In the mid 1930’s, several laboratories had developed equipment to accelerate protons and α-particles to high energies. When these elementary particles were used as projectiles to bombard stable atoms, new isotopes were formed. Some of these isotopes were radioactive. Another very efficient particle used in these experiments was the neutron. This particle has no charge and will consequently not be influenced by the electric field around the atomic nucleus. The neutron readily penetrates the atom, forming new isotopes. Reactors are excellent sources of neutrons and are used for the production of radioactive isotopes needed for biomedical research and the treatment of disease. The number of artificial isotopes increased rapidly in the years after 1934. By 1937, approximately 200 isotopes were known, in 1949 the number was 650 and today more than 1,300 radioactive isotopes have been produced.