Rays or Particles?
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
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.
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.