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
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.

Last modified August 26, 2009 by Randy Russell.

You might also be interested in:

Traveling Nitrogen Classroom Activity Kit

Check out our online store - minerals, fossils, books, activities, jewelry, and household items!...more

Radioactive Decay

Some materials are radioactive. They emit radiation. When an atom of a radioactive substance emits radiation, it is transformed to a new type of atom. This process is called radioactive decay. There are...more


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


Isotopes are different "versions" of a chemical element. All atoms of an element have the same number of protons. For example, all hydrogen atoms have one proton, all carbon atoms have six protons, and...more

Element (Chemical Element)

An element (also called a "chemical element") is a substance made up entirely of atoms having the same atomic number; that is, all of the atoms have the same number of protons. Hydrogen, helium, oxygen,...more


Carbon-14 is an isotope of the element carbon. All carbon atoms have 6 protons in their nucleus. Most carbon atoms also have 6 neutrons, giving them an atomic mass of 12 ( = 6 protons + 6 neutrons). Carbon-14...more

Carbon-14 Dating (Radiocarbon Dating)

Carbon-14 dating (also called "radiocarbon dating") is used to determine the age of materials that contain carbon that was originally in living things. It is often used in archeology and some...more

Flying Atoms?

Scientists have found a possible source of the high speed atoms flying through space in the form of cosmic rays. These atoms reach velocities close to the speed of light. It was known that the source...more

Windows to the Universe, a project of the National Earth Science Teachers Association, is sponsored in part is sponsored in part through grants from federal agencies (NASA and NOAA), and partnerships with affiliated organizations, including the American Geophysical Union, the Howard Hughes Medical Institute, the Earth System Information Partnership, the American Meteorological Society, the National Center for Science Education, and TERC. The American Geophysical Union and the American Geosciences Institute are Windows to the Universe Founding Partners. NESTA welcomes new Institutional Affiliates in support of our ongoing programs, as well as collaborations on new projects. Contact NESTA for more information. NASA ESIP NCSE HHMI AGU AGI AMS NOAA