caesium-137

caesium-137 is a radioactive isotope of caesium formed as a fission product by the nuclear fission of uranium-235 and other fissile nuclides in nuclear reactors and nuclear weapons. It is one of the most problematic fission products due to its intermediate half-life of about 30 years, high solubility, and chemical similarity to potassium, which allows it to readily enter the biosphere. The decay of this isotope yields the metastable nuclear isomer barium-137m, which subsequently emits a characteristic and energetic gamma ray with a photon energy of 662 keV, making it both a significant biological hazard and a useful radiation source.

Properties

Caesium-137 undergoes beta decay with a half-life of 30.05 years to the metastable state of barium-137m, which has a short half-life of 2.55 minutes and decays to stable barium-137 by emitting a prominent 662 keV gamma ray. This decay pathway is a classic example of a radioactive decay chain. The isotope's physical and chemical properties are identical to stable caesium, exhibiting high solubility in water and behaving similarly to potassium in biological systems, which contributes to its environmental mobility and uptake by organisms. Its specific activity is approximately 3.2 terabecquerels per gram.

Production and sources

The primary anthropogenic source of caesium-137 is neutron-induced nuclear fission of fuels like uranium-235 and plutonium-239 in nuclear reactors and during detonations of nuclear weapons. It is a common yield product in both thermal neutron and fast neutron fission spectra. Major historical releases have originated from atmospheric nuclear weapons testing, particularly during the 1950s and 1960s, and from severe accidents at civilian nuclear facilities such as the Chernobyl disaster and the Fukushima Daiichi nuclear disaster. It is also produced in smaller quantities for medical and industrial use in facilities like the High Flux Isotope Reactor at Oak Ridge National Laboratory.

Applications

Due to its consistent gamma emission, caesium-137 is used as a calibration source for gamma spectroscopy equipment in laboratories worldwide, including those operated by the International Atomic Energy Agency. In industrial settings, it is employed in radiography to inspect welds and in gauges for measuring liquid flow, density, and thickness in sectors like mining and construction. Therapeutically, it has been used in brachytherapy sources for treating cancers, though its use has declined in favor of isotopes like cobalt-60 and iridium-192. It also serves as a radiation source in certain types of food irradiation facilities.

Health and environmental effects

Internal contamination with caesium-137 poses a serious health risk as its chemical similarity to potassium leads to uniform distribution throughout the body, particularly in soft tissues and muscles, resulting in whole-body exposure to its beta and gamma radiation. This can cause acute radiation syndrome at high doses and increase the long-term risk of cancer, particularly in organs like the thyroid gland and bone marrow. In the environment, it binds to soils and sediments but remains bioavailable, entering the food chain through plants and fungi; notable uptake occurs in organisms like wild boar in contaminated regions of Belarus and Norway.

Incidents and contamination

The largest single release of caesium-137 into the environment resulted from the 1986 Chernobyl disaster in the Soviet Union, which heavily contaminated areas in Ukraine, Belarus, and parts of Scandinavia. Significant deposition also occurred from the 2011 Fukushima Daiichi nuclear disaster in Japan, affecting the Pacific Ocean and surrounding prefectures. Other notable incidents include the Goiânia accident in Brazil, where a stolen radiotherapy source caused widespread contamination, and the Kyshtym disaster at the Mayak Production Association in Russia. Global dispersion from mid-20th century nuclear weapons testing by states like the United States and the Soviet Union has left a detectable layer of caesium-137 in soils and glaciers worldwide.

Category:Caesium Category:Isotopes of caesium Category:Fission products Category:Gamma-ray sources