Tritium is a radioactive isotope of hydrogen. Tritum has a half-life of 12.3 years. The dotted green line shows how much of a 100 kg sample of tritium would remain over time. After 12.3 years, half would have decayed (yellow lines). After 24.6 years (two half-lives), only one-quarter (25 kg) of the original 100 kg of tritium would remain (blue lines).
Original artwork by Windows to the Universe staff (Randy Russell).