Becquerel (Bq): The unit of radioactive decay equal to one disintegration per second. The Becquerel is the basic unit of radioactivity used in the international system of radiation units, referred to as the “SI” units. 37 billion (3.7×10^{10}) becquerels = 1 curie (Ci).

Roentgen: A roentgen is the amount of x-radiation or gamma radiation that produces on electrostatic unit of charge in on cc of dry air at 0 degrees C and 760 mm of mercury atmospheric pressure.

curie (Ci): The original unit used to express the decay rate of a sample of radioactive material. The curie is equal to that quantity of radioactive material in which the number of atoms decaying per second is equal to 37 billion (3.7×10^{10}). It was based on the rate of decay of atoms within one gram of radium. It is named for Marie and Pierre Curie who discovered radium in 1898. The curie is the basic unit of radioactivity used in the system of radiation units in the United States, referred to as "traditional" units.

Dose: A general term used to refer to the effect on a material that is exposed to radiation.

rad (R): The original unit developed for expressing absorbed dose, which is the amount of energy from any type of ionizing radiation (e.g., alpha, beta, gamma, neutrons, etc.) deposited in any medium (e.g., water, tissue, air). A dose of one rad is equivalent to the absorption of 100 ergs (a small but measurable amount of energy) per gram of absorbing tissue. The rad has been replaced by the gray in the SI system of units (1 gray = 100 rad).

Gray (Gy): The international system (SI) unit of radiation dose expressed in terms of absorbed energy per unit mass of tissue. The gray is the unit of absorbed dose and has replaced the rad. 1 gray = 1 Joule/kilogram and also equals 100 rad.

rem (Roentgen Equivalent Man): A rem is the dosage received from the exposure to a rad. It is the number of rads multiplied by the quality factor of the particular source of radiation. The rem and millirem are the commonly used measurement unit of radiation dose in the U.S. 1 rem 1=1 rad.

Sievert (Sv): A sievert is the standard international (SI) measurement of dose. One sirevert is equivalent to 100 rem. A microsievert (mSv) is one millionth of a sievert.

Dose Rate: The radiation dose delivered per unit time.