Tritum has a half-life of 12.3 years. Pretend you started with 100 kilograms of tritium. After 12.3 years (one half-life), only half (50 kg) of the tritium would be left (yellow lines). After 24.6 years (two half-lives), only one-quarter (25 kg) of the original 100 kg of tritium would be left (blue lines).

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Some materials are radioactive. Their atoms give off radiation. When an atom gives off radiation, it turns into a different kind of atom. That is called radioactive decay. Some atoms decay very quickly, in seconds or minutes. Others take a long time to decay... sometimes millions of years! Scientists use the term "half-life" to describe how fast or slow the radioactive decay is.

Let's say you had 100 kilograms of tritium. Tritium is a radioactive form of hydrogen. The half-life of tritium is about 12 years. After 12 years, half of the tritium would be "gone". It would have given off radiation and decayed... and turned into helium. Only 50 kg of tritium would be left. After another 12 years (a second half-life), half of what was left would decay. There would only be 25 kg of tritium left after 24 years from the start. That's one-quarter (half of half) of the 100 kg we started with.

Different radioactive materials have different half-lives. Carbon-14 has a half-life of nearly 6, 000 years. The half-life of uranium-235 is more than 700 million years! On the other hand, the half-life of nitrogen-13 is less than 10 minutes!

Scientists use radioactive materials with different half-lives in various ways. Carbon-14 dating is used to find out how old things that were once alive are. The more radioactive carbon-14 that is "missing" from a sample, the longer ago it must have died. Doctors use radioactive materials to treat some diseases. They use materials with short half-lives so the radiation doesn't hang around in the body too long. Old fuel from **nuclear power plants** can be a problem if it has a long half-life. It is hard to find a safe place to store radioactive materials with long half-lives.

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Radiation comes in two basic types: electromagnetic radiation transmitted by photons, and particle radiation consisting of electrons, protons, alpha particles, and so forth. Electromagnetic radiation...more