It is well known that the discovery of nuclear fission in 1938 had far reaching, even revolutionary effects on world politics. It certainly led to a change of thinking about warfare. Less known is the fact that the discovery of nuclear fission involved a major change of thinking about nuclear reactions, though not of the scale that relativity or quantum mechanic led to. Let us go back to the 1920's. At that time, many atomic system became understood with the help of quantum mechanics. However, little wa known about nuclei, except some rough information about nuclear binding, and the existence of some natural radioactivity.

In the early years of nuclear physics, before the discovery of the neutron in 1932, it was believed that nucleon are made of protons and electrons only. There was no model of the nucleu such as we have today. However there was one success of quantum mechanic in nuclear physics: the theory of alpha decay. As was shown by Gamow, the long lives of alpha-decaying substances could be understood in terms of the alpha particle having to tunnel through a potential barrier, due to the Coulomb interaction between the alpha particle and the rest of the nucleus. Of course, in classical physics, tunnelling cannot occur as it involve a temporary violation ofenergy conservation. Even in quantum mechanics, such tunnelling only occurs with appreciable probability for light particles.

Thus it became part of the conventional wisdom that alpha particle are the heaviest particles that can be emitted in nuclear reactions. While this was reasonable enough, the community of nuclear physicists was reluctant to give it up, even as evidence (from fission) suggested something else.In particular, the discovery of the neutron in 1932 ushered in the age of modern nuclear physics, but it did not lead to a change of thinking about the alpha being the heaviest particle that can be emitted.

In 1934 Fermi studied the reactions induced by slow neutrons on many different nuclei. Among other things, this led to the production of a large number of new isotopes, some subject to beta-decay. An example is the reaction:

Fermi also bombarded Uranium with neutrons, and by analogy with the lighter elements, had some reason to believe that he had made transuranic elements. In fact, he received the Nobel Prize in 1938, for "his demonstration of the existence of new radioactive elements produced by neutron irradiation,and for his related discovery of nuclear reactions brought about by slow neutrons".

It was thought that just like Aluminum can be transmuted to Silicon by slow neutrons, so Uranium could be transmuted into a new element with Z = 93.

The name Neptunium for Z = 93, and, in fact, the discovery came only in 1940 and later. One reason for the earlier plausible, but mistaken conclusions, was that it was not clear what the chemical properties of the transuranic elements should be. Thus in the 1930's, it was not easy to test if they had actually been made.

Now, already in 1934,  Ida Noddack suggested that a different reaction might be occurring, namely breakup of the nucleus into heavy fragments - nuclei of known element lighter than Uranium. She published this suggestion in "Uber das Element 93," Zeitschrift fur Angewandte Chemie 47: 653 (1934). Here she pointed out that chemical analyses could detect such elements were they to be present. Chemical analyses were not done in Fermi's lab and, though she was a chemist, Noddack did not reproduce Fermi's experiments to look for such evidence. Perhaps because her idea went against the conventional wisdom that no particles heavier than alpha particles could be emitted, it was not taken seriously at the time. A few years after Fermi, Irene Joliot-Curie and her husband Frederick also bombarded Uranium with neutrons. Like Fermi, they believed to have made transuranics, but they also found some evidence that Lanthanum(Z = 57) wa produced in the reaction. But they did not connect this with Noddack's idea that the Uranium nucleus was breaking up and this was a fragment of the breakup.

Starting in 1936, Lise Meitner initiated an experimental program to look at the reaction induced by neutrons on Uranium. She invited her old-time friend and collaborator Otto Hahn, a chemist, to join her in investigating properties of the resulting radioactivities. They invited another chemist, Arnold Strassmann, in to join their effort. The results, based on the assumption that the radioactivitie involved nuclei with Z in the vicinity of 92 (Uranium), were quite inconsistent. For example, they thought that one of the elements made was Radium(Z = 88). What they thought was, according to Andersen¹, that Radium had been precipated using Barium as the carrier element. However, the chemists Hahn and Strassman discovered that they could not separate the Radium from its Barium carrier. This was in the fall and winter of 1938 and Lise Meitner had fled to Stockholm from Berlin to avoid Nazi persecution. However, she and Hahn kept in touch by letter and Strassmann reported that she remained the leader of the joint effort.² Finally they felt compelled to draw the conclusion, that the element did not just behave like Barium, it was Barium.

Soon after Hahn and Strassmann reported their results, Lise Meitner and her nephew, Otto Frisch, gave the explanation. They wrote two papers giving a theory of nuclear fission and, infact, coined this phrase. Their theory was immediately taken to be the explanation. It should be pointed out that in 1936/37, Bohr and Kalckar had suggested the possibility of collective excitations involving the nuclear surface. This picture (unlike that of tunnelling through a Coulomb barrier) is consistent with the possibility of nuclear fission. Meitner and Frisch were familiar with the Bohr-Kalckar model,and that made it easier for them to finally overcome the conventional wisdom about emission of heavy particles. The explanation of nuclear fission spread very fast, and early in 1939 this process was soon reproduced in many laboratories. More detailed propertie of fission were worked out by Bohr and Wheeler using the nuclear liquid drop model. That was the beginning of the atomic era. It was one of the more dramatic occasions in Physics when science and politics intersected.

As a postscript, once the prejudice against emission of particles heavier than the alpha particle was broken, it was no great surprise when in the 1980's some clear evidence for decays involving the emission of ¹²C nuclei was established.

1. H. Andersen, "Categorization, Anomalie and the Discovery of Nuclear Fission".  Studies in History and Philosophy of Modern Physics, Vol. 27, 1996, p.463-492. This article discusses the change of thinking which occurred before the idea of fission was accepted by nuclear physicists.

2. Ruth Lewin Sime, "Lise Meitner: A Life in Physics," University of California Press, Los Angeles 1996. This biography of Lise Meitner by a chemist is an excellent account of the history and the scientific history in which Meitner was intimately involved.

A short summary of the fission story is given in an article by E.G. Segre, "The Discovery of Nuclear Fission", Physics Today, July 1989, p.38-43. Emilio Segre was one of the pioneers in nuclear physics of the 1930's.