Radionuclides, Radioactive Decay, and Half-Lives

What are Radionuclides?

A radionuclide (sometimes called a radioisotope) is an atom with an unstable nucleus. An unstable nucleus means that the atom’s ratio of neutrons to protons is unbalanced, which gives it too much energy. To become more stable, the atom changes the number of neutrons or protons, or both. To make this change, the atom emits (gives off) energy in the form of radiation. This process of radiation emission that allows the atom to become stable is calledradioactive decay.

Illustration: Alpha particle leaving the nucleus due to radioactive decay Credit: EPA.gov

Alpha particle leaving the nucleus due to radioactive decay. Credit: EPA.gov (click to enlarge)

How do radioactive decay and radiological half-life work together?

The half-life determines how long it takes for half of the atoms in a given mass to decay into another form. Depending on the radionuclide, the half-life can range from fractions of a second to billions of years.

Radioactive decay will continue until the nucleus is balanced. The rate at which radioactive materials decay varies greatly among radionuclides. A highly radioactive substance (one that has a short half-life) decays more quickly than a less radioactive substance (one that has a longer half-life).

Examples of half-lives of certain radioactive elements:

Carbon-14, which is used for dating fossils, has a half-life of 5,730 years.
Plutonium-239has a half-life of 24,000 years.
Uranium-238, the most prevalent isotope in uranium ore, has a half life of about 4.5 billion years.
Americium-241, which is used in smoke detectors, has a half-life of 432.7 years.
Radon-222 has a half-life of 3.82 days.

Alpha, beta, and gamma rays are released during radioactive decay. Sometimes neutrons orpositrons (a positive electron) are released. Protons from the nucleus of an atom are not released during typical radioactive decay. The U.S. Environmental Protection Agency (EPA) and other regulatory agencies generally limit radiation exposures from specific sources to the public to levels well under 100 millirem (mrem).

Hundreds of different radionuclides exist. Some of the more familiar ones are radon, uranium, and radium. The EPA website Commonly Encountered Radionuclides

provides fact sheets on the 12 most commonly used radionuclides.

Related Information on this web site

Radiation Dictionary

Atom: the smallest particle of an element that can enter into a chemical reaction.

Decay, Radioactive: disintegration of the nucleus of an unstable atom by the release of radiation.

Half-Life: the time any substance takes to decay by half of its original amount. See also biological half-life, decay constant, effective half-life, radioactive half-life.

mrem: One thousandth of a rem (0.001 rem). Definition provided by the United States Nuclear Regulatory Commission (NRC).

Neutron: a small atomic particle possessing no electrical charge typically found within an atom's nucleus. Neutrons are, as the name implies, neutral in their charge. That is, they have neither a positive nor a negative charge. A neutron has about the same mass as a proton. See also alpha particle, beta particle, gamma ray, nucleon, x-ray.

Nucleus: the central part of an atom that contains protons and neutrons. The nucleus is the heaviest part of the atom.

Positrons: particle equal in mass but opposite in charge to the electron. A positive electron. Definition provided by NRC.

Protons: a small atomic particle, typically found within an atom's nucleus, that possesses a positive electrical charge. Even though protons and neutrons are about 2,000 times heavier than electrons, they are tiny. The number of protons is unique for each chemical element. See also nucleon.

Radionuclides: an unstable and therefore radioactive form of a nuclide.