The important thing about so-called ‘atom-splitting’ experiments is that they actually split the nucleus, the tiny central kernel of the atom first identified by Ernest Rutherford (see here). It is the number of positively charged protons in the nucleus that determines the nature of the atom – what element it belongs to. Hydrogen has one proton per atom, helium two, and so on. The electric charge of these protons is balanced by the negative electric charge of an equal number of electrons in a cloud in the outer part of the atom – one for hydrogen, two for helium, and so on. The arrangement of electrons in the cloud determines the chemical properties of an element. The nucleus also contains neutral particles called neutrons, which do not affect the chemistry. Atoms with different numbers of neutrons but the same number of protons are called isotopes of a particular element. For example, some atoms of helium have two protons and one neutron, and are known as helium3; others have two protons and two neutrons and are known as helium4.
The particles in the nucleus are held together by an interaction known as the strong nuclear force, which has only a very short range but which affects both neutrons and protons and is powerful enough at short range to overcome the natural tendency of the positively charged protons to repel one another. But the bigger a nucleus is (the more protons it contains), the harder it is for the strong force to overcome the electric repulsion, which makes it possible for heavy nuclei to split, or fission, into two or more lighter pieces. This is the process that people usually think of today when talking about splitting the atom – the kind of fission involved in a nuclear bomb or a nuclear power station (see here). But it is also possible to force nuclei to split by bombarding them with high-energy particles from outside. This is what John Cockcroft and Ernest Walton did in the first atom-splitting experiments, at the Cavendish Laboratory in Cambridge on 14 April 1932. Rutherford was the Director of the laboratory at the time, and had suggested that Cockcroft and Walton should combine their efforts on the project.
The realization that such a project might be feasible was inspired by a visit to Cambridge by the Russian theorist George Gamow in 1929. Gamow had calculated that under the right circumstances a relatively low energy particle could penetrate an atomic nucleus and trigger interactions there. Cockcroft worked out that a proton accelerator running at a few hundred thousand volts might do the job, and it seemed that such an accelerator might be built with the limited resources available at the Cavendish. As Cockcroft later said, in a speech at a Nobel banquet, new ideas combined with new technology to make many great discoveries possible at that time.
Cockcroft and Walton brought separate skills to the task. Cockcroft was a theorist who worked out what was possible, while Walton was a superb experimenter, a ‘handson’ man, who built an apparatus that accelerated protons (hydrogen nuclei) down a tube across an electric potential of just over 700 kilovolts. This was an early example of a so-called ‘linear accelerator’. The proton beam was directed at a target made of lithium, a soft metal in which each nucleus has three protons and either three or four neutrons. The bombardment produced traces of helium, atoms with only two protons in each nucleus. The helium was in the form of fast-moving alpha particles (essentially helium nuclei), which Walton detected by watching the flashes made by these particles when they hit a paper screen coated with zinc sulphide.
To do this, he sat in a lead-lined box, to protect him from radiation, peering through a microscope focused on the screen. In order to work out how much lead was needed, a zinc sulphide screen was hung on the wall inside. If it glowed, another layer of lead was added! Walton wrote: ‘In the microscope there was a wonderful sight. Lots of scintillations, looking just like stars flashing out momentarily on a clear dark night.’ Rutherford himself had investigated and named alpha radiation at the end of the nineteenth century. When he was brought in to see the experiment, he said: ‘Those look mighty like alpha particles to me.’ A crucial clue was that the flashes occurred in pairs, showing that two alpha particles had been emitted simultaneously from the same source. This became known as ‘splitting’ the atom, but it was actually a two-stage process in which an incoming proton briefly combines with a lithium nucleus to make a nucleus of beryllium, containing four protons, then the beryllium nucleus splits into two helium nuclei. The team shared the Nobel Prize in 1951, ‘for their pioneer work on the transmutation of atomic nuclei by artificially accelerated atomic particles’.