John Cockcroft and Ernest Walton received the Nobel Prize in Physics on November 15, 1951, for building the first particle accelerator and using it to split atomic nuclei — a feat that transformed nuclear physics and laid groundwork for everything from medical isotopes to particle colliders.
The British physicists accomplished the splitting at Cambridge University's Cavendish Laboratory in 1932, bombarding lithium atoms with protons accelerated to energies of about 700,000 electron volts. The experiment confirmed Albert Einstein's mass-energy equation and proved that nuclear reactions could be triggered artificially rather than only occurring in radioactive decay.
Their apparatus, later called the Cockcroft-Walton generator, used a voltage multiplier circuit to accelerate protons through a vacuum tube. The machine was crude by modern standards — roughly 2 meters tall and assembled from glass cylinders, metal electrodes, and sealing wax. It produced a beam of protons that struck a lithium target, splitting each nucleus into two helium nuclei, or alpha particles.
The work carried real physical danger. The high-voltage equipment had exposed conductors, and the two men sometimes had to crawl along the laboratory floor to reach instruments without touching electrified components. Cockcroft later described the setup as operating at voltages that could kill a person instantly if contacted.
Ernest Rutherford, then director of the Cavendish Laboratory, had challenged his colleagues to find a way to accelerate particles to energies high enough to penetrate atomic nuclei. Cockcroft and Walton solved the problem by designing a circuit that converted low alternating current into extremely high direct-current voltage, a principle still used in particle accelerators and high-voltage power supplies today.
The 1951 Nobel Prize was awarded "for their pioneer work on the transmutation of atomic nuclei by artificially accelerated atomic particles." The citation recognized that their method opened the door to a new era of experimental nuclear physics.
The technology they pioneered led directly to cyclotrons, synchrotrons, and eventually the Large Hadron Collider at CERN, which accelerates protons to energies nearly 10 million times higher than the Cockcroft-Walton generator achieved. Medical linear accelerators used in cancer radiation therapy also descend from their voltage multiplier design.
Walton died in 1995 at age 91. Cockcroft died in 1967 at age 70. Their original accelerator is preserved at the Science Museum in London.
