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.
| 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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