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, Cs-137), cesium-137, or radiocaesium, is a radioactive isotope of caesium which is formed as one of the more common fission products by the nuclear fission of uranium-235 and other fissionable isotopes in nuclear reactors and nuclear weapons.
It is among the most problematic of the short-to-medium-lifetime fission products because it easily moves and spreads in nature due to the high water solubility of caesium's most common chemical compounds, which are salts. The remainder directly populates the ground state of barium-137, which is stable.
It is believed that the capsule, originally a part of a measurement device, was lost in the late 1970s and ended up mixed with gravel used to construct the building in 1980. By the time the capsule was discovered, 6 residents of the building had died from leukemia and 17 more had received varying doses of radiation.
In the Acerinox accident of 1998, the Spanish recycling company Acerinox accidentally melted down a mass of radioactive caesium-137 that came from a gamma-ray generator.
Unlike most other radioisotopes, caesium-137 is not produced from the same element's nonradioactive isotopes but as a byproduct of the nuclear fission of much heavier elements; until the building of the first artificial nuclear reactor, the Chicago Pile-1, in late 1942, caesium-137 had not occurred on Earth in significant amounts for about 1.7 billion years.
The Kramatorsk radiological accident happened in 1989 when a small capsule containing highly radioactive caesium-137 was found inside the concrete wall of an apartment building in Kramatorsk, Ukrainian SSR.
A number of techniques are being considered that will be able to strip out 80% to 95% of the caesium from contaminated soil and other materials efficiently and without destroying the organic material in the soil. The caesium precipitated with ferric ferricyanide (Prussian blue) would be the only waste requiring special burial sites.
The aim is to get annual exposure from the contaminated environment down to 1 millisievert (m Sv) above background.
The salts of caesium are also soluble in water, and this complicates the safe handling of caesium. Test explosions "Simon" and "Harry" were both from Operation Upshot–Knothole in 1953, while the test explosions "George" and "How" were from Operation Tumbler–Snapper in 1952 Caesium-134 and caesium-137 were released into the environment during nearly all nuclear weapon tests and some nuclear accidents, most notably the Chernobyl disaster and the Fukushima Daiichi disaster.
Cobalt-60, , is preferred for radiography, since it is chemically a rather nonreactive metal and produces higher energy gamma-ray photons. As of 2005 and for the next few hundred years, caesium-137 is the principal source of radiation in the zone of alienation around the Chernobyl nuclear power plant.
Ba-137m has a half-life of about 153 seconds, and is responsible for all of the emissions of gamma rays in samples of caesium-137.