Radionuclides and Health

Parts of the following are taken from:
Environmental Protection Agency: Facts about Cesium-137 (pdf format)
Department of Physics and Astronomy, Georgia State University: Hyperphysics
Energy Released during Fission Reaction, Chain Reaction

Certain isotopes of elements are unstable, and release excess energy in the form of ionizing radiation, also known as radioactivity. Radioactivity is defined as the release of energy resulting from a spontaneous change in the structure of the nucleus. Radioactive decay is often accompanied by the ejection of a particle from the nucleus. Learn more about elements, isotopes, and radioactivity.

Radioactive isotopes of elements are called radionuclides. Some radionuclides, such as uranium and radon gas, occur naturally. Others, like strontium, cesium, plutonium and tritium, which is a form of hydrogen, are human-made products of nuclear fission (the splitting apart of atoms).

This cartoon shows how a nucleus breaks apart during nuclear fission. In the example shown a neutron hits a uranium-235 nucleus and splits it producing 2 fissure products (Ba-139 and Kr-94), 3 neutrons, and radiation energy (not shown). Fission figure from:

If the reaction continues, the 3 neutrons that were emitted hit 3 uranium nuclei now producing a total of 9 neutrons. The 9 nine neutrons emitted hit 9 uranium nuclei producing 27 neutrons, and so on and so on. The multiple reactions happen over a very short time interval and the process is called a chain reaction. If enough uranium is present (critical mass) a nuclear explosion will occur. Fission figure from:

Radioactive cesium-137 is produced 6% of the time when uranium or plutonium undergoes fission (source: In other words 6 atoms of cesium-137 are produce for every 100 fissions. Therefore, cesium-137 is a common radionuclide produced by nuclear fission in a nuclear reactor or atomic bomb.

Cesium-137 decays in the environment by emitting beta particles. It decays to a short lived decay product, barium-137m (the 'm' means metastable). The barium-137 isotope emits gamma radiation of moderate energy, which further decays to a stable form of barium.

Cesium-137 is significant because of its prevalence, relatively long half-life (30 years), and its potential effects on human health. In addition, cesium-137 has the property of being mistaken for potassium by living organisms and taken up as part of the fluid electrolytes (substances that form ions in water, essential for good health). This means that it is passed on up the food chain and re-concentrated from the environment by that process.

Cesium-137 presents both an external and internal health hazard. The strong external gamma radiation associated with its short-lived decay product barium-137m makes external exposure a concern, and shielding is often needed to handle materials containing large concentrations of cesium. Cesium-137 can also enter the body by being inhaled or ingested. After radioactive cesium is ingested, it is distributed fairly uniformly throughout the body's soft tissues. Slightly higher concentrations are found in muscle; slightly lower concentrations are found in bone and fat. While in the body, cesium poses a health hazard from both beta and gamma radiation, and the main health concern is associated with the increased likelihood for inducing cancer.

Ionizing radiation has enough energy to change normal cellular functioning and even cause cells to die or transform into a cancerous cell. Ionizing radiation is generally categorized by strength or energy level into three main categories:

  1. alpha particles. Most densely ionizing, but weakest form of ionizing radiation. These particles travel a few inches through air, but can be stopped by a sheet of paper. This means that cells can be protected or shielded from damage by alpha particles by clothing. Even your skin will protect you from damage from alpha particles. However, if alpha particles are inhaled or ingested or get into a cut on the skin, they can cause damage to cells. As alpha particles decay inside the body, the surrounding cells absorb the radiation.
  2. beta particles. More energetic. These particles can travel several feet through air, but are stopped with denser materials such as wood, glass or aluminum foil.
  3. gamma rays. High-energy electromagnetic energy waves and the most penetrating type of radiation. They travel at the speed of light through the air. Cells must be shielded from gamma rays with concrete, lead or steel.

Figure from web site

Large amounts of cesium-137 were produced during atmospheric nuclear weapons tests conducted in the 1950s and 1960s. As a result of atmospheric testing and radioactive fallout, the cesium was dispersed and deposited world wide. Sources of exposure from cesium-137 include fallout from previous nuclear weapons testing, soils and waste materials at radioactively contaminated sites, radioactive waste associated with the operation of nuclear reactors, spent fuel reprocessing plants, and nuclear accidents. Cesium-137 is also a component of low level radioactive waste at hospitals and research facilities.