|Natural abundance||0 (artificial element)|
|Half-life||30.17 y ± 0.03 y|
|Parent isotopes||137Xe (β−)|
|Isotope mass||136.907 u|
|Decay mode||Decay energy|
|beta, gamma||1.176  MeV|
55Cs, 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.
It has a half-life of about 30.17 years, and decays by beta emission to a metastable nuclear isomer of barium-137: barium-137m (137mBa, Ba-137m). (About 95 percent of the nuclear decay leads to this isomer. The other 5.0 percent directly populates the ground state, which is stable.) Ba-137m has a half-life of about 153 seconds, and it is responsible for all of the emissions of gamma rays. One gram of caesium-137 has an activity of 3.215 terabecquerel (TBq).
The photon energy of Ba-137m is 662 keV. These photons can be useful in food irradiation and in the radiotherapy of cancer. Caesium-137 is not widely-used for industrial radiography because it is quite chemically reactive, and hence, difficult to handle. Also the salts of caesium are very soluble in water, and this complicates the safe handling of caesium. Cobalt-60, 60
27Co, is preferred for radiography, since it is chemically a rather nonreactive metal offering higher energy gamma-ray photons. Caesium-137 can be found in some moisture and density gauges, flow meters, and related sensors.
Caesium-137 has a small number of practical uses. In small amounts, it is used to calibrate radiation-detection equipment. It is used as a gamma emitter for oilfield wireline density measurements. It is used as a relative-dating material for assessing the age of sedimentation occurring after 1954. It is also sometimes used in cancer treatment, and it is also used industrially in gauges for measuring liquid flows and the thickness of materials.
Radioactive caesium in the environment 
Small amounts of caesium-134 and caesium-137 were released into the environment during nearly all nuclear weapon tests and some nuclear accidents, most notably the Goiânia accident and the Chernobyl disaster.
As of 2005, caesium-137 is the principal source of radiation in the zone of alienation around the Chernobyl nuclear power plant. Together with caesium-134, iodine-131, and strontium-90, caesium-137 was among the isotopes distributed by the reactor explosion that constitute the greatest risk to health. The mean contamination of caesium-137 in Germany following the Chernobyl disaster was 2000 to 4000 Bq/m2. This corresponds to a contamination of 1 mg/km2 of caesium-137, totaling about 500 grams deposited over all of Germany. In Scandinavia, some reindeer and sheep exceeded the Norwegian legal limit (3000 Bq/kg) 26 years after Chernobyl.
In April 2011, elevated levels of caesium-137 were also being found in the environment after the Fukushima Daiichi nuclear disasters in Japan. In July 2011, meat from 11 cows shipped to Tokyo from Fukushima Prefecture was found to have 1,530 to 3,200 becquerels per kilogram of Cs-137, considerably exceeding the Japanese legal limit of 500 becquerels per kilogram at that time. In March 2013, the Japanese utility that owns the tsunami-damaged nuclear power plant said that it had detected a record 740,000 becquerels per kilogram of radioactive cesium in a fish caught close to the plant. That is 7,400 times the government limit for safe human consumption.
Caesium-137 is reported to be the major health concern in Fukushima. The government is under pressure to clean up radioactivity from Fukushima from as much land as possible so that some of the 110,000 people can return. 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. These include Hydrothermal blasting. 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 (mSv) above background. The most contaminated area where radiation doses are greater than 50 mSv/year must remain off limits but some areas that are currently <5mSv/year may be decontaminated allowing 22,000 residents to return.
Caesium-137 in the environment is anthropogenic (man-made). 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, meaning that until the building of the first artificial nuclear reactor, the Chicago Pile-1, in late 1942, it had not occurred on Earth for billions of years. By observing the characteristic gamma rays emitted by this isotope, it is possible to determine whether the contents of a given sealed container were made before or after the first atomic bomb explosion (Trinity test, June 16, 1945), which spread some of it into the atmosphere, quickly distributing trace amounts of it around the globe. This procedure has been used by researchers to check the authenticity of certain rare wines, most notably the purported "Jefferson bottles". It is also possible to date soils and sediments, given the short life of Cs137 across the Earth's entire surface.
Health risk of radioactive caesium 
|244Cm||241Puƒ||250Cf||227Ac№||10–22 y||medium||m is
|249Cfƒ||242mAmƒ||251Cfƒ||140 y –
No fission products
|248Cm||4n+1||234U№||211–348 ky||99Tc||₡ can capture||126Sn||79Se|
|232Th№||238U№||235Uƒ№||0.7–14 Gy||fission product yield|
Caesium-137 reacts with water producing a water-soluble compound (caesium hydroxide), and the biological behavior of caesium is similar to that of potassium and rubidium. After entering the body, caesium gets more or less uniformly distributed throughout the body, with higher concentration in muscle tissues and lower in bones. The biological half-life of caesium is rather short at about 70 days. A 1972 experiment showed that when dogs are subjected to a whole body burden of 3800 μCi/kg (140 MBq/kg, or approximately 44 μg/kg) of Cesium-137 (and 950 to 1400 rads), they die within thirty-three days, while animals with half of that burden all survived for a year.
The improper handling of caesium-137 gamma ray sources can lead to release of this radioisotope and radiation injuries. Perhaps the best-known case is the Goiânia accident of 1987, in which an improperly-disposed-of radiation therapy system from an abandoned clinic in the city of Goiânia, Brazil, was scavenged from a junkyard, and the glowing caesium salt sold to curious, uneducated buyers. This led to four deaths and serious injuries from radiation exposure.
Caesium gamma-ray sources that have been encased in metallic housings can be mixed-in with scrap metal on its way to smelters, resulting in production of steel contaminated with radioactivity.
In 2009, a Chinese cement company (in Tongchuan, Shaanxi Province) was demolishing an old, unused cement plant and did not follow standards for handling radioactive materials. This caused some caesium-137 from a measuring instrument to be included with eight truckloads of scrap metal on its way to a steel mill, where the radioactive caesium was melted down into the steel.
See also 
- National Institute of Standards and Technology. "Radionuclide Half-Life Measurements". Retrieved 2011-11-07.
- The Lund/LBNL Nuclear Data Search. "Nuclide Table". Retrieved 2009-03-14.
- "NIST Nuclide Half-Life Measurements". NIST. Retrieved 13 March 2011.
- Michael Sandelson; Lyndsey Smith (21 May, 2012). "Higher radiation in Jotunheimen than first believed". The Foreigner. Retrieved 2012-05-21.
- "High levels of caesium in Fukushima beef". Independent Online. 9 July 2011.
- "Fish Near Fukushima Reportedly Contains High Cesium Level". Huffington Post. 17 March 2013.
- Dennis Normile, "Cooling a Hot Zone," Science, 339 (1 March 2013) pp. 1028-1029.
- Takeshi Okumura (October 21, 2003). "The material flow of radioactive cesium-137 in the U.S. 2000". http://www.epa.gov/. US Environmental Protection Agency.
- Note: This is the heaviest isotope with a half-life of at least ten years before the "Sea of Instability".
- Note: Radium (element 88) is actually a sub-actinide, but it immediately precedes actinium (89) and follows a three element gap of instability after polonium (84) where no isotopes have half-lives of at least ten years (the longest-lived isotope in the gap is radon-222 with a half life of less than four days). Radium's longest lived isotope, at 1600 years, thus merits inclusion here.
- Note: specifically from thermal neutron fission of U-235, e.g. in a typical nuclear reactor.
- R. Nave. "Biological Half-life". Hyperphysics.
- H.C. Redman et al. (1972). "Toxicity of 137-CsCl in the Beagle. Early Biological Effects". Radiation Research 50 (3): 629–648. doi:10.2307/3573559. JSTOR 3573559. PMID 5030090.
- The Radiological Accident in Goiânia. IAEA. 1988.
- "Radioactive Scrap Metal". NuclearPolicy.com. Nuclear Free Local Authorities. October 2000.
- J.M. LaForge (1999). "Radioactive Caesium Spill Cooks Europe". Earth Island Journal (Earth Island Institute) 14 (1).
- "Chinese 'find' radioactive ball". BBC News. 27 March 2009.