This not only changed our view of the nucleus, but also provided a new, relatively inexpensive means of probing the nucleus. Two protons and 2 neutrons made a helium nucleus while 92 protons and 146 (or 143) neutrons made uranium, the heaviest known element. It increased the mass of elements without adding electrical charge. Chadwick had found a new elementary particle, the third basic component of the nucleus. (Not to worry in 1935, they received the Nobel Prize in chemistry for their discovery of artificial radioactivity.) It is interesting to note that the Joliot-Curies’ misinterpretation of their results cost them the Nobel Prize. You can read his lecture as he received his Nobel prize. In 1935, Sir James Chadwick received the Nobel Prize in physics for this work. He called it the neutron in a paper published in the February 17, 1932, issue of Nature.
By comparing the energies of recoiling charged particles from different targets, he proved that the beryllium emissions contained a neutral component with a mass approximately equal to that of the proton. He not only bombarded the hydrogen atoms in paraffin with the beryllium emissions, but also used helium, nitrogen, and other elements as targets. When James Chadwick reported to Lord Rutherford on the Joliot-Curies’ results, Lord Rutherford exclaimed, "I do not believe it!" Chadwick immediately repeated the experiments at the Cavendish Laboratory in Cambridge, England. We now know that gamma photons do not have enough energy to eject protons from paraffin. The trouble was that the electron was 1,836 times lighter than the proton and, therefore, recoiled much more easily than the heavier proton after a collision with a \(\gamma\) photon. They used the analogy of the Compton Effect, in which photons impinging on a metal surface eject electrons. However, the Joliot-Curies interpreted the results as the action of photons on the hydrogen atoms in paraffin. This discovery was amazing because photons have no mass. They found that this radiation ejected protons from a paraffin target. In 1932, Irene Joliot-Curie, one of Madame Curie’s daughters, and her husband, Frederic Joliot-Curie, decided to use their strong polonium alpha source to further investigate Bothe’s penetrating radiation. (Courtesy of the American Institute of Physics) They bombarded beryllium with alpha particles emitted from polonium and found that it gave off a penetrating, electrically neutral radiation, which they interpreted to be high-energy gamma photons. In 1928, a German physicist, Walter Bothe, and his student, Herbert Becker, took the initial step in the search. However, its electrical neutrality complicated the search because almost all experimental techniques of this period measured charged particles. This postulation stimulated a search for the particle. Twelve years earlier, Lord Ernest Rutherford, a pioneer in atomic structure, had postulated the existence of a neutral particle, with the approximate mass of a proton, that could result from the capture of an electron by a proton. (Courtesy of the American Institute of Physics) (left), Lord Rutherford at Cambridge (right)