April 14, 1932: Zounds! We've Split the Atomic Nucleus
John Cockcroft (left) and Ernest Walton flank their boss, Ernest Rutherford, soon after their successful experiment. Rutherford won his Nobel Prize in 1908; Cockroft and Walton received theirs in 1951.
Courtesy Cavendish Laboratory, University of Cambridge
Courtesy Cavendish Laboratory, University of Cambridge
1932: John Cockcroft and Ernest Walton focus a proton beam on lithium and bust its nucleus. The era of accelerator-based experimental nuclear physics is born.
Ernest Rutherford, who first postulated the concept of atomic nucleus in 1910, had called for "a million volts in a soapbox" to advance nuclear research. Working in a vacant room at Rutherford's Cavendish Laboratory at Cambridge University, Englishman Cockcroft and Irishman Walton used spare parts along with some wood and nails to build the world's first nuclear-particle accelerator in 1929. At the heart of the Cockcroft-Walton generator, a system of capacitors and thermionic rectifiers upped the voltages to 600,000 volts. It wasn't a million volts, but it proved sufficient.
The research duo used their new-found juice to accelerate hydrogen nuclei in a discharge tube and have them bombard a layer of lithium. Sure enough, when the hydrogen nucleus (one proton) collided with the lithium nucleus (three protons and four neutrons), the lithium broke into two helium nuclei (with two protons and two neutrons each) shooting off in opposite directions. POW!
First Walton, then Cockcroft, then Rutherford himself observed the results on a zinc sulfide screen: the wave patterns characteristic of the helium nucleus -- the alpha radiation Rutherford had discovered. It wasn't changing lead into gold, as the medieval alchemists had dreamt. But it was the first nuclear transmutation of one element (lithium) to another (helium) under full human control (as opposed to using natural radiation, as Rutherford had already done).
Even better, Cockcroft and Walton measured the total kinetic energy of the helium nuclei. It was greater than that of the original hydrogen and lithium nuclei. But they also observed a loss in the total mass of the nuclei. This provided confirmation of Einstein's law (E = mc2) on the equivalence of mass and energy. The initial computations were subject to experimental error, but later scientific work in this vein confirmed the equation in detail.
Particle accelerators advanced rapidly in power and design -- like Ernest Lawrence's circular accelerator, or cyclotron. (Earnest researchers, these chaps: Rutherford, Walton, Lawrence.)
Cockcroft and Walton received the Nobel Prize in Physics in 1951. The citation from the Royal Swedish Academy of Sciences said their work had "opened up a new and fruitful field of research which was eagerly seized upon by scientific workers the world over ... profoundly influenced the whole subsequent course of nuclear physics [and] stands out as a landmark in the history of science."
The Cockcroft-Walton circuit is still used to supply voltage in large particle accelerators.
The very word atom is from the Greek for indivisible.
