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Explaining neutron bombardment and resonance theory


Neutron bombardment and resonance theory: Fermi's discoveries
Hans Bethe Scientist
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The first artificial radioactivity however had been found by Curie and Joliot, when carbon was bombarded and made Nitrogen-13. But when Fermi found radioactivities all over and often several in the same target, and Fermi then found something... two very interesting things. One was that slow neutrons were much more effective than fast neutrons in getting into a nucleus. Of course slow protons never get into a nucleus because they are repelled by the... by the electric forces, but slow neutrons have an easy time, and slow neutrons having a very long wave length can get into the nucleus very easily. And Fermi himself brought the theory to that as well. But then he found also another interesting phenomenon; namely... he could pbut absorbers between the neutron source and the target, and there was one overall absorber which was boron, or one absorber for only slow neutrons which was cadmium, but then he also irradiated, let's say silver, and put a silver foil in between. And it turned out that the silver foil absorbed the neutrons which made radioactivity in silver much more strongly than it absorbed other neutrons. So this was explained by Bohr as a resonance phenomenon, and the resonance theory was immediately adopted by Fermi and by everybody else and I tried to explain these resonances by just taking the wave function of a neutron in the field of a nucleus, and then just as a one body problem there would be resonances with the spacings of maybe 1 million electron volt. But that wasn't at all what Fermi had observed, so I... then Bohr came and said 'No, that's not at all the way it works. The neutron gets caught in the nucleus and stays in the nucleus for a long time, maybe a thousand times the transit time of a neutron, and then finally it comes out again. And in between it forms a compound nucleus.' So the question that then arose was how many levels would such a compound nucleus have. The theory of scattering with the compound nucleus was made by Breit and Wigner and was obviously right and is still right, but now the question was how many resonances would there be.

The late German-American physicist Hans Bethe once described himself as the H-bomb's midwife. He left Nazi Germany in 1933, after which he helped develop the first atomic bomb, won the Nobel Prize in Physics in 1967 for his contribution to the theory of nuclear reactions, advocated tighter controls over nuclear weapons and campaigned vigorously for the peaceful use of nuclear energy.

Listeners: Sam Schweber

Silvan Sam Schweber is the Koret Professor of the History of Ideas and Professor of Physics at Brandeis University, and a Faculty Associate in the Department of the History of Science at Harvard University. He is the author of a history of the development of quantum electro mechanics, "QED and the men who made it", and has recently completed a biography of Hans Bethe and the history of nuclear weapons development, "In the Shadow of the Bomb: Oppenheimer, Bethe, and the Moral Responsibility of the Scientist" (Princeton University Press, 2000).

Tags: Irène Joliot-Curie, Frédéric Joliot-Curie, Enrico Fermi, Niels Bohr, Gregory Breit, Eugene Wigner

Duration: 4 minutes, 19 seconds

Date story recorded: December 1996

Date story went live: 24 January 2008