Terrestrial radiation exposure, mainly in the form of gamma radiation, is
due to so-called primordial radionuclides in the earth crust. Because
of their extremely long half-lives, they have existed since the beginning
of Earth. Especially potassium 40 and the nuclides of the uranium-radium
and thorium series are relevant with respect to radiation exposure of the
population. Uranium and thorium disintegrate, via several decay products,
to stable lead. One of the decay products is radon, a naturally occurring
radioactive noble gas with a half-life of 3.8 days.
Because of the variable composition of rock and soil in different regions,
there are strong fluctuations of terrestrial radiation. Certain regions of
the world have particularly high absorbed dose rates due to ambient radiation
produced by high activity concentrations of natural radioactive substances
in the soil (thorium containing monazite sand in Brasil and India, granite
rock with high thorium and uranium contents in France). The annual radiation
exposure from external sources in Germany is about 0.4 mSv.
Naturally occurring radionuclides are also ingested through drinking water
and food or are inhaled, causing an internal radiation exposure of about
1.7 mSv per year. The main source of this exposure is Radon-222 and
its decay products, which contribute approximately 1.4 mSv. The atmospheric
radon levels near the ground depend on geological properties, soil porosity,
and construction materials used.
What is radioactivity?