Radioactive Decay | US EPA (2024)

Radioactive decay is the emission ofenergy in the form of ionizing radiationRadiation with so much energy it can knock electrons out of atoms. Ionizing radiation can affect the atoms in living things, so it poses a health risk by damaging tissue and DNA in genes.. The ionizing radiation that is emitted can include alpha particlesA form of particulate ionizing radiation made up of two neutrons and two protons. Alpha particles pose no direct or external radiation threat; however, they can pose a serious health threat if ingested or inhaled., beta particlesA form of particulate ionizing radiation made up of small, fast-moving particles. Some beta particles are capable of penetrating the skin and causing damage such as skin burns. Beta-emitters are most hazardous when they are inhaled or swallowed. and/or gamma raysA form of ionizing radiation that is made up of weightless packets of energy called photons. Gamma rays can pass completely through the human body; as they pass through, they can cause damage to tissue and DNA.. Radioactive decayoccurs in unbalanced atomscalled radionuclides.

Elements in the periodic table can take on several forms. Some of these forms are stable; other forms are unstable. Typically, the most stable form of an element is the most common in nature. However, all elements have an unstable form. Unstable forms emit ionizing radiation and are radioactive. There are some elements with no stable form that are always radioactive, such as uranium. Elements that emit ionizing radiation are called radionuclides.

When it decays, a radionuclide transforms into a different atom - a decay product. The atoms keep transforming to new decay products until they reach astable state and are no longer radioactive. The majority of radionuclides only decay once before becoming stable. Thosethat decay in more than one step are called series radionuclides. The series of decay products created to reach this balance is called the decay chainThe series of decays or transformations that radionuclides go through before reaching a stable form. For example, the decay chain that begins with Uranium-238 culminates in Lead-206, after forming intermediates such as Uranium-234, Thorium-230, Radium-226, and Radon-222. Also called the "decay series.".

Each series has its own uniquedecay chain. The decay products within the chain are always radioactive. Only thefinal, stable atom in the chain is not radioactive. Some decay products are a different chemical element.

Every radionuclide has a specific decay rate,which is measured in terms of "half-lifeThe time required for half of the radioactive atoms present to decay or transform. Some radionuclides have half-lives of mere seconds, but others have half-lives of hundreds or millions of years.." Radioactive half-life is the time required for half of the radioactive atoms present to decay. Some radionuclides have half-lives of mere seconds, but others have half-lives of hundreds or millions or billions of years.