Terme de Glossaire
Radioactivité
Radioactivity is the transformation of atomic nuclei which are accompanied by the emission of corpuscles. They can be natural (uranium) or artificial, when stable nuclei are bombarded with particles (protons, photons, neutrons, etc.) to make them unstable.
The period (or half-life) of a radioactive atom is a statistical concept; it is the length of time at the end of which an atom's nucleus has one chance in two of disintegrating. For a range of atoms (matter), this period is the time required for half the atoms to disintegrate naturally.
Three types of radioactivity can be identified, depending on the nature of the particle emitted by the nucleus: alpha (α), beta (β) and gamma (γ). In alpha radioactivity, a particle "α" composed of two protons and two neutrons is expelled from the nucleus. The initial nucleus thus becomes the nucleus of another chemical element (hence radium 228 becomes radon 224). The "α" rays do not penetrate matter and a sheet of paper is sufficient to stop them. In "β" radioactivity, a neutron from the nucleus transforms into a proton and emits an electron (with another particle known as an antineutrino). One element is thus transformed into another as the number of protons has changed (for example, neptunium becomes plutonium). β rays are also easy to stop. Finally, γ radioactivity is produced when the nucleus does not change in nature (it remains the same element), but it loses some of its mass through emission of a high energy photon. This photon is not emitted in visible light, it is much too energetic. This is why γ rays are dangerous, even more so than X rays. Only lead plate can stop them.
The period (or half-life) of a radioactive atom is a statistical concept; it is the length of time at the end of which an atom's nucleus has one chance in two of disintegrating. For a range of atoms (matter), this period is the time required for half the atoms to disintegrate naturally.
Three types of radioactivity can be identified, depending on the nature of the particle emitted by the nucleus: alpha (α), beta (β) and gamma (γ). In alpha radioactivity, a particle "α" composed of two protons and two neutrons is expelled from the nucleus. The initial nucleus thus becomes the nucleus of another chemical element (hence radium 228 becomes radon 224). The "α" rays do not penetrate matter and a sheet of paper is sufficient to stop them. In "β" radioactivity, a neutron from the nucleus transforms into a proton and emits an electron (with another particle known as an antineutrino). One element is thus transformed into another as the number of protons has changed (for example, neptunium becomes plutonium). β rays are also easy to stop. Finally, γ radioactivity is produced when the nucleus does not change in nature (it remains the same element), but it loses some of its mass through emission of a high energy photon. This photon is not emitted in visible light, it is much too energetic. This is why γ rays are dangerous, even more so than X rays. Only lead plate can stop them.
