for social responsibility."

In early 1954, Hahn authored the article "Cobalt 60 – Danger or Blessing for Mankind?", which addressed the misuse of atomic energy. This piece was extensively reprinted and broadcast on radio in Germany, Norway, Austria, and Denmark, with an English version disseminated globally by the BBC. The international response proved encouraging. The subsequent year, he initiated and organized the Mainau Declaration of 1955. In this declaration, Hahn and other international Nobel laureates highlighted the perils of atomic weapons and issued an urgent warning to global nations against resorting to "force as a final resort." This declaration was released one week after the comparable Russell-Einstein Manifesto. In 1956, Hahn reiterated his appeal, supported by the signatures of 52 Nobel colleagues from various international locations.

Hahn was a key contributor and co-author of the Göttingen Manifesto, issued on April 13, 1957. In this document, he, alongside 17 prominent German atomic scientists, formally protested against the proposed nuclear armament of the West German armed forces (Bundeswehr). Consequently, Hahn received an invitation to meet with German Chancellor Konrad Adenauer and other high-ranking officials, including Defense Minister Franz Josef Strauss, and Generals Hans Speidel and Adolf Heusinger, both of whom had served as generals during the Nazi era. The two generals contended that the Bundeswehr required nuclear weapons, a recommendation that Adenauer endorsed. A communiqué was subsequently drafted, stating that the Federal Republic would not produce nuclear weapons nor solicit its scientists to do so. Instead, German forces were supplied with US nuclear weaponry.

On November 13, 1957, at the Konzerthaus (Concert Hall) in Vienna, Hahn issued a warning regarding the "dangers of A- and H-bomb-experiments," asserting that "today war is no means of politics anymore – it will only destroy all countries in the world." His widely praised speech was broadcast internationally by the Austrian radio service, Österreichischer Rundfunk (ÖR). On December 28, 1957, Hahn reiterated his appeal in an English translation for Bulgarian Radio in Sofia, which was subsequently broadcast across all Warsaw Pact states.

In 1959, Hahn co-founded the Federation of German Scientists (VDW) in Berlin, a non-governmental organization dedicated to promoting responsible scientific practice. Members of the Federation are committed to considering the potential military, political, and economic ramifications, as well as the possibilities of atomic misuse, in their scientific research and educational endeavors. Through its interdisciplinary efforts, the VDW engages not only the general public but also policymakers across all societal and political strata. Until his death, Otto Hahn consistently cautioned against the perils of the nuclear arms race among major powers and the global threat of radioactive contamination.

Lawrence Badash observed:

The crucial aspect is not that scientists might diverge on the precise location of their societal responsibility, but rather their awareness of such a responsibility, their willingness to articulate it, and their expectation that their pronouncements will influence policy. Otto Hahn, it appears, transcended merely being an exemplar of this twentieth-century conceptual shift; he was a pivotal leader in its development.

Hahn was among the signatories of the accord to convene a convention aimed at drafting a global constitution. Consequently, a World Constituent Assembly was convened, marking the first instance in human history where such a body gathered to formulate and ratify a Constitution for the Federation of Earth.

Personal Life

In June 1911, during a conference in Stettin, Hahn encountered Edith Junghans (1887–1968), then a student at the Royal School of Art in Berlin. They reconnected in Berlin and became engaged in November 1912. The couple married on March 22, 1913, in Stettin, the city where Edith's father, Paul Ferdinand Junghans, served as a high-ranking legal official and President of the City Parliament until his passing in 1915. Following a honeymoon at Punta San Vigilio on Lake Garda in Italy, they traveled to Vienna and subsequently to Budapest, where they resided with George de Hevesy.

Their only child, Hanno Hahn, was born on April 9, 1922. Hanno enlisted in the military in 1942, serving as a panzer commander on the Eastern Front during World War II. He sustained the loss of an arm in combat. Post-war, he pursued a career as an art historian and architectural researcher (at the Hertziana in Rome), gaining recognition for his findings concerning early 12th-century Cistercian architecture. In August 1960, during a research trip in France, Hanno, along with his wife and assistant Ilse Hahn (née Pletz), tragically died in an automobile accident. They were survived by their fourteen-year-old son, Dietrich Hahn.

In 1990, the Hanno and Ilse Hahn Prize was instituted to commemorate Hanno and Ilse Hahn, recognizing exceptional contributions to Italian art history and supporting emerging, talented art historians. This award is conferred biennially by the Bibliotheca Hertziana – Max Planck Institute for Art History in Rome.

Demise and Legacy

Passing

In October 1951, Hahn was shot in the back by a disgruntled inventor who sought to draw attention to the perceived disregard of his concepts by established scientists. Hahn sustained injuries in a motor vehicle accident in 1952, followed by a minor myocardial infarction the subsequent year. In 1962, he published the book Vom Radiothor zur Uranspaltung (lit.'From Radiothorium to Uranium Fission'). This work was subsequently released in English in 1966 under the title Otto Hahn: A Scientific Autobiography, featuring an introduction by Glenn Seaborg. The positive reception of this publication may have encouraged him to compose a more comprehensive autobiography, Otto Hahn. Mein Leben; however, prior to its publication, he sustained a fractured cervical vertebra while exiting a vehicle. His health progressively declined, and he passed away in Göttingen on July 28, 1968. His wife, Edith, survived him by merely two weeks. He was interred in the Stadtfriedhof in Göttingen. The Max Planck Society issued the subsequent obituary notice the day following his death:

On July 28, in his 90th year, Honorary President Otto Hahn passed away. His name will be indelibly inscribed in human history as the progenitor of the atomic age. In his passing, Germany and the global community have lost a scholar distinguished equally by his integrity and profound humility. The Max Planck Society mourns its founder, who perpetuated the mission and traditions of the Kaiser Wilhelm Society post-war, and also grieves for a benevolent and cherished individual whose memory will endure among all who had the opportunity to meet him. His contributions will persist. He is remembered with deep gratitude and admiration.

Fritz Strassmann stated:

The number of individuals who had close proximity to Otto Hahn was limited. His conduct was entirely authentic for him, yet for subsequent generations, he will serve as an exemplary figure, regardless of whether one admires his humane and scientific sense of responsibility or his personal fortitude.

Otto Robert Frisch recounted:

Hahn maintained a modest and informal demeanor throughout his life. His disarming candor, unwavering kindness, sound judgment, and playful wit will be cherished by his numerous friends worldwide.

The Royal Society in London noted in an obituary:

It was remarkable how, following the war, this rather unassuming scientist, who had dedicated a lifetime to laboratory work, transformed into an effective administrator and a significant public figure in Germany. Hahn, celebrated as the discoverer of nuclear fission, was respected and trusted for his human qualities, straightforward manner, transparent honesty, practical judgment, and steadfast loyalty.

Legacy

Hahn is recognized as the progenitor of radiochemistry and nuclear chemistry. He is primarily remembered for his discovery of nuclear fission, which forms the basis of both nuclear power and nuclear weaponry. Glenn Seaborg asserted that "very few individuals have been afforded the opportunity to contribute to science and humanity on the scale achieved by Otto Hahn." His conferral of the 1944 Nobel Prize in Chemistry acknowledged this pivotal discovery. Nevertheless, subsequent commentators have argued that Lise Meitner's exclusion reflected prevailing sexism and antisemitism within the Nobel Committee. Conflict between chemists and physicists, as well as between theorists and experimentalists, also played a role. Hahn's post-war endeavors to rehabilitate Germany's international image have also been critically examined. Hahn has been characterized as politically passive during the Nazi era, suggesting that while he was not a party member, he tolerated affiliated colleagues, thereby incurring a degree of moral complicity. In a letter addressed to James Franck on February 22, 1946, Meitner articulated:

Hahn is, unequivocally, an honorable individual possessing numerous commendable attributes. His deficiencies lie solely in a lack of circumspection and potentially a certain fortitude of character; these are minor imperfections in ordinary circumstances, but in the complex contemporary era, they carry profound ramifications.

Honours and Awards

Throughout his lifetime, Hahn received numerous orders, medals, scientific accolades, and fellowships from Academies, Societies, and Institutions globally. In late 1999, the German news magazine Focus conducted a survey among 500 prominent natural scientists, engineers, and physicians concerning the most influential scientists of the 20th century. In this poll, Hahn was ranked third (with 81 points), following theoretical physicists Albert Einstein and Max Planck, thereby establishing him as the preeminent chemist of his era.

In addition to the Nobel Prize in Chemistry (1944), Hahn received the following distinctions:

• the Emil Fischer Medal from the Society of German Chemists (1922),
• the Cannizaro Prize from the Royal Academy of Science in Rome (1938),
• the Copernicus Prize from the University of Konigsberg (1941),
• the Gothenius Medal from the Akademie der Naturforscher (1943),
• the Max Planck Medal from the German Physical Society, shared with Lise Meitner (1949),
• the Goethe Medal from the city of Frankfurt-on-the-Main (1949),
• the Golden Paracelsus Medal from the Swiss Chemical Society (1953),
• the Faraday Lectureship Prize and Medal from the Royal Society of Chemistry (1956),
• the Grotius Medal from the Hugo Grotius Foundation (1956),
• the Wilhelm Exner Medal from the Austrian Industry Association (1958),
• the Helmholtz Medal from the Berlin-Brandenburg Academy of Sciences and Humanities (1959),
• and the Harnack Medal in Gold from the Max Planck Society (1959).

In 1962, Hahn was appointed honorary president of the Max Planck Society.

• He was elected a Foreign Member of the Royal Society in 1957.
• His honorary memberships in international academies and scientific societies included
  • the Romanian Physical Society in Bucharest,
  • the Royal Spanish Society for Chemistry and Physics, the Spanish National Research Council,
  • and academies located in Allahabad, Bangalore, Berlin, Boston, Bucharest, Copenhagen, Göttingen, Halle, Helsinki, Lisbon, Madrid, Mainz, Munich, Rome, Stockholm, the Vatican, and Vienna.

He also held an honorary fellowship at University College London,

• and was granted honorary citizenship by the cities of Frankfurt am Main and Göttingen in 1959, followed by Berlin in 1968.
• In 1959, Hahn was designated an Officer of the French Ordre National de la Légion d'Honneur.
• Additionally, he received the Grand Cross First Class of the Order of Merit of the Federal Republic of Germany in 1959.
• In 1966, U.S. President Lyndon B. Johnson and the United States Atomic Energy Commission (AEC) jointly presented the Enrico Fermi Award to Hahn, Lise Meitner, and Fritz Strassmann. Hahn's diploma specifically recognized his "pioneering research in the naturally occurring radioactivities and extensive experimental studies culminating in the discovery of fission."
• He was awarded honorary doctorates from
  • the University of Göttingen,
  • the Technische Universität Darmstadt,
  • the Goethe University Frankfurt in 1949,
  • and the University of Cambridge in 1957.

Several entities have been named in honor of Hahn, including:

• the NS Otto Hahn, which was the sole European nuclear-powered civilian vessel (launched in 1964);
• a lunar crater (named jointly with his namesake Friedrich von Hahn);
• and the asteroid 19126 Ottohahn.
• Additionally, the Otto Hahn Prize is awarded by both the German Chemical and Physical Societies and the city of Frankfurt am Main;
• the Otto Hahn Medal, which serves as an incentive for young scientists, and the Otto Hahn Award, both conferred by the Max Planck Society;
• and the Otto Hahn Peace Medal in Gold, presented by the United Nations Association of Germany (DGVN) in Berlin in 1988.

Throughout various periods, proposals emerged to name newly synthesized elements in Hahn's honor. American chemists first suggested in 1971 that element 105 be designated hahnium; however, in 1997, the IUPAC officially named it dubnium, referencing the Russian research center in Dubna. Subsequently, in 1992, a German research team discovered element 108 and proposed the name hassium (derived from Hesse). Despite the established convention granting discoverers the right to suggest names, an IUPAC committee in 1994 recommended the name hahnium for element 108. Following objections from the German discoverers, the name hassium (Hs) was internationally adopted in 1997.

• List of peace activists

Publications in English

• Hahn, Otto (1936). Applied Radiochemistry. Ithaca, New York: Cornell University Press.Hahn, Otto (1950). New Atoms: Progress and Some Memories. New York-Amsterdam-London-Brussels: Elsevier Inc.Hahn, Otto (1966). Otto Hahn: A Scientific Autobiography. Translated by Ley, Willy. New York: Charles Scribner's Sons.Hahn, Otto (1970). My Life. Translated by Kaiser, Ernst; Wilkins, Eithne. New York: Herder and Herder.Notes

  References

  • Otto Hahn on Nobelprize.org including the Nobel Lecture on 13 December 1946 From the Natural Transmutations of Uranium to Its Artificial Fission
  • Otto Hahn and the Discovery of Nuclear Fission Archived 1 February 2014 at the Wayback Machine BR, 2008
  • Otto Hahn (1879–1968) – The discovery of fission
  • Otto Hahn – Founder of the Atomic Age Author: Dr Edmund Neubauer (Translation: Brigitte Hippmann) – Website of the Otto Hahn Gymnasium (OHG), 2007.
  • Otto Hahn Peace Medal in Gold Website of the United Nations Association of Germany (DGVN) in Berlin
  • The history of the Hahn Meitner Institute (HMI) Helmholtz-Zentrum, Berlin 2011.
  • Max Planck Society website, 2011, detailing Otto Hahn's leadership of a delegation to Israel in 1959.
  • Biographical account of Otto Hahn (1879–1968).
  • Hiroshima University Peace Lecture, delivered by Dietrich Hahn on 2 October 2013, titled 'Otto Hahn: A Life Dedicated to Science, Humanity, and Peace,' archived on 24 September 2015 at the Wayback Machine.
  • Brandl, Marinus. 'Otto Hahn: Discoverer of Nuclear Fission and Progenitor of the Atomic Bomb.' GMX, Switzerland, 17 December 2013.
  • Archival collection of newspaper clippings concerning Otto Hahn, housed within the ZBW's 20th Century Press Archives.