Broadly speaking, radiation is a way in which energy moves from one place to another. Thus, the energy released when a stone is dropped into water radiates away in circular waves. Sound energy radiates from a speaker's mouth to a listener's ear; light and heat energy radiate from the sun to the earth. Electrons, radiating from a hot wire, provide the energy that forms the picture in a television set. In the first four examples the radiation consists of waves--water waves, sound waes, light waves, heat waves. In the last, the radiation is a stream of minute particles.
One of the major discoveries of modern physics is that the shorter the wave length of any wave radiation, the more energy each unit of it carries. Hence, Xrays and gamma rays are enormously more energetic than light. They penetrate much farther into all kinds of matter, and they produce much larger effects.
In addition to waves, atoms are now know to radiate a great of particles. These are all unimaginably tiny (measured in 100-trillionths of an inch,) unimaginably light, and known to us only indirectly through their effects. Some of the more important particles are:
Electrons
The lightest particles, carrying a negative electric charge. Radiation electrons are sometimes called beta rays.
Protons
About 2,000 times as heavy as electrons and positively charged.
Neutrons
Like protons, but uncharged.
Alpha Particles
Each one is an assemblage of two protons and two neutrons.
Atomic radiation is given off by atoms which have more than the normal complement of energy--"excited" atoms in the physicists' phrase. Every atom is composed of a tiny, positively charged "nucleus" surrounded by a cloud or swarm of negative electrons.
How does an atom "get excited?" One way is to be struck by a projectile. In an X-ray machine, a stream of fast-moving electrons is made to strike a metal target. This excites some of the electrons in the atoms of the target. In the process of giving off the excess energy thus gained, the atomic electrons send out Xrays. With the exception of Xrays, the radiation we are interested in comes from the nuclei of atoms rather than from their electron clouds. The big "atom-smashing" machines one reads about are simply devices for hurling various particles against nuclei. The target nuclei then spew out radiation, including gamma rays and a great variety of particles.
When certain atomic nuclei, notably those of uranium and plutonium, are struck by neutrons, they do not radiate in the usual way. Instead, they split into two roughly equal parts and give off a large burst of energy. It is this energy that provides the explosive power of an atomic bomb, and that can coverted into useful power in an atomic pile.
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