The half-life of radioactive particles

To better understand the decay rate for various radioactive isotopes, natural and man-made, the engineers at M.I.T. wrote an nice overview.  See my *NEWS* page for their website, or check the article on their blog here:


“The chart below lists various important fission products along with their yields – the frequency at which they are produced from fission. For example, 6.3% of fission events (on average) will produce xenon-135 (after the highly unstable fission products rapidly decay). The half-life is a general time scale for how long the listed radioactive fission product will exist before decaying to a more stable fission product. Note that cesium and iodine, which were detected near the Fukushima site, are by far the most frequently occurring radioactive fission product elements.

Yield Fission Product Half-life
6.8% cesium-133/134* 2 years
6.3% iodine-135 / xenon-135 7 hours
6.3% zirconium-93 1.5 million years
6.1% cesium-137 30 years
6.1% molybdenum-99 / technetium-99** 200,000 years
5.8% strontium-90 30 years
2.8% iodine-131 8 days
2.3% promethium-147 3 years
1.1% samarium-149 not radioactive
0.7% iodine-129 15 million years
0.4% samarium-151 90 years
0.4% ruthenium-106 1 year
0.3% krypton-85 11 years
0.2% palladium-107 7 million years

*Cs-133 is stable but has a high fission yield, but it will then produce Cs-134 from absorbing neutrons in the reactor and Cs-134 is radioactive with a ~2 year half-life.

**Half-life reported in the table is for Tc-99.  Mo-99 has a half-life of ~66 hours, which then decays to Tc-99m (metastable form of Tc-99) with a half-life of  ~6 hours.  The Tc-99m then decays to the Tc-99 with the 200,000 year half-life reported in the table.

Note that longer half-lives do not necessarily mean more danger. Some fission products have extremely long half-lives but emit very little radiation at any given time, so they are not dangerous. Other fission products emit huge amounts of radiation but exist for such a short period of time that they are not dangerous. How harmful a given fission product is to humans is a complicated function of half-life, radiation intensity, and various human biology factors.”  [end quote]

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