Radiation and radioactivity

Scientists distinguish radiation from radioactivity, which is a property of some types of matter. Radioactivity causes matter to release certain forms of radiation as the result of changes in the nuclei of the atoms that make up the matter.

To understand radiation and radioactivity, it is necessary to understand how an atom is constructed and how it can change. An atom consists of tiny particles of negative electric charge called electrons surrounding a heavy, positively charged nucleus. Opposite electric charges attract each other, and like charges repel (push away) each other. The positively charged nucleus therefore attracts the negatively charged electrons and so keeps them within the atom.

The nucleus of every element except the most common form of hydrogen consists of particles called protons and neutrons. (A normal hydrogen nucleus is made up of a single proton and no neutrons.) Protons carry a positive charge, and neutrons have no charge. The most common form of helium, for example, has two protons and two neutrons in the nucleus and two electrons outside the nucleus. Protons and neutrons consist of even smaller particles called quarks.

Within the nucleus, the positively charged protons repel one another because they have like charges. The protons and neutrons remain together in the nucleus only because an extremely powerful force holds them together. This force is called the strong nuclear force or the strong interaction.

An atom can change the number of protons and neutrons in its nucleus by giving off or taking in atomic particles or bursts of energy--that is, by giving off or taking in radiation. But any change in the number of protons in the nucleus produces an atom of a different element. Radioactive atoms spontaneously release radiation to take on a more stable form. The process of giving off atomic particles is called radioactive decay. As radioactive elements decay, they change into different forms of the same element or into other elements until they finally become stable and nonradioactive.

Radioactive decay takes place at different rates in different elements or different forms of the same element. The rate of decay is measured by the half-life, the length of time needed for half the atoms in a sample to decay. For example, the half-life of cesium 137, a radioactive form of the metal cesium, is about 30 years. After about 60 years, approximately a fourth of the original cesium 137 remains. After another 30 years, only an eighth remains, and so on. The half-life of radon 222 is about 3.8 days. Half-lives vary from fractions of a second to billions of years.