Radioactivity is present all over the world. It forms an ambient radiological background noise consisting mainly of radionuclides naturally present in the ground or in the materials surrounding us and of radiation of cosmic origin, but also artificially produced in different fields (industrial, medical, military...). In addition to this permanently present background, there are radiological accidents (Chernobyl, Fukushima...). We are therefore permanently exposed to ionizing radiation from natural and artificial.
Telluric radiations
The earth's crust contains radioactive atoms that are the source of natural radioactivity (thorium-232, uranium-235, uranium-238 and their descendants). With very long life spans of up to several billion years, they are still far from having disappeared and are therefore an integral part of our environment.
Depending on the nature of the rocks composing the subsoil, we are therefore more or less exposed to telluric radiation. In some granite regions, for example in France, Limousin or Brittany, the terrestrial radiation can be 4 times higher than in the Paris region. Some black sandy beaches in Kerala, India, rich in monazite (containing thorium), can reach radiation levels 10 to 100 times higher. In these regions, radiation exposure may also be higher inside a building due to the presence of radon, a radioactive gas resulting from the decay of radioelements present in soils or building materials.
Cosmic radiation
Cosmic radiation is very different in nature from terrestrial radiation. It consists mainly of hydrogen nuclei (protons) and light nuclei (He for example) of very high energy. This radiation has two components, one is of galactic or extragalactic origin and comes from the disintegration of stars and the other from the sun, which has a continuous activity, with a cycle of about 11 years, to which are added sporadic eruptions. The particles accelerated during certain eruptions are so energetic that they produce effects at ground level.
Life on Earth is protected by barriers that filter some of these particles: the interplanetary magnetic field emitted by the sun, whose intensity varies during its cycle, and the Earth's magnetic field and atmosphere. The interaction of cosmic radiation with the constituents of the atmosphere produces a continuous "rain" of secondary particles (neutrons, muons, electrons...) which are largely absorbed before reaching ground level.
As a result, the intensity of cosmic radiation varies very strongly with altitude. At ground level, the dose received is about 100 times lower than at flight altitudes of long-haul aircraft.
It should be noted that radionuclides, known as cosmogenic, are also produced permanently by the interaction, in the upper atmosphere, of cosmic radiation with the atoms constituting our atmosphere. The best known is carbon-14.
Although cosmic radiation is strongly attenuated by our atmosphere, it can nevertheless be detected at ground level where the contribution to population exposure is low. At flight altitudes of airliners, dose rates can be up to 100 times higher than those observed at ground level and sometimes more over short periods of time in the event of a major solar flare. Several parameters influence the intensity of cosmic radiation:
- The atmosphere which acts as an attenuator, explaining that exposure is greater at high altitude (up to about 25 km) because the thickness of the air through which the particles pass is less,
- The magnetosphere, which gives more protection to the equator than to the poles. The levels of in-flight exposure are therefore dependent on latitude. Transpolar routes are more exposed than transequatorial routes,
- Cyclic variations of the solar magnetic field (solar cycle of 11 years on average).
Influence of altitude
As they enter the upper layers of the Earth's atmosphere, ions (mainly protons) interact with atoms in the atmosphere. These collisions produce sheaves of particles, some of which, very unstable (pawns and kaons), disintegrate to generate other particles (muons, photons, electrons, neutrons...) detectable at ground level. This set of secondary particles is called secondary radiation. The proportion of each type of particle varies with altitude. At ground level, exposure to cosmic radiation is mainly due to muons and to a lesser extent to neutrons, while at flight altitudes, exposure due to neutrons dominates.
Influence of latitude
The Earth can be considered as a large magnet that produces a magnetic field whose lines of force "enter" through the North Magnetic Pole to "exit" at the South Magnetic Pole: this is called the magnetosphere.
If cosmic particles have an energy above a certain threshold, called magnetic cut-off energy, they can pass through the magnetosphere and then penetrate the upper layers of the atmosphere. Otherwise they are reflected in space. At high latitude, in polar cones, particles, including those of lower energy, can follow the lines of force of the magnetic field and reach the atmosphere more easily. As a result, areas near the poles are exposed to a higher particle flux than in the Equator, which is better protected by the Earth's magnetic field.
Influence of sun activity
The galactic cosmic radiation arriving in our planetary system is deflected by the magnetic field created by the sun, also called the interplanetary magnetic field. Indeed, the Sun constantly releases a flow of particles that fills the entire interplanetary environment, called the solar wind. It induces a magnetic field whose intensity varies with solar activity. Thus, the galactic cosmic radiation reaching the Earth is less when the solar activity is strong. As we know that solar activity follows an average cycle of eleven years, it is possible to predict exposure to galactic radiation over several years.
Rare events
The main contribution of the sun to the exposure of people on board aircraft results from eruptions that are difficult to predict. A solar flare generates particles, mainly protons, in large quantities and lasts a few hours. However, only eruptions generating a flow of sufficiently energetic particles are detectable on the ground or on board an aircraft; such eruptions remain exceptional: on average, there is one per year.
These eruptions are frequent when the Sun's magnetic field is particularly disturbed, resulting in the appearance of many spots on its surface. The most complex groups of sunspots result in large solar flares.
Artificial radiation
Exposure to ionizing radiation in our environment is partly of artificial origin: medical field, consequences of nuclear accidents...