By 1927 the focus of atomic research had moved to the atomic nucleus. In order to prise the nucleus open to examine its internal structure, it must be hit with highly energetic particles of its own size or smaller. Walton’s first job in Rutherford’s laboratory was to build an apparatus capable of accelerating electrons (much smaller particles than the atomic nucleus) to very high speeds. Although this project worked well, the speeds achieved remained too slow.
In 1928 George Gamow had applied the new quantum mechanics to show how particles could tunnel through potential barriers, and how this could explain the decay of nuclei through alpha emission. He also realized that tunnelling could lower the energy required for an incident positively charged particle to overcome the Coulomb barrier of a target nucleus. It was this insight that underpinned the commitment of Cockcroft and Walton.
In 1929 Walton was joined by J.D. Cockroft and together they worked to develop an apparatus to accelerate positively charged particles (electrons are negatively charged) to high velocities. Walton’s great technical skill and experimental ingenuity greatly helped to develop the apparatus despite the scarce resources available – they relied in part on car batteries and bits of petrol pumps. They built an accelerator capable of developing an accelerating voltage up to 700,000 volts for the acceleration of protons.
On 14th April 1932, Walton and Cockroft used their proton accelerator to bombard a target made of lithium, the third lightest natural element with protons, causing their nuclei to split and producing two alpha particles.
Those disintegrations produced little flashes of light on a scintillation screen. This was the first time an artificial disintegration of an atomic nucleus was witnessed. What they did, was combine lithium and hydrogen to form helium-thereby producing the first artificial nuclear reaction. The results were published in the scientific journal Nature on 30th April 1932.
Two years after Cockcroft and Walton completed their accelerator, another, much better instrument was invented in the United States by Robert Van de Graaff of MIT. Van de Graaff worked out the principle of the device by using tin cans, silk ribbons, and a small motor, but it worked magnificently; he was able to produce huge potentials.
Ernest Walton built a particle accelerator in the 1930s
