Otto Hahn completed his doctoral work in organic chemistry at the Chemical Institute in Marburg. Between 1904 and 1906, Hahn performed postdoctoral research at the University College of London (under Sir William Ramsey [1852–1916]) and at the Physical Institute of McGill University in Montreal, Canada (under Ernest Rutherford [1817–1937]).
This research experience introduced Hahn to radiochemistry, and he discovered several new radioactive substances in the process. In London, he discovered radiothorium (RdTh)—now recognized as the thorium isotope thorium-228. While in Montreal, Hahn discovered radioactinium (RdAc)––now called thorium-227. Before the concept of the isotope was introduced in 1913 by the British chemist Frederick Soddy (1877–1956), there was much confusion within the scientific community on how to name all the interesting new radioactive isotopes that belonged to the natural decay chains of uranium and thorium. Hahn’s discoveries, which he originally called radiothorium and radioactinium, reflected the prevailing lack of complete understanding of isotopes.
Hahn returned to Europe in 1906 and moved to Berlin to work at the Chemical Institute of the University of Berlin, directed by Emil Fischer (1852–1919). In 1907, Hahn met the Austrian-Swedish physicist Lise Meitner (1878–1968). Despite the misogynistic work environment created by Fischer at the Chemical Institute, Hahn and Meitner managed to establish a productive collaboration that would go on to span three decades. He was a skilled chemist who had no particular interest or talents in physics or mathematics. She was a skilled physicist who wanted to explore the exciting fields of radioactivity and radiochemistry being pioneered by Marie Curie (1867–1934). In 1912, Hahn and Meitner moved their research work to the new Kaiser Wilhelm Institute for Chemistry at Berlin-Dahlen. Following the move, Hahn became the head of the small but independent department of radiochemistry.
The start of World War I interrupted their collaborative research. Meitner departed the institute to serve as an X-ray technician in field hospitals of the Austrian Army. Hahn served with various units of the German Army as a chemical warfare specialist. Following his wartime service, Hahn announced in 1918 their collaborative discovery of protactinium (Pa; atomic number 91), the long-lived mother substance of the actinium decay series. This discovery earned both Meitner and Hahn recognition within the German scientific community. In the 1920s, she began to concentrate on the physics of beta particles and he pursued specialized avenues in radiochemistry. In 1921, for instance, he discovered “uranium Z”—the first example of a nuclear isomer.
Then, in the early 1930s, the discovery of the neutron in England by James Chadwick (1891–1974) and the pioneering neutron research of Enrico Fermi (1901–1954) in Rome encouraged Hahn and Meitner to collaborate again. This time they began to bombard uranium with slow neutrons in the hope of creating new, transuranic elements. But these were dangerous times for a scientist of Jewish ancestry in Berlin, like Meitner. Her Austrian citizenship afforded a thin layer of protection against the anti-Semitic laws being imposed after the Nazi party rose to power in Germany in 1933. Hahn and Meitner labored on at the university irradiating uranium and thorium with neutrons and exploring the byproduct materials. Suddenly, Meitner had to flee for her life as Nazi troops invaded Austria in July 1938. Hahn enlisted the services of a chemist named Fritz Strassmann (1902–1980) to continue the uranium bombardment experiments.
They soon made an incredible discovery, the full interpretation of which Hahn initially hesitated to suggest. Starting in the fall of 1938, Hahn and Strassmann began to detect quantities of barium in the uranium target material. Although barium is chemically similar to radium, it has a much lower atomic mass. Hahn was puzzled. In mid-December, he sent a letter to his long-time scientific collaborator, Meitner, who was now safely in Stockholm, Sweden. Meitner suspected immediately that the atomic nucleus was splitting, or undergoing “fission.” Hahn and Strassmann published the results of their experiment in the January 6, 1939, issue of Naturalwissenschaften but did not offer any scientific explanation of what had taken place. Meitner charged forward with a theoretical explanation. Over the Christmas holidays, she had discussed Hahn’s findings with her nephew, the Austrian-British physicist Otto Frisch (1904–1979). He agreed with her hypothesis, and they published a world-changing short paper, “Disintegration of Uranium by Neutrons: A New Type of Nuclear Reaction” in the February 11, 1939, issue of Nature. Within a few short weeks, physicists around the world learned about nuclear fission, and a few began to speculate on how this phenomenon could be used in a superweapon.
Throughout World War II, Hahn remained on the technical periphery of the German atomic bomb program. Captured by British troops, he was interned in a country manor (called Farm Hall) near Cambridge, England, with nine other prominent German physicists who had been working on a Nazi atomic bomb. There he learned that he had won the 1944 Nobel Prize in chemistry for “his discovery of the fission of heavy nuclei.” He received the announcement of the atomic bombing of Hiroshima on August 6, 1945, with horror and shock. Hahn went into a deep state of depression because his discovery had led to such a powerful weapon.
Hahn returned to Germany after World War II and was elected president of the Max Planck Society for the Advancement of Science. He delivered his Nobel lecture and received his 1944 prize in December 1946. He became a respected public figure, assisting senior officials in the new federal government during West Germany’s reconstruction. For the remainder of his life, Hahn campaigned vigorously against nuclear weapons testing and any expansion of the nuclear arms race. He died on July 28, 1968, in Göttingen, Germany, as a result of injuries from an accidental fall.
LISE MEITNER (1878 – 1968)
