VOLUME 3 Nuclear Fission 117 New Elements 117 The Discovery of Fission 122 The Nuclear Chain Reaction 127 The Nuclear Bomb 131 Nuclear Reactors 141 Nuclear Fusion 146 The Energy of the Sun 146 Thermonuclear Bombs 148 Controlled Fusion 150 Beyond Fusion 158 Antimatter 158 The Unknown 163 Reading List 165 [Illustration: _Enrico Fermi (left) and Niels Bohr discuss physics as they stroll along the Appian Way outside Rome in 1931._] NUCLEAR FISSION New Elements In 1934 Enrico Fermi began his first experiments involving the bombardment of uranium with neutrons—experiments that were to change the face of the world. Fermi had found that slow neutrons, which had very little energy, were easily absorbed by atomic nuclei—more easily than fast neutrons were absorbed, and certainly more easily than charged particles were. Often what happened was that the neutron was simply absorbed by the nucleus. Since the neutron has a mass number of 1 and an atomic number of 0 (because it is uncharged), a nucleus that absorbs a neutron remains an isotope of the same element, but increases its mass number. For instance, suppose that neutrons are used to bombard hydrogen-1, which then captures one of the neutrons. From a single proton, it will become a proton plus a neutron; from hydrogen-1, it will become hydrogen-2. A new nucleus formed in this way will be at a higher energy and that energy is emitted in the form of a gamma ray. Sometimes the more massive isotope that is formed through neutron absorption is stable, as hydrogen-2 is. Sometimes it is not, but is radioactive instead. Because it has added a neutron, it has too many neutrons for stability. The best way of adjusting the matter is to emit a beta particle (electron). This converts one of the neutrons into a proton. The mass number stays the same but the atomic number increases by one.