Neutron, Discovery of
NEUTRON, DISCOVERY OF
The discovery of the neutron by James Chadwick in 1932 was the central discovery that opened up the field of nuclear physics in succeeding years. Earlier, physicists believed that the nucleus of every atom was composed of only two elementary particles, the positively charged proton (the nucleus of the hydrogen atom) and the much lighter negatively charged electron; now that no longer could be maintained, although the question of whether or not the neutron was a new elementary particle remained open for more than two years after its discovery.
The first suggestion that a neutron, a particle with no electric charge but with a mass comparable to that of a proton, might exist in the nucleus was made by Ernest Rutherford in a Bakerian Lecture before the Royal Society in London on June 3, 1920, a year after he had succeeded J. J. Thomson as Cavendish Professor of Experimental Physics in Cambridge. Rutherford believed that the alpha particle, the doubly charged, mass-4 nucleus of the helium atom, consisted of four protons and two electrons, and he also believed that he had just found evidence for a new doubly charged, mass-3 nuclear particle consisting of three protons and one electron. Thus, he argued, since two electrons could bind four protons, and one electron three protons, one electron should be able to bind two protons, which would be a new mass-2 isotope of hydrogen, and one electron should be able to combine with one proton, which would be a mass-1 neutron. To Rutherford the neutron also was needed to explain how the nuclei of heavy elements could be built up. Convinced therefore that the neutron should exist, he set some of his research students in search for it experimentally. Chadwick, his right-hand man in the Cavendish Laboratory, also joined that search at odd times throughout the 1920s, to no avail.
As it turned out, the discovery of the neutron was a Tale of Three Cities, with Walther Bothe and his assistant Herbert Becker working in the Physikalisch-Technische Reichsanstalt (Imperial Physical-Technical Institute) in Charlottenburg, a suburb of Berlin; Irène Curie and her husband Frédéric Joliot working in the Institut du Radium in Paris; and James Chadwick working in the Cavendish Laboratory in Cambridge. The story reached its crescendo between June 1930 and February 1932.
By June 1930, when Bothe and Becker published a preliminary report of their experiments, Bothe had worked in the field of nuclear physics for three years, bombarding various light elements with the alpha particles emitted by polonium. He had become convinced that the incident alpha particles excited the nuclei of these elements to higher energy levels, and when they dropped back down to lower energy levels, they emitted high-energy gamma rays. By October he and Becker had found experimentally that such gamma rays were emitted by six light nuclei, including beryllium and boron, whose energies were as high as those of the most energetic gamma rays being emitted spontaneously from heavy radioactive elements.
Bothe and Becker's experiments exerted a powerful influence on Irène Curie and Frédéric Joliot, who had begun to collaborate scientifically in 1928, two years after their marriage. They were drawn to Bothe and Becker's work following an international conference on nuclear physics—the first major one of its kind—that Enrico Fermi organized in Rome in October 1931, which Irène's mother Marie Curie attended and where she heard Bothe give a lecture on his experiments. She also heard Niels Bohr from Copenhagen question whether the laws of conservation of energy and momentum remained valid in the nucleus, and she heard Robert A. Millikan from Pasadena argue strenuously that cosmic rays consist of photons of energy even higher than that of gamma rays. On returning to Paris, she reported these ideas to her daughter and son-in-law, and they all exerted perceptible influences on them.
Curie and Joliot first repeated and then extended Bothe and Becker's experiments, bombarding lithium, beryllium, and boron with polonium alpha particles, and finding that the energy of the gamma rays emitted by beryllium, for example, was much higher than Bothe and Becker had reported; indeed, it lay somewhere between the energies of the gamma rays emitted by radioactive elements and the energies of Millikan's cosmic rays. To investigate the gamma rays emitted by beryllium further, Curie and Joliot inserted sheets of lead and other substances in their path and in front of an ionization chamber. Nothing surprising happened—until they inserted thin sheets of paraffin and other hydrogenous substances, which, in the case of paraffin, caused the ionization current to suddenly double. They reasoned, following a suggestion of Marie Curie, that their high-energy gamma rays were striking and dislodging protons in these hydrogenous (proton-rich) substances which then entered their ionization chamber, greatly increasing its current. They measured the energies of the protons and calculated that to produce them the energies of the incident gamma rays from beryllium and boron had to be 50 and 35 million electron volts, respectively—enormous energies comparable to those of Millikan's cosmic rays. The real problem was that they exceeded the energies that were available from the nuclear reactions that presumably had produced them in the first place. Conservation of energy thus was violated—but that, according to Niels Bohr, was possible in the nuclear realm.
Curie and Joliot reported their experimental findings and conclusions on January 18, 1932, and before the end of the month the journal in which they appeared arrived in Cambridge—where James Chadwick was astonished by them. He then showed their paper to Rutherford, who burst out, "I don't believe it"—a reaction, Chadwick recalled, that he never heard before or since. They agreed that Chadwick should repeat Curie and Joliot's experiments immediately. Chadwick did and convinced himself that their observations were correct but that their interpretation of them was not. The radiations from beryllium and boron did not consist of highly energetic gamma rays but of neutrons.
The neutrons, Chadwick reasoned, were being produced by the nuclear reactions
4Be9 + 2He4 → 6C12 + 0n1 and 5B11 + 2He4 → 7N14 + 0n1,
in other words, the alpha particle (2He4) was striking either a beryllium or boron nucleus (4Be9 or 5B11) and producing either a carbon or nitrogen nucleus (6C12 or 7N14) and a neutron (0n1), where the subscripts denote atomic numbers and the super-scripts atomic masses. These neutrons then were striking and dislodging protons from the paraffin and other hydrogenous substances. From the second reaction, knowing the kinetic energy of the incident alpha particle and the masses of the boron, helium, and nitrogen nuclei, and measuring the kinetic energies of the nitrogen nucleus and neutron, Chadwick assumed that energy was conserved and calculated the mass of the neutron from the mass-energy balance of the reaction, finding it to be 1.0067 atomic mass units (amu). He sent off a preliminary note on his discovery on February 17 and a full report around May 10, 1932.
The fundamental question remaining was whether the neutron was a new elementary particle or a stable proton-electron compound particle. Chadwick's discovery had been conditioned psychologically and institutionally by his long and close association with Rutherford while working in the Cavendish Laboratory. Chadwick's answer to the above question also was conditioned by his knowledge that Rutherford had envisioned the neutron as a stable proton-electron compound in his Bakerian Lecture of 1920—and that was precisely what Chadwick took it to be in 1932. Moreover, he had quantitative support for this view because the sum of the masses of the proton and electron was 1.0078 amu, or 0.0011 amu larger than the mass of the neutron at1.0067 amu, which translated into a proton-electron binding energy of 1 to 2 million electron volts, taking into account the experimental uncertainties involved in his calculation. To Chadwick that was convincing evidence that the neutron was a stable proton-electron compound and not a new elementary particle.
This question remained in dispute for over two years as Curie and Joliot and also Ernest O. Lawrence in Berkeley weighed in on it as well. Thus, at the seventh Solvay conference in Brussels in October 1933, Chadwick again argued for his value of 1.0067 amu for the mass of the neutron, while Curie and Joliot presented evidence supporting a much higher value of 1.012 amu, and Lawrence a much lower one of 1.0006 amu. Five months later, in March 1934, Lawrence was forced to withdraw his low value, admitting that he had misinterpreted his experiments. The issue was finally settled after Maurice Goldhaber, who had found refuge from Nazi Germany in Cambridge, pointed out to Chadwick in April 1934 that the energetic gamma rays emitted by a certain radioactive nucleus probably could be used to disintegrate the nucleus of heavy hydrogen (which Harold C. Urey had discovered in December 1931) into a proton and a neutron and therefore that the mass of the neutron could be calculated from the mass-energy balance of this reaction. Chadwick tested Goldhaber's idea in a preliminary way some weeks later, found that it worked, and invited Goldhaber to join him in pursuing it further. They reported their finding in August 1934: the mass of the neutron was 1.0080 amu, not quite as high as Curie and Joliot's value, but definitely higher than Chadwick's earlier value of 1.0067 amu. The mass of the neutron was unquestionably greater than the sum of the masses of the proton and electron at 1.0072 amu. The neutron therefore was not a stable proton-electron compound but a new elementary particle. In fact, it was a new un stable elementary particle that would decay spontaneously into a proton, electron, and neutrino.
See also:Chadwick, James
Chadwick, J. "Some Personal Notes on the Discovery of the Neutron" in Cambridge Physics in the Thirties, edited by J. Hendry (Adam Hilger, Bristol, 1984).
Feather, N. "The Experimental Discovery of the Neutron" in Cambridge Physics in the Thirties, edited by J. Hendry (Adam Hilger, Bristol, 1984).
Stuewer, R. H. "Mass-Energy and the Neutron in the Early Thirties." Science in Context6 , 195–238 (1993).
Stuewer, R. H. "The Seventh Solvay Conference: Nuclear
Physics at the Crossroads" in No Truth Except in the Details: Essays in Honor of Martin J. Klein, edited by A. J. Kox and D. M. Siegel (Kluwer Academic, Dordrecht, 1995).
Roger H. Stuewer
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