James Chadwick

Much of the research in nuclear science in the 1930s focused on implications of radioactivity. By the first decades of the century, scientists had explained the phenomenon of radioactivity—discovered by Henri Becquerel in 1896—as the transmutation of certain chemical elements, with the emission of different kinds of radiation called alpha, beta, and gamma rays. Alpha rays were identified as nuclei of helium atoms, beta rays as electrons, and gamma rays as electromagnetic waves more energetic than X-rays. The discovery of the neutron in 1932 by James Chadwick, a physicist at the Cavendish Laboratory in Cambridge, allowed physicists to theorize that the nucleus was composed of protons and neutrons. The number of protons in the nucleus equaled the atomic number of the element, with the positive charge of the protons balanced by an equal number of electrons in the atom outside the nucleus. Different atoms of the same chemical element, however, could have different numbers of neutrons in the nucleus; a variety of atom with a particular number of neutrons was called an isotope. Certain isotopes were unstable, due to forces arising from particular combinations of protons and neutrons, and would decay radioactively into new elements.

Participants at the seventh Solvay Conference on nuclear physics, held in Brussels, Belgium in October 1933.

John Cockcroft and George Gamow.

Transmutation of chemical elements resulted from the absorption or emission of protons and neutrons and subsequent rearrangement of nuclei. Thus Cockcroft and Walton disintegrated lithium in 1932 by firing a proton into a lithium nucleus to produce two alpha particles. In 1934 the Joliot-Curies in Paris bombarded aluminum with alpha particles to make a new, unstable isotope of phosphorous, which then decayed radioactively to a stable isotope of silicon. They had produced artificially for the first time a radioactive substance, which until then scientists had found only in nature. Lawrence's cyclotron provided a ready means to bombard various elements with neutrons, protons, or deuterons and check the products for radioactivity. The production, detection, and exploration of new radioactive isotopes would form a large part of the program at the Rad Lab, and other nuclear science labs, through the rest of the 1930s. The endeavor led, among other things, to the discovery of nuclear fission by scientists in Europe.

Back