Alpha decay
The process in which a heavy atomic nucleus throws out one alpha particle, a helium nucleus, and becomes the nucleus of a different element.
Heavy nuclei such as uranium and radium emit an alpha particle with a certain probability. The nucleus left behind has two fewer protons and two fewer neutrons, making it the element two places back in the periodic table. When any single nucleus will decay is not fixed; only the probability is known.
In 1927 Ernest Rutherford set out an awkward fact[1]. The uranium nucleus is surrounded by a barrier of repulsion high enough to turn back an incoming alpha particle of 8.57 MeV, yet the alpha particles uranium itself emits carry only about 4.2 MeV. The following year Gamow and others explained it by tunnelling[2]: the particles had not gone over the barrier but through it. The same calculation showed why a slight difference in energy changes the half-life by many orders of magnitude.
Related terms
- Alpha particleA heavy, positively charged particle made of two protons and two neutrons: the same thing as a helium nucleus. It is thrown out by radioactive substances.
- Quantum tunnellingThe appearance of a particle, with some probability, on the far side of a barrier it lacks the energy to climb over. It happens because particles behave as waves.
- Half-lifeThe time it takes for the number of radioactive nuclei to fall by half through decay. It is fixed for each kind of nucleus and does not change with temperature or pressure.