Fission is Explained
Enrico Fermi 1901-1954
Courtesy Chemical and Engineering News
Physicists welcomed the neutron as a bullet that could strike any nucleus, unopposed by electric repulsion. During the middle 1930s, a number of investigators, chief among them the Italian physicist Enrico Fermi, exposed many different isotopes of the chemical elements to beams of neutrons to see what would happen.
What usually happened was that the bombarded nuclei would absorb neutrons, emit alpha, beta, or gamma rays, and change into different isotopes. The identification of the extremely small quantities of isotopes produced required the development of a fantastic new branch of chemistry known as radiochemistry, or, as one chemist put it, “phantom chemistry.”
In some cases the absorption of a neutron by a nucleus was followed by the emission of a negative electron (beta particle). This produced an atom whose nuclear positive charge had been increased by one unit and which therefore belonged at the next higher place on the periodic table. Fermi and others then considered the fascinating possibility of doing the same thing to uranium, the last-known element on the periodic table, to create previously unknown chemical elements. The results of bombarding uranium with neutrons turned out to be extremely complex, but it eventually became clear that “transuranic” elements (those heavier than uranium) could actually be made in this way.[2]
Some of the complex results of bombarding uranium with neutrons formed an intriguing puzzle that kept various investigators busy for several years. In 1939 the German chemists Otto Hahn and Fritz Strassmann and the physicists Lise Meitner and Otto Frisch were able to announce a solution. The absorption of a neutron by a certain uranium nucleus (later shown to be that of the relatively rare isotope uranium-235) can result in a splitting, or fission, of the nucleus into two parts with separate weights that place them somewhere near the middle of the periodic table.
Lise Meitner and Otto Hahn in their laboratory in the 1930s.
Courtesy Addison-Wesley Publishing Co.
The announcement of this discovery created quite a stir among physicists because a nuclear process of this nature must release a very large amount of energy.
Scale model of the CP-1 (Chicago Pile No. 1) used by Enrico Fermi and his associates on December 2, 1942, to achieve the first self-sustaining nuclear reaction. Alternate layers of graphite, containing uranium metal and/or uranium oxide, were separated by layers of solid graphite blocks. Graphite was used to slow down neutrons to increase the likelihood of fissions.
The excitement among physicists became even greater when it was realized that this newly discovered process of fission was accompanied by the release of several free neutrons from the splitting nucleus. Each new neutron could, if properly slowed down by a moderating material, cause another nucleus to split and release more energy and still more neutrons, and so on, as illustrated in [Figure 5]. (A moderator is necessary because fast, newly released neutrons are too readily absorbed by uranium-238 nuclei, which rarely split.) Apparently all that was needed to achieve this spectacular kind of a chain reaction was to assemble enough uranium in one place so that the released neutrons would have a good chance of finding another ²³⁵U nucleus before escaping from the pile. The amount of fissionable material required to sustain a chain reaction is termed the “critical mass.” A team of scientists led by Fermi achieved the first self-sustaining nuclear reaction on December 2, 1942, under the grandstand at the University of Chicago’s athletic field. This date is often referred to as the beginning of the Nuclear Age.
Figure 5 This diagram shows what happens in a chain reaction resulting from fission of uranium-235 atoms.
STRAY NEUTRON ²³⁵U ORIGINAL FISSION FISSION FRAGMENTS One to three neutrons from fission process A NEUTRON SOMETIMES LOST ²³⁸U CHANGES TO PLUTONIUM ²³⁵U ONE NEW FISSION FISSION FRAGMENT One to three neutrons again ²³⁵U ²³⁵U TWO NEW FISSIONS FISSION FRAGMENTS