Pioneering Nuclear Science: The Discovery of Nuclear Fission
At an exhibition organised by the Permanent Mission of the Federal Republic of Germany to mark the 75th anniversary of the discovery of nuclear fission at the Vienna International Centre, a replica of Dr. Otto Hahn's laboratory table on which the fission experiment was performed was displayed. The laboratory replica was on loan from the Deutsches Museum, Munich. Vienna, Austria, 25 November 2013. (Photo: D. Calma/IAEA)
"The pioneering work of Otto Hahn, Lise Meitner and Fritz Strassman was a crucial step in the long scientific journey that led to the development of nuclear technology as we understand it today." With these words, IAEA Director General Yukiya Amano marked the 75th anniversary of the discovery of nuclear fission, celebrating the scientists who deduced the process upon which all nuclear technology depends.
Nuclear fission, the process by which an atom splits into lighter atoms, releasing considerable energy, has had a profound effect on our world in delivering energy, influencing geopolitics and opening new frontiers in science and medicine.
75 years ago three scientists Dr. Otto Hahn, Dr. Lise Meitner and Dr. Fritz Strassman working at the Kaiser Wilhelm Institute for Chemistry in Berlin developed an experiment grounded on the then-evolving concept that splitting an atom of an element would produce two atoms of smaller different elements.
Their historical experiment when compared to examples of today's breakthrough experimental setups seem like nothing more than a high school demonstration, but at the time it represented cutting-edge research.
The Experiment That Changed Everything
The experiment used no more ground breaking technology or equipment than any other physics lab in Europe might have employed at the time. Rather, the key to success was a unique set of curious minds in the pursuit of discovery.
The experiment was spread over three rooms; an irradiation room; a chemistry laboratory; and a measuring room. In the irradiation room, a uranium sample was irradiated by a neutron source (a mixture of radium and beryllium), which was sealed in brass tubes and placed in a paraffin block, which slowed down the neutrons. Neutrons at the time were a relatively new discovery and as they are neutrally charged they can interact with an atom's nucleus with less interference from its electrons and protons. As the neutrons bombarded the uranium sample, nuclear fission occurred.
To measure radioactivity and the extremely small quantities of radioactive substances produced, the measuring room was equipped with home-made Geiger-Müller radioactivity counters to determine the decay of the extremely small quantities of radioactive substances produced. Unstable radioactive isotopes, like uranium used in the experiment, transmute over time into other elements in a process known as decay. Because different elements decay at different rates and release different types of radiation, plotting the decay of the uranium sample as a curve on a graph helped reveal the kind of atoms present, and was essential for determining what elements were produced from the nuclear fission. The Geiger-Müller counters used to detect the radiation produced were powered by large high voltage batteries and transferred impulses to mechanical counters through amplifiers and auxiliary apparatus.
The irradiated uranium sample was then brought to the chemistry laboratory where the subsequent radioactive elements from the nuclear fission were isolated using chemical methods.
In 1938, due to the prevailing political situation in Germany, Meitner's Jewish ancestry made it unsafe for her to live in Berlin and she fled to Sweden. Hahn and Strassman continued working on the on-going experiment alone. They shared the outcome of their testing with Meitner. She and her nephew, physicist Otto Frisch, were then able to correctly interpret the confusing data by hypothesising and articulating how the uranium nuclei had split to form lighter elements, for example, barium and krypton, releasing neutrons and large amounts of energy.
The threesome's experimental hypothesis was based on their assumption that an atom splits in the same fashion as a water droplet, which matched a theory that the famous physicist Niels Bohr had posited at that time. That imagined process ran counter to the prevailing understanding that bombarding the atom with neutrons would chip away at the large uranium atom, leaving it mainly intact. After they correctly explained to the scientific community that the uranium atom had actually split into smaller parts, their model provided an explanation for the release of more neutrons, which is the necessary prerequisite in the creation of a chain reaction and essential in many later nuclear applications. They coined their newly discovered process "nuclear fission" as it was comparable to the fission observed in biology cell division.
The discovery later earned Otto Hahn the 1944 Nobel Prize for Chemistry. Arguably as important as any award or prize, was Hahn's, Meitner's and Strassman's staunch refusal to be involved in the development of nuclear weapons, which their discovery had made possible. They also declared their fierce opposition to the use of nuclear technologies for military purposes. Many technological advances, from computing to flight to dynamite have been used to enable more deadly, more destructive weapons, but the discoverers of nuclear fission were adamant that their finding had its great potential in its peaceful applications.
The IAEA's Statute tasks the Agency with accelerating and enlarging the contribution of atomic energy to peace, health and prosperity, and Director General Amano's statement to the United Nations General Assembly in November 2013 reminds us of the wide range of benefits that nuclear fission has made possible and its relevance in providing for a better future:
"The IAEA gives priority to assisting developing countries in using nuclear technology in areas including health, food and agriculture, and water management. By making nuclear technology available, the IAEA makes a unique and lasting contribution to achieving the Millennium Development Goals."
For a more in-depth look at the discovery of fission and all of those that contributed to it, please visit the American Institute of Physics Website.
-- By Michael Madsen, IAEA Division of Public Information
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