Caesium-137

Caesium-137 is a radioactive isotope of the element caesium. An isotope is a specific version of an element with a unique atomic structure. This particular isotope is a common fission product. It is created during nuclear fission, which is the splitting of atoms like uranium-235. This process happens inside nuclear reactors and nuclear weapons. Trace amounts also come from the spontaneous fission of uranium-238. Because it is a byproduct of nuclear energy, it is a major concern for environmental safety.

The behavior of caesium-137 is defined by its radioactive decay. It has a half-life of approximately 30.04 years. A half-life is the time required for half of the radioactive atoms to decay. During this process, the isotope undergoes beta emission. This means it releases a particle to become a stable form of barium, known as barium-137. About 94.6% of these decays result in a metastable nuclear isomer called barium-137m. This isomer is unstable and lasts for about 153 seconds. When it finally drops to its ground state, it emits photons. These photons are responsible for the gamma ray emissions seen in caesium-137 samples.

Scientists Glenn T. Seaborg and Margaret Melhase discovered this isotope. Since the first atomic bomb test in 1945, caesium-137 has been present in the Earth's atmosphere. Because it is an almost purely synthetic isotope, it did not exist in nature before 1945. This unique history allows researchers to use it for dating. For example, it can be used to check the authenticity of rare wines. It is also used as a relative-dating material. Scientists measure it to assess the age of soil sedimentation occurring after 1945.

Caesium-137 has several practical applications in science and industry. In medicine, it is used for radiation therapy to treat patients.

In industrial settings, it powers flow meters and thickness gauges. It is also used in moisture-density gauges and borehole logging devices. However, it is not widely used for industrial radiography. This is because it is difficult to obtain a high specific activity material in a small, well-defined shape. Additionally, the most common high-activity source is caesium chloride. This salt is highly soluble in water, which creates a high risk of contamination if a source is damaged.

Environmental contamination can occur through nuclear accidents or testing. Atmospheric nuclear tests released caesium-137 into the air. This material can travel very long distances because it becomes volatile at high temperatures. Once it lands on soil, it moves easily because its common chemical salts are highly water-soluble. The 1986 Chernobyl disaster caused widespread contamination. In Germany, the contamination covered an area roughly the size of the state of North Rhine-Westphalia. In Scandinavia, some reindeer and sheep exceeded legal radiation limits 26 years after the accident.

The Fukushima Daiichi disaster in 2011 also released significant amounts of the isotope. By 2014, the radiocaesium had spread throughout the western North Pacific Ocean. It was detected in the ocean surface layer as far south as 35°N and as far west as 155°E. In the food chain, concentrations were found to be high in fungi and leaf litter. In July 2011, meat from cows in Fukushima Prefecture exceeded the Japanese legal limit.

Biological studies show that caesium behaves similarly to potassium and rubidium in the body. It distributes uniformly through soft tissue. While it does not bioaccumulate like radium, it has a biological half-life of about 70 days. If ingested, it can be treated with Prussian blue, which binds to the caesium and reduces its biological half-life to 30 days.

Accidental exposure has occurred in several historical incidents. In 1987, the Goiânia accident in Brazil involved the sale of glowing caesium salt from an abandoned clinic. This led to four deaths and several injuries. In 1989, a capsule was found inside a concrete wall in Kramatorsk, Ukraine. By the time it was discovered, six residents had died. In 1994, a theft in Estonia resulted in a fatal dose for one person. These incidents highlight the danger of improperly managed radioactive sources. Managing these materials requires strict safety protocols to prevent human and environmental harm.