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Hans Bethe - Neutron bombardment and resonance theory: Fermi's discoveries (55/158)

3,380 views · Published 27 June 2017 · 4:19 · Indexed 26 October 2026

Channel: Web of Stories - Life Stories of Remarkable People · 2017 · Science & Technology

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German-born theoretical physicist Hans Bethe (1906-2005) was one of the first scientists to join the Manhattan Project, later strongly advocating nuclear disarmament. In 1967, he was awarded the Nobel Prize in Physics for his theory of stellar nucleosynthesis. [Listener: Sam Schweber; date recorded: 1996]

TRANSCRIPT: 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.

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