ATOMIC BOMBARDMENT
(1932-1938)
Events: Atomic Discoveries, 1890s-1939

In the 1930s, scientists learned a tremendous amount about the structure of the atom by bombarding it with sub-atomic particles.  Ernest O. Lawrence’s cyclotron, the Cockroft-Walton machine, and the Van de Graaff generator, developed by Robert J. Van de Graaff at Princeton University, were particle accelerators designed to bombard the nuclei of various elements to disintegrate atoms.  Attempts of the early 1930s to split atoms, however, required huge amounts of energy because the first accelerators used proton beams and alpha particles as sources of energy.  Since protons and alpha particles are positively charged, they met substantial resistance from the positively charged target nucleus when they attempted to penetrate atoms.  Even high-speed protons and alpha particles scored direct hits on a nucleus only approximately once in a million tries.  Most simply passed by the target nucleus.  Not surprisingly, Ernest Rutherford, Albert Einstein (right), and Niels Bohr regarded particle bombardment as useful in furthering knowledge of nuclear physics but believed it unlikely to meet public expectations of harnessing the power of the atom for practical purposes anytime in the near future.  In a 1933 interview, Rutherford called such expectations "moonshine."  Einstein compared particle bombardment with shooting in the dark at scarce birds, while Bohr, the Danish Nobel laureate, agreed that the chances of taming atomic energy were remote. 

Rutherford, Einstein, and Bohr proved to be wrong in this instance, and the proof was not long in coming.  Beginning in 1934, the Italian physicist Enrico Fermi began bombarding elements with neutrons instead of protons, theorizing that Chadwick's uncharged particles could pass into the nucleus without resistance.  Like other scientists at the time, Fermi paid little attention to the possibility that matter might disappear during bombardment and result in the release of huge amounts of energy in accordance with Einstein's formula, E=mc², which stated that mass and energy were equivalent.  Fermi and his colleagues bombarded sixty-three stable elements and produced thirty-seven new radioactive ones.  They also found that carbon and hydrogen proved useful as moderators in slowing the bombarding neutrons and that slow neutrons produced the best results since neutrons moving more slowly remained in the vicinity of the nucleus longer and were therefore more likely to be captured.  

One element Fermi bombarded with slow neutrons was uranium, the heaviest of the known elements.  Scientists disagreed over what Fermi had produced in this transmutation.  Some thought that the resulting substances were new "transuranic" elements, while others noted that the chemical properties of the substances resembled those of lighter elements.  Fermi was himself uncertain.  For the next several years, attempts to identify these substances dominated the research agenda in the international scientific community, with the answer coming out of Nazi Germany just before Christmas 1938.