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). Click on image for full size (12 Kb) Original artwork by Windows to the Universe staff (Randy Russell).

Physicists use the term "half-life" to describe how long it takes for radioactive materials to decay. When an atom of a radioactive substance decays, it emits radiation and changes into a different type of atom (a different isotope or element). Different radioactive materials decay at different rates; some decay in a matter of seconds or minutes, while others take millions of years. A short "half-life" means the substance decays quickly; a long half-life indicates a slower rate of decay.

Not all atoms of a particular type of radioactive material will decay at the same time. However, for a sample with a large number of atoms the laws of probability allow us to say something about the average decay rate. For any substance, after a certain amount of time, about half of the atoms in the sample will have decayed. That "certain amount of time" is called the half-life of that particular substance. Different radioactive substances have different half-lives. For example, tritium, a radioactive isotope of hydrogen, has a half-life of 12.3 years; while carbon-14 has a half-life of nearly 6,000 years and uranium-235 has a half-life of more than 700 million years.

Let's say you had a sample of 100 kilograms of tritium. After 12.3 years (the half-life of tritium), half of the tritium would be "gone", having emitted beta particles and decayed to form helium-3. The other half of the tritium, 50 kg, would remain. After another 12.3 years (24.6 years from the start), half of the remaining 50 kg of tritium would decay, so only 25 kg of the original 100 kg would still be tritium. Another 12.3 years later (36.9 years from the start), half of the 25 kg of tritium remaining would decay, leaving us with one-eighth (12.5 kg) of the original 100 kg sample.

Knowing the half-life of a radioactive material can help us determine the age of an object. The amount of radioactive carbon-14 left in a once-living object (wood, bones or teeth, etc.) can be used to determine how long ago the creature died. Isotopes with short half-lives are often used in medicine; the fast decay rates of such isotopes prevent radiation from lingering in the body. The challenge of disposal of radioactive isotopes with long half-lives is a major problem associated with the use of nuclear power for electricity.

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). Click on image for full size (12 Kb) Original artwork by Windows to the Universe staff (Randy Russell).

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.

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). Click on image for full size (12 Kb) Original artwork by Windows to the Universe staff (Randy Russell).

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!