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 nucleuswhen 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, 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 neutronsinstead 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.
Enrico Fermi. Photo courtesy of the University of Chicago
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 Germany just before Christmas 1938.
THE DISCOVERY OF FISSION
The English word “atom” derives from the Greek word “atomon” (“ατομον”), which means “that which cannot be divided”. In 1938, the scientific community proved the Greek philosophers wrong by dividing the atom.
Fission, the basis of the atomic bomb, was discovered in Germany less than a year before the beginning of the Second World War. It was December 1938 when the radiochemists Otto Hahn (above, with Lise Meitner) and Fritz Strassmann, while bombarding elements with neutrons in their Berlin laboratory, made their unexpected discovery. They found that while the nuclei of most elements changed somewhat during neutron bombardment, uranium nuclei changed greatly and broke into two roughly equal pieces. They split and became not the new transuranic elements that some thought Enrico Fermi had discovered but radioactive barium isotopes (barium has the atomic number 56) and other fragments of the uranium itself. The substances Fermi had created in his experiments, that is, did more than resemble lighter elements – they were lighter elements. The products of the Hahn-Strassmann experiment weighed less than that of the original uranium nucleus, and herein lay the primary significance of their findings. It folIowed from Albert Einstein’s E=mc2 equation that the loss of mass resulting from the splitting process must have been converted into energy in the form of kinetic energy that could in turn be converted into heat.
Otto Hahn discovered nuclear fission with fellow scientist Fritz Strassmann in 1938. They were awarded the Nobel Prize in Chemistry.
Calculations made by Hahn’s former colleague, Lise Meitner (above, with Otto Hahn) and her nephew, Otto Frisch, led to the conclusion that so much energy had been released that a previously undiscovered kind of process was at work. Frisch, borrowing the term for cell division in biology – binary fission – named the process “fission.” Fermi had produced fission in 1934, he had just not recognized it.
It soon became clear that the process of fission discovered by Hahn and Strassmann had another important characteristic besides the immediate release of enormous amounts of energy. This was the emission of neutrons. The energy released when fission occurred in uranium caused several neutrons to “boil off” the two main fragments as they flew apart. Given the right set of circumstances, perhaps these secondary neutrons might collide with other atoms and release more neutrons, in turn smashing into other atoms and, at the same time, continuously emitting energy. Beginning with a single uranium nucleus, fission could not only produce substantial amounts of energy but could also lead to a reaction creating ever-increasing amounts of energy. The possibility of such a “chain reaction” (left) completely altered the prospects for releasing the energy stored in the nucleus. A controlled self-sustaining reaction could make it possible to generate a large amount of energy for heat and power, while an unchecked reaction could create an explosion of huge force.
Uranium, the heaviest natural element on Earth, was involved in many of these early processes and became a subject of great interest in physics for a few reasons. Uranium is the heaviest natural element with 92 protons. Hydrogen, in contrast, is extremely light and only has one proton. The interesting part about uranium, however, isn’t so much the number of protons – it’s the unusually high number of neutrons in its isotopes. One isotope of uranium, uranium-235, has 143 neutrons and undergoes induced fission very easily.