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Commonly Encountered Radionuclides

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Approximately 2,300 nuclides have been identified; most of them are radioactive.

Which nuclides are radioactive?

Unstable nuclides of any element can exist. However, almost all elements that are heavier than bismuth, which has 83 protons, have an unstable nucleus; they are radioactive and are known as "heavy nuclides. Nuclides with fewer than 83 protons are known as "light nuclides.

While there are hundreds of radionuclides, many of them are rarely encountered. People are much more likely to encounter a few that are used routinely for medical, military, or commercial purposes. The list below and accompanying fact sheets discuss twelve radionuclides. They are the ones most commonly used and most commonly found in Superfund Sites.

Please note: Where an element is listed rather than an individual radionuclide, the element has several radioactive isotopes of interest:

Fact Sheets on Commonly Encountered Radionuclides
Radiation Type
Name Atomic Number Alpha Beta Gamma
americium-241 95 Americium is primarily an alpha particle emitter. It also emits some gamma rays.   Americium primarily emits alpha particles, but also emits some gamma rays.
cesium-137 55   Cesium-137 is a beta particle emitter. Cesium-137 is a gamma emitter.
cobalt-60 27   Cobalt-60 is a beta particle emitter. Cobalt-60 is a gamma emitter.
iodine-129 &-131  53   Iodine-129 and -131 are beta particle emitters Iodine-129 and -131 are gamma ray emitters
plutonium 94 Most plutonium isotopes are alpha particle emitters; some emit other forms of radiation. Most plutonium isotopes are alpha particle emitters; some emit other forms of radiation. Most plutonium isotopes are alpha particle emitters; some emit other forms of radiation.
radium 88 Radium isotopes are alpha particle emitters.   Radium isotopes are also gamma ray emitters.
radon 86 The isotopes of radon are alpha particle emitters.    
strontium-90 38   Strontium-90 is a beta particle emitter.  
technetium-99 43   Technitium-99 is a beta particle emitter. Technitium-99m is a gamma emitter.
tritium *  1   Tritium is a beta particle emitter.  
thorium 90 Thorium emits alpha particles with gamma rays   Thorium emits alpha particles with gamma rays
uranium 92 Uranium is an alpha particle emitter.   Uranium is a gamma ray emitter.
* tritium is a specific isotope, H-3.

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