Shielding: Any material or obstruction that absorbs radiation and thus tends to protect personnel
or materials from the effects of ionizing radiation.
SI: The International System of Units ("Le Systeme International d'Unites") as defined by the
General Conference of Weights and Measures in 1960. These units are generally based on the
meter/kilogram/second units, with special quantities for radiation including the becquerel, gray,
Sievert (Sv): The SI unit of dose equivalent. It is equivalent to 100 rem.
Specific Activity: The total activity of a given nuclide per gram of material. Specific activity is a
function of half-life and is therefore also unique to each radionuclide. There are approximately
10,000 curies in a gram of tritium and 1 curie in a gram of radium-226.
Survey Meter: An instrument used to monitor the presence of radioactivity by detecting the
radiation emitted during the radioactive decay.
Tenth Value Layer (TVL): Amount of shielding material required to reduce radiation exposure by
a factor of 10. One TVL is equal to 3.3 HVL.
Tissue Equivalent Material: Material made up of the same elements in the same proportions as
they occur in a particular biological tissue.
Thermoluminescent Dosimeters (TLD): Dosimeters made of certain crystalline materials that are
capable of both storing a fraction of absorbed ionizing radiation and releasing this energy in the
form of visible photons when heated. The amount of light released can be used as a measure of
radiation exposure to these crystals.
radioactive and has a half-life of 12.3 years.
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