Discovery of Nuclear Fission (1938): Hahn, Meitner, Strassmann

The discovery of nuclear fission in December 1938 revealed that the nucleus of a uranium atom, when struck by a neutron, can split into two lighter nuclei and release an enormous amount of energy. The chemists Otto Hahn and Fritz Strassmann made the decisive observation in Berlin, and the physicists Lise Meitner and Otto Robert Frisch explained it within weeks. Within seven years fission powered the first nuclear reactor and the atomic bombs used against Japan.

Background: the race to build new elements

The story begins with the discovery of the neutron by James Chadwick in 1932. Because neutrons carry no electric charge, they can approach and enter an atomic nucleus without being repelled, and physicists soon used them as probes. Building on the picture of the atom established by Ernest Rutherford’s discovery of the nucleus, researchers wanted to know what happened when heavy nuclei absorbed an extra neutron.

In 1934 Enrico Fermi’s group in Rome bombarded uranium, then the heaviest known element, with neutrons and found new radioactive substances. Fermi believed these were “transuranic” elements, heavier than uranium. The German chemist Ida Noddack suggested the same year that the nucleus might instead break into large fragments, but her idea was not tested and was largely ignored.

In Berlin, the radiochemist Otto Hahn and the Austrian-born physicist Lise Meitner, who had worked together since 1907, took up the problem at the Kaiser Wilhelm Institute for Chemistry. They were joined by the younger analytical chemist Fritz Strassmann. For four years the team catalogued a confusing array of products that did not fit the theory of the time.

Meitner’s flight from Germany

Meitner was of Jewish descent. As an Austrian citizen she had been partly shielded from Nazi racial laws, but after Germany annexed Austria in March 1938 she became subject to them and could no longer leave legally. In July 1938, with help from Hahn and the Dutch physicists Dirk Coster and Adriaan Fokker, she crossed secretly into the Netherlands and then travelled to Sweden, where she took up a modest post at Manne Siegbahn’s institute in Stockholm.

The collaboration continued by letter. Hahn and Strassmann did the chemistry in Berlin; Meitner, from exile, questioned the results and suggested further tests. The political situation was worsening around them: in November 1938 the violence of Kristallnacht made clear what Jewish scientists who stayed in Germany could expect.

What happened in December 1938

In late 1938 Hahn and Strassmann were studying substances they believed to be isotopes of radium formed from uranium. To separate them they used barium as a chemical “carrier”, since radium and barium behave alike. To their surprise, the radioactive material could not be separated from the barium at all. Repeated tests in mid-December, recorded in Hahn’s notebook, pointed to one conclusion: the product was not radium but barium itself, an element roughly half the mass of uranium.

On 19 December Hahn wrote to Meitner describing the result, which as a chemist he was sure of but as a physicist could not explain. On 22 December Hahn and Strassmann sent a paper to the journal Die Naturwissenschaften, cautiously reporting barium among the products, a result they admitted ran against all previous experience in nuclear physics. It was published on 6 January 1939.

Meitner and Frisch’s explanation

Meitner spent the Christmas holidays of 1938 with her nephew, the physicist Otto Robert Frisch, who was working at Niels Bohr’s institute in Copenhagen, in the Swedish town of Kungälv. According to Frisch’s later account, they discussed Hahn’s letter during a walk in the snow, sitting on a tree trunk to do calculations.

They drew on the “liquid drop” model of the nucleus developed by George Gamow and Niels Bohr. A large uranium nucleus, they reasoned, is barely held together; the electrical repulsion between its many protons nearly cancels the surface tension of the “drop”. An absorbed neutron could make it wobble, stretch and divide into two pieces that fly apart. Meitner calculated from known nuclear masses that the two fragments would weigh less than the original nucleus by about one-fifth of a proton mass. Using Einstein’s equation E = mc², that missing mass corresponded to roughly 200 million electron volts of energy, matching the energy the fragments would gain from mutual repulsion.

Back in Copenhagen in early January 1939, Frisch told Bohr, who immediately accepted the idea just before sailing for the United States. Frisch confirmed the process experimentally on 13 January by detecting the large bursts of ionisation produced by the fragments. Borrowing a term from biology for the division of cells, he named the process “fission”. The joint paper by Meitner and Frisch was published in Nature on 11 February 1939.

Key people

• Otto Hahn (1879–1968): German radiochemist who led the Berlin experiments. His institute did some research for the German uranium programme, but he took no significant part in weapons work and was interned by the Allies at Farm Hall in England in 1945, where he heard the news of Hiroshima.
• Fritz Strassmann (1902–1980): analytical chemist whose careful separations were essential. He refused to join Nazi organisations, and he and his wife sheltered a Jewish woman in their Berlin flat; Yad Vashem later recognised them as Righteous Among the Nations.
• Lise Meitner (1878–1968): Austrian-Swedish physicist, co-leader of the research for three decades, who provided the theoretical interpretation from exile. She later declined to take part in bomb work.
• Otto Robert Frisch (1904–1979): her nephew, who confirmed fission experimentally and coined the name. In 1940 he co-wrote the Frisch–Peierls memorandum in Britain showing that a uranium bomb was feasible.

Consequences: from laboratory to chain reaction

Bohr announced the news at a conference in Washington, D.C., on 26 January 1939, and laboratories in the United States, France and elsewhere reproduced the result within days. The crucial next question was whether fission also released neutrons. In spring 1939 Frédéric Joliot-Curie, Hans von Halban and Lew Kowarski in Paris, and independently Leo Szilard and Fermi’s group in New York, showed that it did, on average two to three per fission. That made a self-sustaining chain reaction possible.

Later in 1939 Bohr and John Wheeler showed that the rare isotope uranium-235 was the one that fissioned readily with slow neutrons. Scientists in several countries realised that a weapon of unprecedented power might be built. In Germany a research effort known as the Uranverein began in 1939. In the United States, Szilard drafted and Einstein signed a letter to President Franklin D. Roosevelt in August 1939 warning of the danger, a step on the road to the Manhattan Project.

Recognition and controversy

In November 1945 the Swedish Academy announced that Otto Hahn had been awarded the 1944 Nobel Prize in Chemistry “for his discovery of the fission of heavy nuclei”. Meitner and Frisch received no share. Later research in the Nobel archives, particularly by the historian Ruth Lewin Sime, suggested that the decision reflected a narrow view of chemistry and physics, the difficulty of judging interdisciplinary work, and the disruption of the war. Hahn, in his public statements after the war, tended to present fission as a chemical discovery made in Berlin after Meitner had left.

Later honours went some way to correcting the record. Meitner shared the Enrico Fermi Award with Hahn and Strassmann in 1966, and element 109 was named meitnerium in 1997. Today most accounts treat the discovery as the joint achievement of the four scientists.

Myths and facts

• Myth: Hahn “split the atom” in 1938. In 1932 John Cockcroft and Ernest Walton had already broken apart light lithium nuclei with accelerated protons, often described as “splitting the atom”. What was new in 1938 was the division of a heavy nucleus into two large fragments with a huge release of energy.
• Myth: the discovery was a solo achievement. Hahn and Strassmann provided the chemical proof, but they could not explain it; the physical interpretation came from Meitner and Frisch.

Timeline

• 1932: James Chadwick discovers the neutron.
• 1934: Enrico Fermi’s group bombards uranium with neutrons; Ida Noddack suggests the nucleus may break apart.
• 1938, July: Lise Meitner escapes from Germany and settles in Stockholm.
• 1938, mid-December: Hahn and Strassmann identify barium among the products of irradiated uranium.
• 1938, 22 December: Their paper is submitted to Die Naturwissenschaften.
• 1938, late December: Meitner and Frisch work out the explanation in Kungälv, Sweden.
• 1939, 6 January: Hahn and Strassmann’s paper is published.
• 1939, 26 January: Niels Bohr announces fission at a conference in Washington, D.C.
• 1939, 11 February: Meitner and Frisch’s paper naming “fission” appears in Nature.
• 1945, November: Hahn is announced as sole winner of the 1944 Nobel Prize in Chemistry.

Why it matters

Nuclear fission turned the nucleus from an object of pure research into a source of practical energy. It underlies every nuclear power station and the first atomic bombs, and it shaped the politics of the twentieth century. The episode is also a lasting example of scientific collaboration across borders under persecution, and of how credit for discoveries can be unevenly shared.