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Name, symbol Caesium-137,137Cs
Neutrons 82
Protons 55
Nuclide data
Natural abundance 0 (artificial element)
Half-life 30.07 years
Isotope mass 136.907 u
Spin 11/2-
Decay mode Decay energy
Beta 1.174 [1] MeV

Caesium-137 (also spelled cesium) is a radioactive isotope of caesium which is formed mainly as a fission product by nuclear fission. It has a half-life of 30.07 years, and decays by beta decay to a metastable nuclear isomer of barium-137 (Ba-137m). (95% of the decay leads to this isomer; the other 5% directly populates the ground state.) Barium-137m has a half-life of 2.55 minutes and is responsible for all of the gamma ray emission. The ground state of barium-137 is stable. 1g of pure Cs-137 has an activity of 3.4 TBq.

Caesium-137 is water-soluble and extremely toxic in minute amounts. Once released into the environment, it remains present for many years as its radiological half-life is 30.07 years. It can cause cancer 10, 20 or 30 years from the time of ingestion, inhalation or absorption provided sufficient material enters the body. [2]

Cs-137 decay scheme.

The photon energy of Ba-137m is 662 keV. These photons can be used in food irradiation, or in radiotherapy of cancers. Cs-137 is not widely used for industrial radiography because it is chemically unstable. For example, its salts are easily soluble in water which complicates safe handling. Cobalt-60 is preferred for radiography, as it is a chemically stable metal offering higher gamma energies and higher activities. Cs-137 can be found in some moisture and density gauges, flow meters, and other sensor equipment.


Caesium in environment

Cs-137 γ-spectrum: 660 keV γ- and 30keV Ba K-lines.
The ten highest depositions of caesium-137 from US nuclear testing at the Nevada Test Site. Shots "Simon" and "Harry" were both from Operation Upshot-Knothole in 1953, while shots "George" and "How" were from Operation Tumbler-Snapper in 1952

Small amounts of Cs-134 and Cs-137 were released into the environment during nuclear weapon tests and some nuclear accidents, most notably the Chernobyl disaster. As of 2005, Cs-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, it was among the isotopes with greatest health impact distributed by the reactor explosion.

The mean contamination of Cs-137 in Germany after Chernobyl was 2000-4000Bq/m², some parts in the south even 10 times higher. This corresponds to a contamination of 1mg of Cs-137 per square kilometer or around 500g Cs-137 deposited all over Germany.

Health risk

Actinides Half-life Fission products
244Cm 241Pu f 250Cf 243Cmf 10–30 y 137Cs 90Sr 85Kr
232 f 238Pu f is for
69–90 y 151Sm nc➔
4n 249Cf  f 242Amf 141–351 No fission product
has half-life 102
to 2×105 years
241Am 251Cf  f 431–898
240Pu 229Th 246Cm 243Am 5–7 ky
4n 245Cmf 250Cm 239Pu f 8–24 ky
233U    f 230Th 231Pa 32–160
4n+1 234U 4n+3 211–290 99Tc 126Sn 79Se
248Cm 242Pu 340–373 Long-lived fission products
237Np 4n+2 1–2 my 93Zr 135Cs nc➔
236U 4n+1 247Cmf 6–23 107Pd 129I
244Pu 80 my >7% >5% >1% >.1%
232Th 238U 235U    f 0.7–12by fission product yield

Biological behavior of Cs-137 is similar to potassium. After entering the organism, all caesium gets more or less uniformly distributed through the body, with higher concentration in muscle tissue and lower in bones. The biological half-life of caesium is short at 70 days [3].

Improper handling of Cs-137 sources can lead to release of the isotope and radiation contamination and injuries. Perhaps the best known case is the Goiânia accident, when a radiation therapy machine from an abandoned clinic in Goiânia, Brazil, was scavenged and the glowing caesium salt sold to curious buyers. Metallic caesium sources can be also accidentally mixed with scrap metal, resulting in production of contaminated steel;[4] a notable example is the case from 1998, when Recycler Acerinox in Cadiz, Spain accidentally melted a source.[2] In 2009, a Chinese demolition company in north-western Shaanxi province did not follow environmental standards, causing some Cs-137 from a measuring instrument to be melted down with other pieces of scrap into slag.[5]

See also

fission products
Q *
155Eu 4.76 .0803 252 βγ
85Kr 10.76 .2180 687 βγ
113mCd 14.1 .0008 316 β
90Sr 28.9 4.505 2826 β
137Cs 30.23 6.337 1176 βγ
121mSn 43.9 .00005 390 βγ
151Sm 90 .5314 77 β


External links


Simple English

Caesium 137 is an isotope of caesium. It is radioactive and often is a by-product of nuclear fission. It has a half life of roughly 30 years.


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