Basics

The basic building block of all materials on earth is the atom. The atom is composed of a central nucleus surrounded by an outer cloud of orbital electrons. The nucleus of the atom is composed of neutrons and protons. The total number of protons contained in a specific atomic nucleus is referred to as the atomic number or Z-number, of that particular type of atom. 

Atoms can be either stable or unstable. Unstable atoms spontaneously decay to a more stable configuration by the emission of particulate radiation and/or electro-magnetic radiation (photons). Unstable atoms are said to be radioactive. Depending on the type of decay, an atom may remain an atom of the same element or it can become an entirely different element.

By definition, there are two broad categories of radiation. The ionizing radiation category includes alphas, betas, protons, x-rays, gamma radiation, and neutrons. Non-ionizing radiations include ultra-violet, infrared, radio waves and microwave radiation. The main difference between these two basic types of radiation is that ionizing radiation has enough energy to remove orbital electrons from the atoms of the material being irradiated. The process of removing orbital electrons is called ionization. Ionization can cause biological damage in living tissue. This manual is only concerned with ionizing radiation.

The energy of ionizing radiation is usually given in electron volts. The electron volt (eV) is defined as the energy of an electron that been accelerated through an electrical potential of l volt. The eV is a very small amount of energy; therefore, keV (thousand electron volts) and MeV (million electron volts) are used as the units of measurement for the energies associated with the emissions from radioactive materials or machines. The energy of visible light is about 2 to 3 eV.

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