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.

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.

Dose: A general term for the quantity of radiation or energy absorbed.

Electromagnetic radiation: an emission of energy with no mass and no charge across a spectrum of wavelengths (frequency). [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.]

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.

Gamma rays, or gamma radiation: Electromagnetic radiation of high energy.

Gray: The SI unit of absorbed dose; 1 gray = 100 rads.

Inverse square law: The relationship that states that electromagnetic radiation intensity is inversely proportional to the square of the distance from a point source.

Ionization: Production of charged particles in a medium. An orbital electron is stripped from a neutral atom, producing an ion pair (a negatively charged electron and a positively charged atom).

Ionizing radiation: Electromagnetic (X ray and gamma) or particulate (alpha, beta) radiation capable of producing ions or charged particles.

Particulate radiation – ionizing radiation with a mass and a charge,   Electron beams, beta , neutrons, and alpha particles.

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.