9. Rays or Particles?

The reason Cockcroft and Walton were hoping to see gamma rays was due to an experiment performed by the Curie-Joliots in Paris. They had bombarded beryllium with alpha-particles, and found that very penetrating gamma rays were emitted. Cockcroft and Walton thought that similar rays should be observed when they bombarded light elements with protons.

Unfortunately, the Curie-Joliots were mistaken in identifying the 'radiation'. In January 1932 James Chadwick repeated their experiments and found that the reaction produced neutral particles, neutrons, that Rutherford had predicted in 1920. A few months after Chadwick's discovery Rutherford came to Cockcroft and Walton and told them they 'ought to put in a fluorescent screen and get on with the job'. Rutherford was clearly hoping to see alpha-particles, and a fluorescent screen would be the best way to detect them.

On 14 April 1932 Walton set up the tube and bombarded lithium with high energy protons. He then crawled into the little observation cabin set up under the apparatus and immediately saw scintillations of the fluorescent screen. The reaction was giving off alpha-particles.

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