Cockcroft, John Douglas (1897–1967)
At this time there was considerable interest in the possibility of learning more about the properties of the atom by using changed particles such as alpha particles as bullets traveling at high speed to fire at the atom.
In 1919 Rutherford succeeded in transmuting atoms of nitrogen into atoms of oxygen using high speed alpha particles emitted by a naturally radioactive material. It was thought that particles accelerated to high speeds by artificial means could also enter the nucleus.
Realizing the possible importance of such methods, Cockcroft teamed up with another worker in the Cavendish Laboratory, Ernest T. S. Walton, to construct a machine that would be able to accelerate alpha particles to higher speeds than those of particles emitted from radioactive substances. The fact that such a machine would also accelerate protons (more efficient probes of the nucleus than alpha particles) and deuterons (see deuterium) to high speeds promised important results.
In 1929, Cockcroft and Walton designed and constructed an electrical circuit which provided several hundred thousand volts and which might form the basis of such a machine. In order to accelerate charged particles like protons they required a source of these particles and also a means of accelerating them.
Hydrogen gas was used as the source of protons. It was ionized by passing an electric arc through it in an ionization chamber. The electron was stripped from the hydrogen atom, and the hydrogen nucleus – the positively-charged proton – remained.
In 1932, Cockcroft and Walton used their machine to accelerate protons, and directed the beam of protons at a sample of lithium. This resulted in changing lithium atoms into two helium atoms. They had disintegrated – "smashed" – the lithium atom by means of artificially accelerated protons.
Cockcroft and Walton were thus the first to succeed in producing high speed protons in the laboratory and to show that these protons were capable of causing atomic disintegrations.
In 1936 Cockcroft became a Fellow of the Royal Society, and he was knighted in 1948. In 1946 he became Director of the Atomic Energy Research Establishment at Harwell. In 1951, Cockcroft shared with Walton the Nobel Prize in Physics awarded for their work in artificial acceleration of particles. In 1960, Cockcroft became the first Master of Churchill College, Cambridge. Cockcroft also made contributions to the development of nuclear reactors.
Related categories• PHYSICISTS
• ATOMIC AND NUCLEAR PHYSICS
• PARTICLE PHYSICS
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