General Information About K-40

Paul Frame, Oak Ridge Associated Universities

Potassium-40(K-40) is a naturally-occurring radionuclide. Wherever there is potassium, there is potassium-40. If there is enough potassium, the K-40 can be detectable with a simple survey instrument.  


K-40 Half-life:  1.28 x 109 years

K-40 Decay Mode:

     Beta decay (89.3%).  The beta maximum energy is 1.31 MeV

     Electron Capture (10.7%).

     Gamma Rays:  1461 keV (10.7%)

Daily intake of potassium element: 3.3 grams

Amount of potassium element in body:  140 grams  (1.5 pCi/g or 55 Bq/kg of body weight)

Typical K-40 activity in body: 0.1 uCi;   This means that there are over 200,000 atoms of K-40 that decay in the body each minute!

Typical K-40 activity in soil:  10 to 20 pCi/g

Dose from Potassium-40

The dose to a typical member of the population is approximately 15-20 mrem/year due to the K-40 in the body and 10 mrem/year due to the gamma rays emitted by K-40 in the environment (primarily the soil).

The human body maintains relatively tight homeostatic control over potassium levels. This means that the consumption of foods containing large amounts of potassium will not increase the body’s potassium content. As such, eating foods like bananas does not increase your annual radiation dose. If someone ingested potassium that had been enriched in K-40, that would be another story.

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Last updated: 01/20/09
Copyright 1999, Oak Ridge Associated Universities