• Radioactive: Atoms that are unstable. An element may have several forms, or isotopes. A radioactive isotope of an element may be called "radioactive". However, the correct term is radionuclide.
  • Radiation: Emissions given off by way of mass, energy or both from unstable atoms or high-voltage devices.
  • Decay: When an atom emits an alpha or beta particle or gamma ray, it becomes a different type of atom. Radioactive substances may go through several stages of decay before they change into a stable, or non-ionizing, form. For example: U-238 has 14 different stages of decay before it stabilizes.
  • Half-life: Each radionuclide has a characteristic half-life, which is the time require for half of the quantity of material to decay.
  • Background: Radiation the occurs naturally everywhere in our environment.
  • Dose: A general term used to refer to the effect on a material that is exposed to radiation. It is used to refer either to the amount of energy absorbed by a material exposed to radiation (see Dose, absorbed) or to the potential biological effect in tissue exposed to radiation (see Dose, equivalent).
  • Dose Rate: The rate at which radiation is being absorbed.


    Radiation Types

    Ionizing Radiation

    Ionizing radiation is radiation that changes the structure of individual atoms by ionizing them. The ions produced in turn ionize more atoms. Substances that produce ionizing radiation are called radioactive.

    Radioactivity is a natural phenomenon. Nuclear reaction take place continuously on the sun and all stars. The emitted radiation travels through space, and a small fraction reaches Earth. Natural sources of ionizing radiation also exist in people and in the ground. The most common of these are uranium and its decay products.

    Non-Ionizing Radiation

    Non-ionizing radiation ranges from extremely low frequency radiation through the audible, microwave, and visible portions of the spectrum into the ultraviolet range.



    Activity: A measure of the emissions.


    • 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 (3.7×1010) 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 (3.7×1010). 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.


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


    Dose Rate: The radiation dose delivered per unit time.


    • Sv/hr: Sieverts per hour
    • rem/hr: rem per hour
    • R/hr: rads per hour