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Half Lives Explained

Time is one of the three basic principles of radiation exposure. Hearing that cesium has a half-life of 30 years can be concerning. However, there are different types of half-lives that describe different things. The 30 years is the physical half-life. Here are the different half-life measures and what they mean.

Half Lives

  • Physical Half Life is the time for a quantity of radioisotope to decay by half (Cs-137 = 30 years)
  • Biological Half Life is the time for 1/2 of the amount of a radionuclide to be expelled from the body (CS-137 = 70 to 100 days)
  • Effective Half Life takes into account both physical & biological half lives

1/T effective= 1/T physical + 1/T biological

The below table presents some of the radioactive isotopes associated with a nuclear power plant with their physical and biological half-lives.

Physical and Biological Half Lives of Select Isotopes
Isotope Physical Half-Life Biological Half-Life
Cesium (Cs-137) 30 years 70 days
Cs-134 2 years 70 days
Iodine-131 (I-131) 8.1 days 138 days
Sr-90 28 years 18,000 days

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