The phenomenon of ionization is the mechanism by which radioactivity acts on matter. Alpha, beta, gamma and X-rays disrupt the organization of living matter. Atoms placed in their path can lose one or more electrons. These atoms are then transformed into "ions" which, as they are electrically charged, will, in turn, disrupt the organization of the molecules or cells of which they are the component parts. This is why radioactive radiation is said to be "ionizing".
When the Earth was first formed, about 5 billion years ago, matter was made up of both radioactive and stable elements. Since then, the radioactivity has continued to decrease as many radioactive atoms have been transformed into stable elements. Some continue to change while others are still forming.
2/3 of the radioactivity to which humans are exposed is natural
1/3 of the radioactivity to which humans are exposed is artificial
Natural radioactivity comes mainly from four sources :
1° Cosmic radiation
This originates from the sun and space. The strength of cosmic radiation increases with altitude: it doubles every 1,500 m. When traveling by plane at an altitude of 26,000 feet, the dose received is almost 100 times greater than that at sea level. Exposure thus rises from 0.5 mSv (millisievert) per person per year at sea level to around 2 mSv/year at ground level, to 50 mSv/year at altitude, to 500 mSv/year in orbit and to 1,000 mSv/year in space.
2° Ambient air
The air gives off radon fumes; radon is a radioactive gas that results from the decay of uranium in the earth's crust. The average equivalent dose in French homes is 1.43 mSv per person per year. It varies, depending on the nature of the ground and the methods of construction and ventilation.
3° Telluric radiation
This is emitted by many radioactive elements present in the earth's crust, such as uranium and thorium. It varies according to the type of ground and thus changes from one region to another. Exposure to telluric radiation is, on average, 0.62 mSv per person per year in France.
4° Food and drink
Both contain radioactive elements which, after ingestion, attach themselves to organic tissues and bones. These are elements that are essential for life and have radioactive isotopes such as potassium 40 or carbon 14. Internal irradiation is, on average, 0.55 mSv per person per year.
There are many applications of ionizing radiation.
1° Medical irradiation
This is the source of the greatest exposure, thanks to the development of nuclear medicine, scanning and radiography. In the most industrialized countries, a person receives an equivalent dose of 1.8 mSv every year. The global average is 0.6 mSv per person per year.
2° Technical and industrial applications
They, too, constitute a source of radioactivity although their induced dose is negligible. Extractive mining, atmospheric fallout from military tests or, more daily, exposure to radiation emitted by televisions or computer screens, lead to an equivalent dose of 0.1 mSv per person per year. All the radiation generated by the nuclear power industry accounts for less than 0.01 mSv per person per year.
Radioactivity is a quantifiable phenomenon. There are three international units of measurement.
Each relates to data of a different type:
Radioactive activity is measured in becquerel (Bq). It quantifies the number of radioactive nucleus decays that occur every second in a sample. For example, around 9,000 atoms in the body of a 70 kg person disintegrate every second, so their activity is 9,000 Bq. The old unit is the curie, equivalent to 37 billion becquerels.
The amount of radiation absorbed by an organism or an object exposed to radiation is measured in gray (Gy).
It is a measure of energy, representing 1 joule per kilogram of matter. The gray replaced the rad (1/100th of a gray) in 1986.
The biological effects of radiation on an exposed organism are measured in sievert (Sv). It is a radiation protection unit. It is expressed in "equivalent dose" and takes into account the characteristics of the radiation and of the irradiated organ. The sievert replaced the rem (1/100th of a Sv) in 1986. The millisievert (mSv), or one thousandth of a sievert, is used very frequently.
Radioactivity is detected and measured using equipment (Geiger counters, ionization chambers, scintillators) or devices (photographic films) that use the properties of radiation. These measurements achieve very high precision.
The primary safety measure is the distance between the organism and the radioactive source.
A security zone is thus defined around the exposed sites and all manipulations are carried out remotely within that zone.
The duration of exposure to radiation is monitored in exposed areas.
The harmfulness of the radiation depends on the dose received, which increases the longer the exposure to the radiation.
Protective screens made of lead, metal or concrete and of suitable thickness stop the radiation.
A few meters of water also provide effective protection. Wearing protective suits insulates professionals from the risk of contamination by unsealed radioactive sources.