20th century / Science

Lise Meitner : The Nucleus of Fission

By Nicola Davies

‘In terms of 19th century stereotypes or rhetorical idealizations, a woman scientist was a contradiction in terms… Women scientists were thus caught between two almost mutually exclusive stereotypes: as scientists, they were atypical women; as women they were unusual scientists.’

Lise Meitner’s name is to many an unfamiliar one, occasionally found somewhere amid the pages of a text on nuclear physics and seldom with great acclaim. In truth, Lise Meitner was the mother of nuclear fission explaining the process by which atoms may be split to release huge quantities of energy – knowledge which has been harnessed to develop both nuclear reactors and nuclear bombs.

Born in Vienna in 1878, Meitner’s talent for mathematics and physics quickly became evident, however academic circles at the beginning of the twentieth century were hostile to the inclusion of women in research and forging a career as a female physicist was a constant struggle. Nevertheless Meitner preserved and after completing undergraduate and doctoral studies at the University of Vienna, Meitner moved to the prestigious University of Berlin where she felt her talents would develop and mature. In Berlin the resistance to female academics was strong, indeed for many years Meitner was forbidden to work in the main laboratories. Eventually however, Meitner’s groundbreaking work in radioactivity could not be ignored and she rose through the academic ranks aided by the recognition and support of the eminent Max Planck and eventually became head of the physics section at the renowned Kaiser Wilhelm Institute for Chemistry.

With Chemist Otto Hahn, Meitner discovered the element Protactinium in 1917 as well as developing new ways of investigating radioactive decay by recoil methods, focusing on the nature of beta rays and the decay of many elements. In 1923 she revealed the cause of the Auger effect which became a vital tool for the probing of the structure and composition of surfaces. The process was subsequently named after Pierre Auger, who independently reported the effect after Meitner had published her results; a prime example of Meitner’s contributions being pushed into the shadows of scientific research.

By 1930 Meitner had published over eighty papers and her scientific reputation was growing. The Meitner-Hahn team was nominated for the Nobel Prize eight times between 1924 and 1934. During this inter-war period Meitner became the first woman to be awarded the silver medal of the prestigious Leibniz Prize, as well as being awarded the AAAWS ‘Women’s Nobel Prize’.

However, the growing political unease of Germany cast many obstacles in the way of academic research, with Jewish scientists being dismissed on the basis of anti-semitic laws. Meitner, who was of Jewish ancestry, continued her work even after many of her rights had been suppressed. Yet after the Anschluss in 1938, Meitner fled undercover to the Netherlands and then to Sweden where she took up a post at Manne Siegbahn’s laboratory. In exile, Meitner continued her work in the field on radioactivity, working with the distinguished Danish scientist Neils Bohr, whilst corresponding with Hahn and discussing her results with her nephew Otto Frisch.

Before arriving in Sweden, Fermi’s experiments involving neutron bombardment of uranium had interested Meitner. She and Hahn had devised and carried out their own experiments to elucidate theories on the existence of transuranic elements. After she had left Germany, Hahn continued to undertake experiments prompted by Meitner, in which he fired slow neutrons at an uranium 235 nucleus. He was surprised and puzzled when he detected the formation of the much lighter atom barium. Hahn wrote to Meitner to tell her of the strange results. Coupling the news from Hahn with the fact that Bohr had found that the amount of energy released upon the bombardment of a uranium nucleus with neutrons was far greater than he had predicted, Meitner and Frisch realised that nuclear fission was occurring. This was astonishing as previously it had been thought that there was not enough energy in the bombarding neutrons to break off components larger than a Helium nucleus. Meitner was the first person able to explain the process occurring.

The greatest aspect of Meitner’s rationalisation is the justification of the production of such a huge quantity of energy during fission. She remembered that a nucleus always has a smaller mass than the sum of its constituent parts due to the ‘conversion’ of some of the mass into energy which is required to hold the nucleus together: this is called the ‘packing fraction’ of the nucleus. Using this knowledge, she realised that the nuclei formed from the fission of a nucleus have different packing fractions. Invoking Einstein’s famous E = mc2 equation, she grasped the essential fact that this difference in mass corresponds to a huge release of energy. This means that when the delicate balance of forces inside the nucleus is disturbed, as it is during bombardment with neutrons, the nucleus deforms and splits so that the two atoms created are repelled from each other with a very large energy.

The significance of this discovery was overwhelming: here was a method of generating enormous amounts of energy just from uranium and a beam of neutrons. Hahn published his results from the neutron bombardment experiments in January 1939. Sadly, Meitner, having carried out so much groundwork, and Fritsch, who had with her interpreted the data, were in danger of missing out on their due credit due to a series of delays in the publication of their work and the omission of citations in the work of others.

Meitner’s contributions to physics were both major and diverse, yet she was repeatedly overlooked when it came to formal recognition due to politics, race, gender or her own natural modesty. In 1946 it was Hahn alone who was awarded the Nobel Prize in Chemistry for the discovery of nuclear fission. Meitner worked tirelessly for many years after the war, following the devastation inflicted by atomic warfare, to address the moral and ethical responsibilities of scientists. Indeed her epitaph captures the true essence of Lise Meitner: ‘A physicist who never lost her humanity’.

References:
Rife. P, Lise Meitner and the Dawn of the Nuclear Age (New York 1999)
Lewin Sime. R, Lise Meitner – A Life in Physics (California 1996)
http://www.washingtonpost.com/wp-srv/style/longterm/books/reviews/lisemeitner.htm
http://physics.bu.edu/py106/notes/RadioactiveDecay.html
http://www.users.bigpond.com/Sinclair/fission/LiseMeitner.html

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