NP30/Fission, S.A. Moszkowski, 2-18-99

VI. NUCLEAR FISSION

(Much of this section is based on an article "Categorization, Anomalies and the Discovery of Nuclear Fission" by H. Andersen in "Studies in History and Philosophy of Modern Physics, Vol 27, 463-492, 1996)

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 mechanics led to. Let us go back to th 1920's. At that time, many atomic systems became understood with the help of quantum mechanics. However, little was known about nuclei, except some rough information about nuclear binding, and the existence of some natural radioactivity.
There was no nuclear model. It was believed that nucleons are made of protons and electrons only. However, there was one success of quantum mechanics 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, such tunnelling could not occur, and in quantum mechanics, such tunnelling only occurs with appreciable probability for light particles.
Thus it became part of the conventional wisdom that alpha particles 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 many new isotopes, some subject to beta-decay. An example is the reaction:
n + 27Al -> 28Al* -> 28 Si + e- + nu
Fermi also bombarded Uranium with neutrons, and by analogy with the lighter elements, believed that he had made transuranic elements. In fact, that was what was credited with, in the award of his Nobel Prize in 1938!
n + 238U -> 239U* -> 239 Np + e- + nu
(The name Neptunium for Z = 93, came of course, only later!)
Now at the time, it was not clear what the chemical properties of the transuranic elements should be. Thus it was not easy to test if they had actually been made. At the time, it was suggested by Ida Noddack that a different reaction was occurring, namely nuclear fission into two parts, both quite heavy. However, Noddack could not give any supporting evidence for her speculative, though remarkably prescient conjecture. Of course, her idea went against the conventional wisdom that no particles heavier than the alpha could be emitted, thus it was not taken seriously by anyone else at the time. A few years after Fermi, Irene and Frederick Joliot-Curie also bombarded Uranium with neutrons. Like Fermi, they believed to have made transuranics, but they also found some evidence that Lanthanum (Z = 57) was produced in the reaction. But they did not connect this with fission.
In 1936, Hahn and Meitner also looked at the reactions induced by neutrons on Uranium, and investigated the properties of the resulting radioactivities. The results, based, of course, on the assumption that these involved nuclei in the vicinity of Z = 92, (Uranium) were quite inconsistent, with several paradoxes. (For the paradoxes, see the hyperlink to PARADOXES.) Also, they found quite convincing evidence that Barium (Z = 56) was produced in the neutron-Uranium reaction. About this time Lise Meitner had to leave Germany to avoid Nazi persecution, but she and Hahn kept in touch by letter. Soon afterwards Meitner and Frisch came up with the explanation in terms of nuclear fission, which is now taken for granted. 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 tunneling 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 this process was soon reproduced in many laboratories. And Bohr and Wheeler worked out more detailed properties of fission using the nuclear liquid drop model.

PARADOXES IN THE NON-FISSION MODEL OF NEUTRON URANIUM REACTIONS

DECAY WITH EMISSION OF HEAVY NUCLEI

In recent years it has been established that there also decays in which a particle heavier than the alpha particle is emitted.
Thus there is clear evidence for some decays involving the emission of 12 C nuclei.

Go to Introduction

Go to Early Nuclear Physics (1896-1930)