James Chadwick and the Alsos Digital Library for Nuclear Issues

Today in History – February 27, 1932 - Chadwick publishes his discovery of the neutron. Until 1932, the atom was known to consist of a positively charged nucleus surrounded by enough negatively charged electrons to make the atom electrically neutral. Most of the atom was empty space, with its mass concentrated in a tiny nucleus. The nucleus was thought to contain both protons and electrons because the proton (otherwise known as the hydrogen ion, H+) was the lightest known nucleus and because electrons were emitted by the nucleus in beta decay.”

So how was the neutron discovered? After four years as a prisoner of war in WWI in Germany, James Chadwick returned to his native England to rejoin the mentor of his undergraduate days, Ernest Rutherford, who was now head of Cambridge University’s nuclear physics lab. Chadwick received a PhD under Rutherford in 1921 and then became his assistant director of the lab.

In 1919 Rutherford discovered the proton, a positively charged particle within the atom’s nucleus. But Rutherford and Chadwick and other researchers were finding that the proton did not seem to be the only particle in the nucleus.

As they studied atomic disintegration, they kept seeing that the atomic number (number of protons in the nucleus, equivalent to the positive charge of the atom) was less than the atomic mass (average mass of the atom). Take the helium atom, for example, with an atomic mass of 4, but an atomic number (or positive charge) of 2. Since electrons have almost no mass, it seemed that something besides the protons in the nucleus were adding to the mass. One leading explanation was that there were electrons and additional protons in the nucleus as well — the protons still contributed their mass but their positive charge was canceled out by the negatively charged electrons. So in the helium example, there would be four protons and two electrons in the nucleus to yield a mass of 4 but a charge of only 2. Rutherford also put out the idea that there could be a particle with mass but no charge. He called it a neutron, and imagined it as a paired proton and electron. There was no evidence for any of these ideas.

Chadwick kept the problem in the back of his mind while working on other things. Experiments in Europe caught his eye, especially those of Frederic and Irene Joliot-Curie. They used a different method for tracking particle radiation. Chadwick repeated their experiments but with the goal of looking for a neutral particle — one with the same mass as a proton, but with zero charge. His experiments were successful. He was able to determine that the neutron did exist and that its mass was about 0.1 percent more than the proton’s. He published his findings with characteristic modesty in a first paper entitled “Possible Existence of Neutron.” He received the  Nobel Prize in physics in 1935 for this discovery.

See the Engineering Pathway’s educational resources on James Chadwick and the neutron. or visit the Nuclear Engineering Education and the Chemical Engineering Education community sites for more information.