Gray: The SI unit of absorbed dose; 1 gray = 100 rads
Rad: The unit of radiation absorbed dose
Rem: A measure of radiation dose related to biological effect
Roentgen: The unit of exposure from X or gamma rays (see exposure)
Sievert: The SI unit of dose equivalent; 1 Sv = 100 rem
X-rays: Penetrating electromagnetic radiation whose wavelengths are shorter than those of visible light.
Radioactivity: The spontaneous emission of radiation from the nucleus of an unstable atom. As a result of this emission, the radioactive atom is converted, or decays, into an atom of a different element that might or might not be radioactive.
Gamma rays, or gamma radiation: Electromagnetic radiation of high energy. Gamma rays are the most penetrating type of radiation and represent the major external hazard.
Exposure: A quantity used to indicate the amount of ionization in air produced by x- or gamma-ray radiation. The unit is the roentgen (R). For practical purposes, one roentgen is comparable to 1 rad or 1 rem for X and gamma radiation. The SI unit of exposure is the coulomb per kilogram (C/kg). One R = 2.58 x 10-4 C/kg.
Electromagnetic radiation: A traveling wave motion that results from changing electric and magnetic fields. Types of electromagnetic radiation range from those of short wavelength, like x rays and gamma rays, through the ultraviolet, visible, and infrared regions, to radar and radio waves of relatively long wavelengths.
Dose rate: The dose delivered per unit of time. It is usually expressed as rads per hour or in multiples or sub multiples of this unit such as millirads per hour. The dose rate is commonly used to indicate the level of hazard from a radioactive source.
Cosmic rays: High-energy radiation that originates outside the Earth's atmosphere.
Curie: A unit of measure used to describe the amount of radioactivity in a sample of material.
Beta particle: A small particle ejected from a radioactive atom. It has a moderate penetrating power and a range of up to a few meters in air. Beta particles will penetrate only a fraction of an inch of skin tissue.
Becquerel: The SI unit of activity 1 disintegration per second; 37 billion Bq = 1 curie. (See conversion factors in the Measurement section.
Background radiation: The radiation in man's natural environment, including cosmic rays and radiation from the naturally radioactive elements, both outside and inside the bodies of humans and animals. It is also called natural radiation. Man-made sources of radioactivity contribute to total background radiation levels.
Alpha particle: A specific particle ejected from a radioactive atom. It has low penetrating power and short range. Alpha particles will generally fail to penetrate the skin. Alpha-emitting atoms can cause health effects if introduced into the lungs or wounds.