Shock wave: Expanding front of high pressure and strong winds produced by an explosion. Spontaneous fission: Natural fission, not induced by a neutron. The half-life for this process in uranium²³⁸ is 8 × 10¹⁵ years. Stratosphere: The atmosphere above the weather zone. The altitude of the stratosphere varies from thirty to fifty thousand feet depending on latitude and season. Stratospheric fallout: World-wide fallout from big bombs whose clouds rise into the stratosphere. On the average the radioactivity remains in the stratosphere for about ten years and is then deposited more or less uniformly over the surface of the earth. Strontium⁹⁰: A radioactive fission product. It has a half-life of 28 years and emits two electrons of average total energy 1.2 million electron-volts. Strontium is chemically similar to calcium and gets deposited in bones. Thermal radiation: Electromagnetic radiation, mainly visible, but also ultraviolet and infrared, emitted from the fireball of a nuclear explosion and transmitted long distances in the surrounding cold air. Thermonuclear bomb: A bomb which derives a significant fraction of its energy from the fusion of hydrogen isotopes. Thermonuclear reaction: A fusion reaction induced by high temperature. Thorium: Element with charge 90. The principal isotope has a weight of 232 and emits an alpha particle with a half-life of 14 billion years. Trigger process: A small cause which leads to a big effect. Tritium: An isotope of hydrogen. Its nucleus (called a triton) consists of one proton and two neutrons. Tritons are radioactive beta emitters having a half-life of 12.25 years. Troposphere: The weather portion of the atmosphere, from sea level to about forty thousand feet. Tropospheric fallout: World-wide fallout, mainly from small bombs (less than a megaton) whose clouds remain in the troposphere. This fallout occurs on the average two weeks to a month after the explosion and stays in a latitude close to the latitude of the explosion. Uranium: Element with charge 92. Natural uranium contains 1 part of U²³⁵ to 139 parts of U²³⁸. U²³⁵ is a fissionable material and U²³⁸ can be converted to plutonium, which is fissionable. X-ray: Penetrating electromagnetic radiation, usually made by bombarding a metal target with energetic electrons. X-rays and gamma rays are really the same thing. FOOTNOTES [1]The word “noble” is perhaps a misnomer—these atoms do not even seek the company of each other. [2]Quotes are put around the word atom because, having lost one of its electrons, it is no longer an ordinary neutral atom in its ground state. [3]Yet.