Radioactivity
Radioactivity is the transformation of an unstable atomic nucleus into one or several nuclei of other elements, and the emission of heat during this transformation as well as an alpha particle (a helium nucleus), an electron (beta particle) or an electromagnetic radiation (X ray or gamma ray).
- Alpha radiation has a very low power of penetration into the air. A simple sheet of paper is able to stop it; however, these particles can be inhaled or ingested.
- Beta radiation travels a few metres in the air. An aluminium sheet a few millimetres thick can stop it.
- Gamma radiation can travel several hundred metres in the air. A very thick slab of concrete or lead is necessary to stop it
Radioactive atoms disintegrate according to a specific law. Considering an (n) amount of atoms at time T=0, the half-life is the period of time after which only half of the (n) atoms remain radioactive. The others have disintegrated to create new elements. Not all radioactive elements have the same half-life. Certain half-lives are extremely short (fractions of a second) and others extremely long (4.47 billion years for uranium 238). Carbon 14 is radioactive. Its half-life is 5,730 years.
Effects on man
Ionising radiation, regardless of its origin, natural or artificial, has enough energy to rip electrons from the atoms of the matter it comes in contact with. When it acts upon the components of living cells, it can alter the molecular structures, destroy or modify the cells and, consequently, generate two categories of biological effects.
Certain effects, referred to as deterministic effects (for example burns, nausea etc.) systematically appear, generally early on when the dose of radiation received exceeds a certain threshold, specific to each effect. The seriousness of the damage increases with the dose.
Random effects, referred to as stochastic effects (mainly cancers) appear arbitrarily, always a few years after the individuals have been exposed. The probability of these effects appearing increases according to the dose received but their seriousness is independent of the dose.
Sources of exposure for man
Nowadays, 60 to 70% of the radioactivity to which man is exposed is of natural origin. It essentially has four sources:
- Ambient air is the primary source of natural irradiation, as it contains radon, a radioactive gas resulting from the disintegration of the uranium present in the earth’s crust. Exposure to radon largely depends on the nature of the subsoil; it therefore varies dramatically over the territory.
- Cosmic radiation, from which we are protected by the atmospheric layer; therefore man’s exposure increases with altitude.
- Terrestrial radiation emitted by many naturally radioactive elements present in the earth’s crust such as uranium and thorium. It varies according to the nature of the soil and changes from one region to the next.
- Food and drink absorbed also contain radioactive elements which attach themselves to the tissue and bones once ingested.
Man is also subject to radiation from artificial sources. The effects on living matter are, for identical doses, similar to the effects of natural radiation. On average, this artificial exposure mostly results from medical exposure (radiography, scan etc.); however, most of the exposure concerns a limited number of individuals. 3% comes from other sources such as the fallout from nuclear weapon tests, the fallout from the Chernobyl accident as well as industrial discharge.
In France, the average impact of industrial discharge in normal conditions (nuclear reactors, combustible energy, waste treatment) is lower than a few thousandths of the natural radioactivity. The overall balance of the radioactive environment in France, all origins combined, represents an average exposure of 4 mSv per year, of which 2.4mSv is due to natural exposure sources.
Order of magnitude of the natural radioactivity found in certain milieus or products
- Granite terrain: 3,000 Bq/kg
- Potatoes: 150 Bq/kg
- Milk: 80 Bq/l
- An average individual emits 8,200 Bq (8,200 disintegrations per second)
