Nuclear Fission

The fission reaction was discovered in l938 by two German scientists, Otto Hahn (1879-1968) and Fritz Strassmann (1902-1980). They had been doing a series of experiments in which they used neutrons to bombard various elements. If they bombarded copper, for example, a radioactive form of copper was produced. Other elements became radioactive in the same way. When uranium was bombarded with neutrons, however, an entirely different reaction seemed to occur. The uranium nucleus apparently underwent a major disruption.

The evidence for this supposed process came from chemical analysis. Hahn and Strassmann published a scientific paper showing that small amounts of barium (element 56) were produced when uranium (element 92) was bombarded with neutrons. It was very puzzling to them how a single neutron could transform element 92 into element 56.

Lise Meitner (1878-1968), a long-time colleague of Hahn who had left Germany due to Nazi persecution, suggested a helpful model for such a reaction. One can visualize the uranium nucleus to be like a liquid drop containing protons and neutrons. When an extra neutron enters, the drop begins to vibrate. If the vibration is violent enough, the drop can break into two pieces. Meitner named this process "fission" because it is similar to the process of cell division in biology. It takes only a small amount of energy to start the vibration which leads to a major breakup.

Scientists in the United States and elsewhere quickly confirmed the idea of uranium fission, using other experimental procedures. For example, a cloud chamber is a device in which vapor trails of moving nuclear particles can be seen and photographed. In one experiment, a thin sheet of uranium was placed inside a cloud chamber. When it was irradiated by neutrons, photographs showed a pair of tracks going in opposite directions from a common starting point in the uranium. Clearly, a nucleus had been photographed in the act of fission.

Another experimental procedure used a Geiger counter, which is a small, cylindrical tube that produces electrical pulses when a radioactive particle passes through it. For this experiment, the inside of a modified Geiger tube was lined with a thin layer of uranium. When a neutron source was brought near it, large voltage pulses were observed, much larger than from ordinary radioactivity. When the neutron source was taken away, the large pulses stopped. A Geiger tube without the uranium lining did not generate large pulses. Evidently, the large pulses were due to uranium fission fragments. The size of the pulses showed that the fragments had a very large amount of energy.

A nuclear chain reaction: the successive fissioning of ever-increasing numbers of uranium-235 atoms. Illustration by Argosy. The Gale Group.

To understand the high energy released in uranium fission, scientists made some theoretical calculations based on Albert Einstein's famous equation E=mc². The Einstein equation says that mass m can be converted ito energy E, and the conversion factor is a huge number, c, which is the velocity of light squared. One can calculate that the total mass of the fission products remaining at the end of the reaction is slightly less than the mass of the uranium atom plus neutron at the start. This decrease of mass, multiplied by c, shows numerically why the fission fragments are so energetic.