Worlds Within Worlds: The Story of Nuclear Energy, Volume 2

Was there nothing that could be done to make nuclear bombardment more efficient and increase the chance of obtaining useful energy out of nuclear reactions? In 1928 the Russian-American physicist George Gamow (1904-1968) suggested that protons might be used as bombarding agents in place of alpha particles. Protons were only one-fourth as massive as alpha particles and the collision might be correspondingly less effective; on the other hand, protons had only half the positive charge of alpha particles and would not be as strongly repelled by the nuclei. Then, too, protons were much more easily available than alpha particles. To get a supply of protons one only had to ionize the very common hydrogen atoms, i.e., get rid of the single electron of the hydrogen atom, and a single proton is left. 87 Artificial radioactivity. 88 Of course, protons obtained by the ionization of hydrogen atoms have very little energy, but could energy be imparted to them? Protons carry a positive charge and a force can therefore be exerted upon them by an electric or magnetic field. In a device that makes use of such fields, protons can be accelerated (made to go faster and faster), and thus gain more and more energy. In the end, if enough energy is gained, the proton could do more damage than the alpha particle, despite the former’s smaller mass. Combine that with the smaller repulsion involved and the greater ease of obtaining protons—and the weight of convenience and usefulness would swing far in the direction of the proton. Physicists began to try to design “particle accelerators” and the first practical device of this sort was produced in 1929 by the two British physicists John Douglas Cockcroft (1897-1967) and Ernest Thomas Sinton Walton (1903- ). Their device, called an “electrostatic accelerator”, produced protons that were sufficiently energetic to initiate nuclear reactions. In 1931 they used their accelerated protons to disrupt the nucleus of lithium-7. It was the first nuclear reaction to be brought about by man-made bombarding particles. Other types of particle accelerators were also being developed at this time. The most famous was the one built in 1930 by the American physicist Ernest Orlando Lawrence (1901-1958). In this device a magnet was used to make the protons move in gradually expanding circles, gaining energy with each lap until they finally moved out beyond the influence of the magnet and then hurtled out of the instrument in a straight line at maximum energy. This instrument was called a “cyclotron”. 89 Inventors of one of the first accelerators, Ernest T. S. Walton, left, and John D. Cockcroft, right, with Lord Ernest Rutherford at Cambridge University in the early 1930s. The bombardment of lithium-7 with protons was the first nuclear reaction caused by man-made particles. 90 The cyclotron was rapidly improved, using larger magnets and increasingly sophisticated design. There are now, at this time of writing, “proton synchrotrons” (descendants of that first cyclotron) that produce particles with over a million times the energy of those produced by Lawrence’s first cyclotron. Of course, the first cyclotron was only a quarter of a meter wide, while the largest today has a diameter of some 2000 meters. As particle accelerators grew larger, more efficient, and more powerful, they became ever more useful in studying the structure of the nucleus and the nature of the subatomic particles themselves. They did not serve, however, to bring the dream of useful nuclear energy any closer. Though they brought about the liberation of vastly more nuclear energy than Rutherford’s initial bombardments could, they also consumed a great deal more energy in the process. It is not surprising that Rutherford, the pioneer in nuclear bombardment, was pessimistic. To the end of his days (he died in 1937) he maintained that it would be forever impossible to tap the energy of the nucleus for use by man. Hopes that “nuclear power” might some day run the world’s industries were, in his view, an idle dream. 91 Ernest O. Lawrence holds a model of the first cyclotron in 1930, a year after its conception. 92