Radioactivity Alpha (α)
How heavy nuclei lose weight ...
Exemple de la désintégration historique du radium-226
Ce gros noyau de 226 nucléons, dont 88 protons et 138 neutrons, émet une particule alpha composée de deux protons et deux neutrons. Il se transforme alors en noyau de radon-222, lui-même radioactif, contenant deux protons et deux neutrons de moins. La désintégration libère 4,6 millions d’électronvolts d’énergie. La particule alpha en emporte les 222/226 ièmes et le radon 4/226 ièmes. Il faut attendre la désintégration 2300 ans en moyenne.
Alpha (α) radiation was first observed as an unknown type of ray that curved in the presence of electric and magnetic fields. The direction of curvature revealed that it had to be carried by particles carrying positive electrical charge, and in 1908 Ernest Rutherford was able to identify these ‘alpha particles’ as helium nuclei, with a resulting electric charge of +2e. Later it was found, after the neutron’s, discovery that the helium nucleus consists of 2 protons and 2 neutrons.
The emission of alpha particles is a property of the heaviest nuclei, such as uranium 238 with its 92 protons and 136 neutrons the heaviest natural nucleus observed.
These unstable nuclei emit a light helium nucleus in order to reduce their mass and hence increase their stability. It turns out that expelling two protons and two neutrons in this manner is more energy efficient than expelling the four particles individually.
Analogy with the recoil experienced by a firearm:
The kinematics of an alpha decay are quite similar to those of a firearm where a light bullet takes away most of the energy of the explosion. The momenta (products of mass times velocity) of the recoiling nucleus and the alpha particle are exactly equal. The respective speeds and energies are inversely proportional to the masses, and as a result are highly unequal. In the case of a radium nucleus, the alpha particle takes away 98.3% of the available energy. These alpha particles are always emitted with the same energy values.
The energy released in alpha decay takes the form of kinetic energy shared between the released alpha particle and the nucleus that expelled it. Much like in artillery fire, where the shell absorbs much of the energy of the explosion, the alpha particle takes away about 98% of the energy and the original nucleus (like the recoil of a cannon firing) gets the rest. The energy of the alpha particle is larger than for beta and gamma decay processes, and is usually of the order of four million electronvolts (MeV).
An example of alpha decay is the historically important transformation of radium 226 into radon 222 through the emission of an alpha particle. This reaction releases 4.6 MeV, and leaves behind a radioactive noble gas (radon), which is what allowed Rutherford to observe the process in Montreal in 1898.
The ‘half-lives’ of alpha disintegrations are often very long, and alpha emitters such as thorium 232 and uranium 238 can take billions of years to completely decay. Radium 226 decays with a half-life of 1600 years, so half of the radium nuclei present at the sacking of Rome have yet to decay – a nucleus which is less radioactive than commonly assumed.
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