In the previous chapter evidence has been presented indicating that the familiar power sources are either highly polluting or quite unable to provide the huge amounts of power that are needed to sustain our standard of living and also to raise that of people in the developing countries. There is however another energy source, the nucleus of the atom. This has been made possible by researches starting with the discovery of radioactivity by Bequerel in 1896 that eventually led to the discovery of the nucleus by Rutherford in 1911. In the following years there were many studies of different nuclei and their interactions.
Rutherford used the radiations from radioactive materials to bombard a range of nuclei, and he established that in some circumstances nuclear reactions can occur. In a nuclear reaction the incident particle can break up another nucleus when it collides with it or it can lead to the transfer of one or more particles from the projectile to the target nucleus, or from the target nucleus to the projectile. In nuclear reactions the sum of the masses of the interacting particles is usually different from the sum of those after the reaction. If it is greater, the excess mass is transformed into energy, which appears as kinetic energy of the emerging particles. The amount of energy released in one reaction is very small, so it was at first not considered practicable to extract useable energy from nuclear reactions.
The radiations from radioactive materials are very unsatisfactory tools for the study of nuclear reactions. They are rather low in energy and so are unable to overcome the electrostatic repulsion of the heavier target nuclei. They are not well-collimated and their energy cannot be continuously varied. This led to the development of nuclear accelerators that produced well-collimated monoenergetic beams that made possible the determination of the cross-sections and other features of a wide range of nuclear reactions. Lawrence built the first cyclotron in the USA, and Cockcroft and Walton made an electrostatic accelerator in Cambridge. This machine could accelerate protons to 700 KeV and they used them to break up the nucleus of lithium. This was the first time an atom had been split using a man-made accelerator.
Neutrons, discovered by Chadwick in 1932, are electrically neutral, and so they are not repelled by the electrostatic field of the nucleus. They therefore easily enter a nucleus, increasing its mass by one unit. Fermi in Italy bombarded a whole series of nuclei with neutrons and discovered that many new nuclei are produced. Two German chemists, Hahn and Strassmann, found that among the nuclei produced when uranium was used was one that seemed to have the same properties as barium. Meitner and Frisch realised that the nuclei of uranium had separated into two fragments, one of which was barium. They verified this explanation by calculating the energy released as the fragments recoil due to electrostatic repulsion and found it to be the same as would be expected from the masses of the participating nuclei. Making use of a biological term applied to the division of a cell into two they called the process fission. During the fission process two or three neutrons are also emitted, and these can enter nearby uranium nuclei and cause them to fission as well. This immediately raises the possibility of a chain reaction that results in the fission of a large number of the uranium nuclei. This happens very rapidly and causes a violent explosion and is the basis of the atomic bomb.
Uranium occurs naturally in the form of two isotopes, with the same nuclear charge but different numbers of nucleons, 235 and 238 respectively. It is only the uranium 235 that undergoes fission in this way, and it constitutes only 0.7% of natural uranium. In natural uranium so many neutrons are captured by the uranium 238 without causing fission that the chain reaction cannot take place. To make a bomb it is necessary to separate the uranium 235 from the uranium 239, a very difficult process that was achieved in the USA, thus making the bomb possible.
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