In January 1932, a young British researcher, James Chadwick, read a paper published in the journal of the French Academy of Sciences, Comptes Rendus by a husband-and-wife team, Frederick and Irene1 Joliot-Curie. In that paper, they described how they had bombarded a piece of the gray, toxic metal beryllium with α (alpha) particles2 shot out of a sample of the radioactive element polonium. What emerged from the experiment was a highly energetic yet intangible form of radiation. Although they couldn’t detect the radiation directly, they were able to detect protons punched out of a range of materials placed in the path of the new radiation. What puzzled them was this new form of radiation passed through almost anything more freely than any other form of radiation they had previously encountered. Earlier experiments by Walter Bothe and his student Herbert Becker in Germany had shown that the radiation would pass through 200mm of lead. Not bad, considering it takes less than 1 mm of lead to stop the most energetic form of energy then known, y (gamma) rays.3 This new radiation was ‘so hard that one can hardly doubt their nuclear origin’.4 Both teams assumed that the unknown radiation must in fact be an even more energetic form of gamma rays; after all, it was known that gamma rays could knock electrons out of metals, so why not protons out of the nuclei of other materials? But this was only part of the story; Chadwick knew the rest. He knew what they had found but had not identified. Electrified into action, he repeated and extended the experiments and, in the course of three weeks of frantic, intense experimentation, demonstrated the existence of the third component of the atom, the neutron. The discovery was to have profound consequences for events in the heavens and here on Earth.
KeywordsNeutron Star White Dwarf Geiger Counter White Dwarf Star Energetic Form
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- 1.Daughter of Pierre and Marie Curie.Google Scholar
- 2.Helium nuclei, now known to consist of two protons and two neutrons.Google Scholar
- 3.γ rays are even more energetic and penetrating than the more familiar X-rays used by the medical profession to explore the nature of broken bones.Google Scholar
- 4.Quoted in Pais (1986, p. 398).Google Scholar
- 5.The word ‘neutron’ had been used as early as 1898 to describe various particles, including proton-electron pairs.Google Scholar
- 6.Quoted in Pais p. 398.Google Scholar
- 7.Named after a Russian physicist Piotr (Peter) Leonidovich Kapitza.Google Scholar
- 8.It depends on the composition and rotation of the white dwarf, but the details don’t matter to our story.Google Scholar
- 9.Thanks to Kenneth Brecher for his insights into this piece of history.Google Scholar
- 10.Astronomers refer to anything heavier than lithium as ‘metals’: they are the product of highly evolved stars and supernovae.Google Scholar
- 11.Much of the biographical data on Zwicky was published in In Search of Dark Matter by Ken Freeman and Geoff McNamara, Springer, 2005.Google Scholar
- 12.Baade, W. & Zwicky, F. 1934, ‘Cosmic Rays from Super-Novae’ Proc. Natl. Acad. Sci, USA, Vol. 20, p. 259.Google Scholar