John Douglas Cockroft | Biography

Todmorden, in Lancashire, had been the home of the Cockroft family for more than 400 years when John was born there on May 27, 1897. The Cockrofts had been involved in the weaving and cotton-manufacturing process for most of that time. Young John attended local schools in Todmorden and then entered the University of Manchester in 1914. Within a year, however, he left school to join the Royal Field Artillery in World War I.

After the war, Cockroft became an apprentice at the Metropolitan Vickers Electric Company. He then enrolled at St. John's College, Cambridge, where he earned his bachelor's degree in 1924. Four years later, he received his doctorate from Cambridge.

While working with Ernest Rutherford at Cambridge, Cockroft became interested in the artificial transmutation of the atomic nucleus. Rutherford had experienced some success in this kind of research using alpha particle s from naturally radioactive materials. In 1919, for example, he had bombarded nitrogen gas with alpha particles, obtaining an isotope of oxygen in the reaction:

₇N¹⁴ + ₂He⁴ ₁H¹ + ₈O¹⁷

In 1929, George Gamow showed mathematically that other particles might be more efficient in producing such changes provided that a way could be found to accelerate them. Working with Ernest Walton, Cockroft designed a machine that could do so. The Cockroft-Walton accelerator was able to produce proton beams with energies up to 600,000 volts. They used these proton beams to bombard a variety of targets.

In 1932, Cockroft and Walton directed protons at a lithium target. They found that tracks produced in a cloud chamber as a result of this reaction were characteristic of alpha particles. They concluded that protons had brought about a nuclear transformation in which alpha particles were the only product:

₃Li⁷ + ₁H¹ ₂He⁴ + ₂He⁴

The Cockroft-Walton experiment was significant because it was the first time a nuclear transformation had been produced by completely synthetic means. It immediately stimulated similar research by other scientists using not only Cockroft-Walton type machines, but also the cyclotron invented by E. O. Lawrence in the early 1930s.

For their invention of the accelerating machine and the production of artificial nuclear transformations, Cockroft and Walton shared the Nobel Prize for physics in 1951. Cockroft was appointed Jacksonian Professor of Natural Philosophy at Cambridge in 1939. During World War II he worked on the development of radar and nuclear weapons. After the war, he became director of the United Kingdom's new Atomic Energy Research Establishment at Harwell. From 1961 to 1965 he served as chancellor of the Australian National University in Canberra. He died in Cambridge on September 18, 1967.