Nuclear Fission

The second important discovery, which was soon tested and found to be correct, was Einstein's theory that energy and mass were different forms of the same thing. This theory is usually expressed in the famous formula ( E=mc², E stands for energy, m for the mass，and c for the speed of light. Notice that the mass of an object is multiplied by the speed of light squared; in other words, the speed of light is multiplied by itself. This means that an enormous amount of energy is locked up in the nucleus of each atom. If heavy atoms could be split, scientists reasoned, this energy could be released, as it is when the radioactive elements disintegrate spontaneously. The scientific name lor splitting the nucleus of an atom is nuclear fission; fission is another word for splitting.

In 1939, just before the outbreak of World War I , it was recognized that an atom of uranium could be split into two nearly equal parts. This was a key discovery because it indicated for the first time that the energy contained in the nuclei of atoms could be released in a usable and practical form.

Many difficulties had to be overcome before this theoretical possibility became a practical reality. One of them involved the kind of uranium that could be used. We have already mentioned isotopes，and the uranium that existed in nature was primarily a mixture of two isotopes-uranium-235 and uranium-238. The uranium— 235 was found to be easily fissionable (it could be split), but it existed in a very small proportion in natural uranium--- less than one percent of the total amount. Before 1939, only very tiny amount of pure uranium-235 had been obtained, barely enough for experimental purpose.

Theoretically, it seemed possible in 1939 to start a chain reaction. When a uranium-235 atom was split, it emitted two or three neutrons which then struck two or three more atoms, which in turn emitted still more neutrons to keep the reaction going. However, the neutrons shot out of the nuclei at a very high speed,whereas the nucleus had been split by a relatively slow-moving neutron. Therefore, some substance had to be found to slow down the neutrons enough to keep the reaction going.

Another problem involved the critical mass of fissionable material necessary to produce a chain raction. This refers to the minimum amount of uranium-235 that had to be used in order to prevent the neutrons from escaping untouched into the surrounding environment. Remember that the interior of the atom is for the most part empty space. As the mass increases, however, the number of neutrons that can escape from it decreases.

Still another problem involved control. It was necessary to find a substance that could absorb neutrons so that the reaction could be stopped or kept at a level at which the release of energy did not cause the system to melt.

Because of the urgency of the war, all these problems were solved in a very short period of time——between 1939 and early 1945, in fact, when the first nuclear device was exploded in New Mexico.