Fireball: The luminous ball of hot air and bomb material which expands and cools as the shock wave races out.
Fission: The breaking-up of a heavy nucleus into two or more fragments. A large amount of energy and some free neutrons are released in the process.
Fissionable material: Isotopes which undergo fission when bombarded by slow neutrons: uranium²³⁵ and plutonium²³⁹.
Fission products: Fission fragments and their daughters, including hundreds of different radioactive species, among them strontium⁹⁰ and cesium¹³⁷.
Fusion: The combining of light nuclei into heavier ones with a release of energy. For example, deuteron + triton → alpha + neutron. About 18 million electron-volts are released in this process.
Gamma ray: Energetic, penetrating electro-magnetic radiation emitted by certain radioactive nuclei, frequently after a beta emission.
Genes: Parts of the chromosomes. They are big molecules that determine heredity.
Ground state: The state of least energy and greatest stability of atoms, molecules, and nuclei.
Ground zero: The point on the surface of the earth directly above or below a nuclear explosion.
Half-life: The time required for one half of a large number of identical radioactive nuclei to disintegrate.