He immediately repeated the experiments, using many different elements as radiation targets besides paraffin. The lab was directed by Ernest Rutherford, and reportedly when Chadwick relayed the Joliot-Curie results and interpretation to Rutherford, he exclaimed "I do not believe it!"Ĭhadwick himself was certainly suspicious. James Chadwick was working at the Cavendish laboratory in Cambridge at that time. Nevertheless, the Joliot-Curies stuck to their interpretation that high-energy photons were striking the hydrogen atoms in paraffin to eject protons. It was well known that photons could strike a metal surface and eject electrons (as occurs in the then-recently-discovered Compton Effect, proving the particle nature of light) and the Joliot-Curies believed something similar was happening in their experiments.īut protons are 1,836 times heavier than electrons-and that much harder to budge. It was asking quite a lot for a massless particle to eject relatively heavy protons. The Joliot-Curie radiation discovery was amazing, because photons have no mass. In 1932, Irène and Frédéric Joliot-Curie performed experiments with this radiation, and showed that if it fell on paraffin or other hydrogen-containing compound, it could eject protons with very high energy from that substance. However, this radiation was more penetrating than any gamma radiation known. They interpreted this radiation to be high-energy gamma rays (photons). In 1930, the physicists Walther Bothe and Herbert Becker bombarded beryllium with alpha particles (helium nuclei) emitted from the radioactive element polonium, and they found that the beryllium gave off an unusual, electrically neutral radiation. Detection methods of that day mainly relied on the electrical charges of particles revealing their presence-but neutrons, having no electrical charge, would leave no trace. Such a "neutron" would prove difficult to detect with 1920s equipment. One major problem with Rutherford's "neutron theory"-not much evidence.Įvidence was difficult to come by. Rutherford imagined a paired proton and electron somehow joined in one particle. Ernest Rutherford in 1921 postulated a particle called the "neutron," having a similar mass as a proton but electrically neutral. Eventually, however, calculations using Heisenberg's uncertainty principle showed it was not possible for electrons to be contained in the nucleus. The extra protons were thought to provide the extra atomic mass, while the additional electrons would cancel out their positive charge, leaving the atom electrically neutral. The theory at the time was that there were "nuclear electrons" in the atomic nucleus, along with additional protons. What could account for all this additional mysterious mass? However, it was also well-known that atomic mass is generally twice the atomic number (i.e., the number of protons), and that almost all the mass of an atom is concentrated in the nucleus. Negatively-charged electrons, orbiting a tiny atomic nucleus composed of positively-charged protons, like a miniature solar system-this model explained atoms being electrically neutral, using only protons and electrons, the two fundamental atomic particles known at the time. When Ernest Rutherford discovered the proton in 1918, scientists at the time might have thought that they had finally figured out atomic structure once and for all. A misinterpretation of data perhaps cost the Joliot-Curies an earlier Nobel Prize, but instead led to James Chadwick taking the Nobel podium two days after the Joliot-Curies, on December 12, 1935, to receive the Nobel Prize in physics for discovering the neutron. The September installment of Nuclear Pioneers explored the artificial radioactivity research of Irène and Frédéric Joliot-Curie, for which they were awarded the Nobel Prize in Chemistry on December 10, 1935.
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