New Particles

In December 1931, Harold Urey discovered deuterium (and its nucleus, the deuteron) by spectroscopically detecting the faint companion lines in the Balmer spectrum of atomic hydrogen that were produced by the heavy hydrogen isotope. In February 1932, James Chadwick, stimulated by the claim of the wife-and-husband team of Irène Curie and Frédéric Joliot that polonium alpha particles cause the emission of energetic gamma rays from beryllium, proved experimentally that not gamma rays but neutrons are emitted, thereby discovering the particle whose existence had been predicted a dozen years earlier by Chadwick’s mentor, Ernest Rutherford. In August 1932, Carl Anderson took a cloud-chamber photograph of a positron traversing a lead plate, unaware that Paul Dirac had predicted the existence of the anti-electron in 1931. These three new particles, the deuteron, neutron, and positron, were immediately incorporated into the experimental and theoretical foundations of nuclear physics.

The Neutron – The Curie Family's Legacy

This article is concerned with the scientific developments that led to the discovery of the neutron by Sir James Chadwick at the Cavendish Laboratory Cambridge in 1932. The Rutherford atom with a heavy nucleus and the problem of the ‘intra-nuclear’ electrons (needed to reconcile nuclear mass and charge) coupled with Marie Curie's discovery of radium as a prime example of natural radioactivity coming from the nucleus were key milestones. Frédéric Joliot and Irène Curie–Joliot almost discovered the neutron in 1931. But the predisposition of the thinking in Chadwick's laboratory allowed conclusive identification of the emission of a heavy neutral particle to be published about a month after the Curie–Joliot experiment. Their Nobel Prize came a few years later with the discovery of artificial radioactivity.