This appendix presents basic facts about radiation. Radiation comes from natural and humanmade sources. People are exposed to naturally occurring radiation constantly.
All matter is made up of atoms.
The number of protons in the nucleus determines an element's atomic number or chemical identity. With the exception of hydrogen, the nucleus of each type of atom also contains at least one neutron. Unlike protons, the neutrons may vary in number among atoms of the same element. The number of neutrons and protons determines the atomic weight. Atoms of the same element that have different numbers of neutrons are called isotopes.