A nuclear phenomenon that escaped the hands of physicists

Otto Hahn and Lise Meitner
Otto Hahn and Lise Meitner frequently worked together, as can be seen in this 1918 photograph made when they discovered the protactinium. Twenty years later, in 1938, they made the discovery of nuclear fission that won them international fame. German physicist and chemist, Otto Han (1879-1968) won the Nobel Prize in 1945. Lise Meitner (1878-1968) was born in Vienna and studied at university there as well as in Berlin. She was a professor of physics at the Berlin University between 1926 and 1933, leaving Germany in 1938 to join the atomic research staff at Stockholm University. In 1939, Lise Meitner published the first scientific article explaining nuclear fission.
© DR

In the years spanning 1934 to 1938, Enrico Fermi with his team in Rome, Otto Hahn and Lise Meitner in Berlin, Irene Curie and Paul Savitch in Paris discovered a large number of new radioisotopes. They all attempted to create heavier, ‘transuranic’, elements by bombarding uranium with neutrons.

It was in 1938 that Otto Hahn and Fritz Strassman, using previous works of Irène Curie and Savitch, were able to make precise chemical analyses of these heavier elements, and discover that bombarded uranium nuclei give sometime rise to two new, surprisingly lighter, elements.

Lise Meitner, now forced to leave Germany for a life in Sweden, was visited by her nephew Otto Frisch, who was then living in Copenhagen. Together, the two became the first to interpret this observed phenomenon, and concluded that the two products of the uranium splitting were of approximately equal size. Their results were published in the magazine ‘Nature’ in February 1939.

A series of elegant experiments carried out by Frederic Joliot on late January 1939 were able to provide the necessary physical evidence. Within the month, he was able to observe products of the uranium fission reaction in the Wilson chamber in his laboratory at the Collège de France in Paris.

Joliot immediately began wondering about the possibility of setting off a chain reaction. Uranium contains a higher neutron-proton ratio than in both the lighter fragments it produces, which means that some free neutrons must be emitted by these fragments richer than normal in neutrons. These neutrons could then, in turn, induce one or more new fission reactions under conditions which would have to be carefully determined. With Hans Halban and Lew Kowarski, Joliot was able to carry out experiments to examine the possibility of starting a chain reaction.

On May 1939 (the 1st, 2nd and 4th of May, to be exact), the three physicists were joined by Francis Perrin to take out three patents in the name of the ‘Caisse Nationale de la Recherche Scientifique’: Device for energy production, Method for stabilizing a device for energy production and Method for perfecting explosive charges.

The war erupted in 1939. Scientists were conscious of the fantastic energy released by chains of fission reactions. Leo Szilard, an Hungarian born physicist, convinced Albert Einstein to use his authority as a leading scientist to alert the US president F.D. Roosevelt. The physics research being conducted in Europe crossed to the Atlantic, escaping the hands of physicists. Enrico Fermi build the world first ‘atomic pile’ in Chicago in 1942. This was the beginning of the Manhattan project that led to the atomic bomb in 1945.

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