Otto Hahn

Otto Hahn was born on March 8, 1879, in Frankfurt am Main, the youngest of four sons of Heinrich Hahn, a prosperous glazier and businessman, and his wife Charlotte ^[1]. The family expected the boys to enter practical professions, and Heinrich hoped Otto might become an architect. The younger Hahn had other interests. As a schoolboy he carried out simple chemistry experiments in the family laundry room, and by the time he finished secondary school he had settled on chemistry as a career ^[1].

In 1897 he enrolled at the University of Marburg, where he studied chemistry with minors in mineralogy, physics, and philosophy. He spent two semesters at the University of Munich before returning to Marburg, where he completed a doctorate in organic chemistry in 1901 under Theodor Zincke, writing a dissertation on derivatives of isoeugenol ^[2]. After a year of compulsory military service, he returned to Marburg as Zincke's assistant. Nothing in this conventional training pointed toward the new science of radioactivity that would define his life.

Hahn's turn toward radiochemistry happened almost by accident. Hoping to improve his English before taking an industrial job, he went to London in 1904 to work in the laboratory of Sir William Ramsay at University College London. Ramsay set him the task of separating radium from a barium salt, and in the course of that work Hahn identified a new radioactive substance he called radiothorium, later recognized as an isotope of thorium ^[2]. The result made his name among the small community of radioactivity researchers.

Encouraged by this success, Hahn spent the winter of 1905 to 1906 at McGill University in Montreal with Ernest Rutherford, then the leading figure in the field. There he discovered radioactinium and absorbed Rutherford's direct, experiment-driven style ^[3]. In 1906 he returned to Germany and joined Emil Fischer's chemistry institute at the University of Berlin, where he was given a converted woodworking shop in the basement as a laboratory. He discovered mesothorium 1, an isotope of radium, in 1907, a finding with commercial value in medical radiation sources ^[2].

That same year Hahn began a collaboration that shaped the rest of his career. The Austrian physicist Lise Meitner arrived in Berlin, and the two formed a working partnership that lasted more than three decades. In 1912 Hahn moved to the newly founded Kaiser Wilhelm Institute for Chemistry in Berlin-Dahlem, where he eventually headed the department of radiochemistry and, from 1928, served as director of the institute ^[3]. During the First World War he was assigned to the German chemical warfare program under Fritz Haber, work involving poison gas that he later described with regret in his memoirs ^[1]. In 1917 and 1918, working with Meitner, he identified protactinium, a new chemical element with atomic number 91 ^[2].

The central achievement of Otto Hahn's life grew out of a puzzle set by Enrico Fermi. In 1934 Fermi's group in Rome bombarded uranium with neutrons and reported evidence of new elements heavier than uranium. Hahn, Meitner, and the young analytical chemist Fritz Strassmann set out to characterize these supposed transuranic elements chemically, a program that occupied them for four years ^[4].

Politics broke the team apart before the puzzle was solved. Meitner, who was of Jewish descent, lost the protection of her Austrian citizenship when Germany annexed Austria in March 1938. That July she fled to the Netherlands and then to Sweden, with Hahn helping to arrange her escape and giving her a diamond ring to use in an emergency ^[4]. Hahn and Strassmann continued the experiments in Berlin while keeping Meitner informed by letter.

In December 1938 the two chemists obtained a result that seemed impossible. Careful fractional crystallization showed that one product of neutron-bombarded uranium behaved exactly like barium, an element barely half the mass of uranium ^[5]. Hahn wrote to Meitner describing the finding and asking whether physics could explain it. Over the Christmas holiday, Meitner and her nephew Otto Robert Frisch worked out that the uranium nucleus had split into two lighter nuclei, releasing enormous energy, and Frisch named the process fission ^[4]. Hahn and Strassmann published the chemical evidence in the journal Naturwissenschaften in January 1939, and Meitner and Frisch published the physical interpretation in Nature the following month ^[5]. Within weeks laboratories around the world had confirmed the result, and physicists immediately grasped that a chain reaction might release energy on an unprecedented scale.

Hahn remained in Germany through the Second World War. He never joined the Nazi Party, and he had protested the dismissal of Jewish colleagues in 1933 by resigning his lectureship at the University of Berlin, though he kept his institute post ^[3]. During the war his institute contributed basic fission research to the German uranium program, but Hahn's own work stayed focused on identifying fission products rather than on weapon design, and Germany never came close to building a bomb ^[6].

In the spring of 1945 Allied forces detained Hahn along with nine other German scientists, including Werner Heisenberg and Max von Laue, and interned them at Farm Hall, a country house near Cambridge in England. Hidden microphones recorded their conversations. When news of the atomic bombing of Hiroshima reached the group in August 1945, Hahn was recorded as being devastated, feeling personally implicated because his discovery had made the weapon possible ^[6]. His colleagues reportedly feared for his state of mind that night.

While still interned, Hahn learned from a newspaper that he had been awarded the Nobel Prize in Chemistry for 1944, given for the discovery of the fission of heavy atomic nuclei ^[7]. He could not attend the December 1945 ceremony and received the award in Stockholm the following year. The prize went to Hahn alone, a decision that historians have debated ever since, since Meitner and Strassmann had been essential partners in the work. Meitner was nominated many times for both the physics and chemistry prizes but never received one ^[4].

In 1911, at a conference in Stettin, Hahn met Edith Junghans, an art student and daughter of the city's parliament chairman. They married in 1913 and had one son, Hanno, born in 1922 ^[1]. Hanno Hahn became a respected art historian specializing in Cistercian architecture. His death in a car accident in France in 1960, together with his wife Ilse, was a blow from which his parents never fully recovered ^[1].

Colleagues described Hahn as modest, cheerful, and straightforward, with little taste for pomp. He kept up his friendship with Lise Meitner despite the strains of exile, war, and the Nobel controversy, and the two corresponded until the end of his life. Both died in 1968, months apart, and Meitner's gravestone carries the epitaph written by Frisch: a physicist who never lost her humanity ^[4]. Hahn was a lifelong gymnastics enthusiast in his youth and remained an avid walker into old age.

After his release from Farm Hall in January 1946, Hahn returned to Germany and was elected president of the Kaiser Wilhelm Society, the country's main research organization. Under Allied pressure the society was renamed, and in 1948 Hahn became the founding president of its successor, the Max Planck Society for the Advancement of Science, a post he held until 1960 ^[3]. In that role he helped rebuild German science from wartime ruin and worked to restore its international standing.

The last two decades of his life were marked by public opposition to nuclear weapons. In 1955 he initiated the Mainau Declaration, in which Nobel laureates warned against the misuse of atomic energy, and in 1957 he was one of the Göttingen Eighteen, a group of leading German scientists who publicly refused to cooperate in arming West Germany with nuclear weapons ^[7]. He was repeatedly nominated for the Nobel Peace Prize for these efforts. Honors accumulated in his final years, including the Enrico Fermi Award shared with Meitner and Strassmann in 1966, the first time the American award went to non-Americans ^[7]. Otto Hahn died in Göttingen on July 28, 1968, at the age of 89, after a fall and a period of declining health ^[1].

Any list of Otto Hahn achievements begins with fission, but his influence runs wider. He helped found radiochemistry as a discipline, discovered or co-discovered numerous radioactive species including protactinium, and in 1921 identified nuclear isomerism in uranium Z, a phenomenon not fully explained until years later ^[2]. Generations of German chemists trained in his institute, and the Max Planck Society he led remains one of the world's foremost research organizations.

His name is attached to institutes, schools, prizes, and streets across Germany, to the element meitnerium's one-time proposed rival name hahnium, and to the nuclear-powered cargo ship Otto Hahn launched in 1964 ^[3]. The chemical element with atomic number 105 was called hahnium by American researchers for decades before international agreement settled on dubnium, a reminder of how contested scientific naming can be.

Historians still argue over the fission story: how credit should be divided among Hahn, Meitner, Strassmann, and Frisch, and how to weigh Hahn's conduct in Nazi Germany, where he protected some colleagues while continuing to lead a major institute under the regime ^[4]. What is not in dispute is the scale of the discovery itself. The experiments Hahn and Strassmann completed in December 1938 revealed an energy source locked inside the atomic nucleus, and every reactor and every arms-control treaty since carries the imprint of that Berlin laboratory. For readers of any Otto Hahn biography, the essential Otto Hahn facts are these: a meticulous chemist, an accidental revolutionary, and in his later years a persistent advocate for keeping his discovery out of weapons ^[7].