The fundamental principle of radioactive decay is that each radioactive element decays at its own characteristic rate. This rate of decay is known as the half-life.

The half-life of an element is the amount of time in which half of the original quantity has decayed. The element tritium has a half-life of about 12 years. If we originally have a 10 gram sample of tritium, 12 years later we will have a 5 gram sample.