Since its inception, people are exposed to a certain dose of radiation issued from the environment in which they live, from the food they eat, and the air they breathe. These doses are known as natural environmental radiation doses. These natural doses do not pose a noticeable danger, as their quantities are usually within limits that are not high, and man has lived in them since the beginning of creation. The cosmic rays and the local radiation emitted by the soil, as well as the radioactive materials present within the formation of the bodies of living organisms, are among the most important sources of natural radiation doses.
1- Cosmic rays:
Large amounts of ionizing cosmic rays reach the Earth's atmosphere, coming from outer space and from the sun. These rays contain different types of nuclear particles with high energy, such as protons, neutrons, etc., and are known as primary cosmic rays. The radiation that humans are exposed to from cosmic rays is based on several factors, which are the latitude of the Earth and the height above the sea level. To increase its density the closer we are to the poles compared to the equator at the same altitude from the sea surface. As for the neutrons, they are not affected by the magnetic field, and therefore the doses resulting from the neutrons are equal at different latitudes.
And height above sea level plays a huge role in changing the dose produced by the cosmic rays.
As for the neutrons, the resulting dose at sea level is in the range of 30 μSv per year, which is a limited dose. However, the role of neutrons is growing rapidly, starting from a height of 1 km above sea level. As for protons, they play a remarkable role, starting from the sea level, where the effective dose resulting from them, at sea level, is about 240 microsiverters per year. This dose increases slowly with altitude, then increases very rapidly with altitude starting from 1 km.
Cosmic rays have a direct effect on living organisms, as they lead to the production of some radioactive materials in the atmosphere as a result of their interaction with the components of this atmosphere. For example, radioactive carbon 14 is formed in the atmosphere as a result of the interaction of cosmic ray neutrons with nitrogen 14, which is the main component of the atmosphere.
Carbon-14 is dispersed in the atmosphere until it reaches the surface of the earth, as it enters the synthesis of all living things on the earth at a constant rate. Also, some other isotopes are formed, such as calcium 41 (half-life 105 x 1.1 years), chlorine 36 (half-life 105 x 3.08) and others.
Radiation from soil:
The earth's crust contains trace amounts of long-lived radioactive elements such as uranium 338, uranium 235, and thorium 232, and this crust also contains small amounts of the radioactive isotope potassium 40, which has a half-life of 109 x 1.28 years and rubidium 87, which has a half-life of about 105 × 4.7 years and these elements decay to emit alpha or beta particles, and this may be followed by the emission of gamma rays. Alpha particles do not pose any radiation hazards to humans living above Earth due to their short range. Also, beta particles do not represent a significant risk. As for gamma rays with a high penetrating power, they represent the main contribution to the radiation dose released by the soil.
The annual dose of gamma radiation depends on the type of soil and the concentration of radioactive elements in it. The concentrations of radioactive elements vary greatly according to the type of soil, as the concentration of uranium increases, for example, in granitic rocks.
Radioactive materials present inside the body of a living being:
The body of a living organism contains trace amounts of radioisotopes such as carbon 14 and potassium 40 (half-life 109 x 1.28 years). Potassium 40 is present in nature with stable potassium 39 in a ratio of 11.7: 100,000. Whereas a standard adult human body mass of 70 kg contains about 140 grams of potassium 39, this latter mass contains 0.16 grams of radioactive potassium 40.
This amount of radioactive potassium emits about 4200 decomposition per second, of which 89% is in the form of beta particles with a maximum energy of (1.461 mev) and absorbs the energy of beta particles completely inside the human body and absorbs 50% of the energy of gamma rays inside it. An annual equivalent of 0.2 mSv