John Cockcroft (1897-1967), British physicist. In 1932, Cockcroft performed the first artificial disintegration of an atomic nucleus. He bombarded a lithium nucleus with protons in a particle accelerator, and observed two alpha particles (helium nuclei) being produced. Study of the energies of these particles verified Einstein's theory of mass-energy equivalence. He was awarded the 1951 Nobel Prize in Physics for this work, along with Ernest Walton.