Half-life
The 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.
In 1900 Ernest Rutherford found that the activity of a radioactive gas given off by thorium halved about every minute[1]. The time to halve is the same whatever the starting amount. When any one nucleus decays is pure chance, but a large number together decline in this regular way.
Half-lives span an extreme range, from tiny fractions of a second to billions of years. In 1911 Hans Geiger and John Nuttall found that in alpha decay, the more energetic the alpha particles a nucleus emits, the shorter its half-life[2]. The extremeness of that relation was later explained by tunnelling. Because half-lives are fixed, they can be used to date rocks and ancient objects.
Related terms
- Alpha decayThe process in which a heavy atomic nucleus throws out one alpha particle, a helium nucleus, and becomes the nucleus of a different element.
- 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.
- Atomic nucleusThe tiny, heavy core at the centre of an atom, made of protons and neutrons and carrying a positive charge.