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GLOSSARY

NATURAL

NORM

RADON

  General Terms
The following list of general terms are used to describe aspects of radiation science. Click on the desired term or scroll down through the page.
 
    ALARA
The guiding principle behind radiation protection is that radiation exposures should be kept "As Low As Reasonably Achievable (ALARA)," economic and social factors being taken into account. This common sense approach means that radiation doses for both workers and the public are typically kept lower than their regulatory limits.
   
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    Atom
Atoms are the smallest part of any material that cannot be broken up by chemical means. Each atom has a center (the nucleus) which contains protons and neutrons. Electrons orbit around the nucleus. In an uncharged atom the number of electrons orbiting the nucleus equals the number of protons in the nucleus. The atom is primarily empty space. If the nucleus of an atom was the size of the button on a baseball pitcher's cap, the electrons would be like dust particles revolving around the outside of the baseball stadium at nearly the speed of light.
   
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    Background Radiation
Radiation is a part of our natural world. People have always been exposed to radiation that originates from within the Earth ("terrestrial" sources) and from outer space ("cosmogenic" or "galactic" sources).
   
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    Electron
Electrons are very small particles with a single negative charge. They are a part of the atom and orbit around the nucleus. Electrons are much smaller than protons or neutrons. The mass of an electron is only about one two-thousandth of a proton or neutron.
   
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    Half-Life
The time required for a population of atoms of a given radionuclide to decrease, by radioactive decay, to exactly one-half of its original number is called the radionuclide's half-life. No operation, either chemical or physical, can change the decay rate of a radioactive substance. Half-lives range from much less than a microsecond to more than a billion years. The longer the half-life the more stable the nuclide. After one half-life, half the original atoms will remain; after two half-lives, one fourth (or 1/2 of 1/2) will remain; after three half-lives one eighth of the original number (1/2 of 1/2 of 1/2) will remain; and so on.
   
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    Ions, Ionization
Atoms which have the same number of electrons and protons have zero charge since the number of positively charged protons equals the number of negatively charged electrons. If an atom has more electrons than protons, it has a negative charge, and is called a negative ion. Atoms which have fewer electrons than protons are positively charged, and are called positive ions. Some forms of radiation can strip electrons from atoms. This type of radiation is appropriately called "ionizing radiation".
   
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    Ionizing Radiation
Ionizing radiation is radiation which has enough energy to cause atoms to lose electrons and become ions. Alpha and beta particles, as well as gamma and x-rays, are all examples of ionizing radiation. Ultraviolet, infrared, and visible light are examples of nonionizing radiation.
   
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    Neutron
Neutrons are part of the nucleus of an atom. Neutrons are, as the name implies, neutral in their charge. That is, they have neither a positive nor a negative charge. Neutrons are about the same size as protons.
   
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    Proton
Protons, along with neutrons, make up the nucleus of an atom. Protons have a single positive charge. While protons and neutrons are about 2,000 times heavier than electrons, they are still very small particles. A grain of sand weighs about a hundred million trillion (100,000,000,000,000,000,000) times more than a proton or a neutron.
   
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    Radiation
Radiation is energy in the form of waves or particles (see types of radiation). Radiation comes from sources such as radioactive material or from equipment such as X-ray machines, or accelerators.
   
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    Radioactive Decay
Radioactive decay describes the process where an energetically unstable atom transforms itself to a more energetically favorable, or stable, state. The unstable atom can emit ionizing radiation in order to become more stable. This atom is said to be "radioactive", and the process of change is called "radioactive decay".
   
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    Risk
In many health fields, risk means the probability of incurring injury, disease, or death. Risk can be expressed as a value that ranges from zero (no injury or harm will occur) to one (harm or injury will definitely occur).
   
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    Types of Radiation
Radiation is energy in the form of waves or particles. X-rays and gamma rays are electromagnetic waves of radiation, as is visible light. Particulate radiation includes alpha and beta radiation. The energy associated with any radiation can be transferred to matter. This transfer of energy can remove electrons from the orbit of atoms leading to the formation of ions. The types of radiation capable of producing ions in matter are collectively referred to as "ionizing radiation".
   
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    Alpha Particle
Alpha particles are composed of two protons and two neutrons. Alpha particles do not travel very far from their radioactive source. They cannot pass through a piece of paper, clothes or even the layer of dead cells which normally protects the skin. Because alpha particles cannot penetrate human skin they are not considered an "external exposure hazard" (this means that if the alpha particles stay outside the human body they cannot harm it). However, alpha particle sources located within the body may pose an "internal" health hazard if they are present in great enough quantities. The risk from indoor radon is due to inhaled alpha particle sources which irradiate lung tissue.
   
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    Beta Particle
Beta particles are similar to electrons except they come from the atomic nucleus and are not bound to any atom. Beta particles cannot travel very far from their radioactive source. For example, they can travel only about one half an inch in human tissue, and they may travel a few yards in air. They are not capable of penetrating something as thin as a book or a pad of paper.
   
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    Gamma Rays
Gamma rays are an example of electromagnetic radiation, as is visible light. Gamma rays originate from the nucleus of an atom. They are capable of traveling long distances through air and most other materials. Gamma rays require more "shielding" material, such as lead or steel, to reduce their numbers than is required for alpha and beta particles.
   
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    X-Rays
X-rays are an example of electromagnetic radiation which arises as electrons are deflected from their original paths or inner orbital electrons change their orbital levels around the atomic nucleus. X-rays, like gamma rays are capable of traveling long distances through air and most other materials. Like gamma rays, X-rays require more shielding to reduce their intensity than do beta or alpha particles. X- and gamma rays differ primarily in their origin: x-rays originate in the electronic shell, gamma rays originate in the nucleus.
   
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    Terms Associated with the Concept of Radiation Dose
The effect that radiation has on any material is determined by the "dose" of radiation that the material receives. Radiation dose is simply the quantity of radiation energy deposited in a material. There are several additional descriptive terms used in radiation protection which provide precise information on how the dose was deposited, the method used to calculate the dose, and the how the radiation energy deposited in tissue will effect humans. Some of these terms are defined below.
   
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    Absorbed Dose
Absorbed dose is the amount of energy deposited in any material by ionizing radiation. The unit of absorbed dose, the rad, is a measure of energy absorbed per gram of material. The unit used in countries other than the U.S. is the gray. One gray equals 100 rad.
   
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    Radiation Dose
The effect of radiation on any material is determined by the "dose" of radiation that material receives. Radiation dose is simply the quantity of radiation energy deposited in a material. There are several terms used in radiation protection to precisely describe the various aspects associated with the concept of dose and how radiation energy deposited in tissue effects humans.
   
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    Exposure
Radiation exposure is a measure of the amount of ionization produced by x-rays or gamma rays as they travel through air. The unit of radiation exposure is the roentgen (R), named for Wilhelm Roentgen, the German scientist who in 1895 discovered x-rays.
   
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    Equivalent Dose
The equivalent dose is a measure of the effect which radiation has on humans. The concept of equivalent dose involves the impact that different types of radiation have on humans. Not all types of radiation produce the same effect in humans. The equivalent dose takes into account the type of radiation and the absorbed dose. For example when considering beta, x-ray, and gamma ray radiation, the equivalent dose (expressed in rems) is equal to the absorbed dose (expressed in rads). For alpha radiation, the equivalent dose is assumed to be twenty times the absorbed dose.
   
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     Glossary | Natural Radioactivity | NORM | Radon

Copyright HPS, 1997

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