.jpg) A sealed caesium-137radioactive source | |
| General | |
|---|---|
| Symbol | 137Cs |
| Names | caesium-137 |
| Protons(Z) | 55 |
| Neutrons(N) | 82 |
| Nuclide data | |
| Natural abundance | 0 (trace) |
| Half-life(t1/2) | 30.04 years[1] |
| Isotope mass | 136.907Da |
| Spin | 7⁄2+ |
| Parent isotopes | 137Xe (β−) |
| Decay products | 137mBa 137Ba |
| Decay modes | |
| Decay mode | Decay energy (MeV) |
| β- (beta decay) | 0.5120[2] |
| γ (gamma-rays) | 0.6617 |
| Isotopes of caesium Complete table of nuclides | |
Caesium-137 (137
55Cs),cesium-137 (US),[7] orradiocaesium, is aradioactiveisotope of caesium that is formed as one of the more commonfission products by thenuclear fission ofuranium-235 and otherfissionable isotopes innuclear reactors andnuclear weapons. Trace quantities also originate fromspontaneous fission of uranium-238. It is among the most problematic of the short-to-medium-lifetime fission products. Caesium has a relatively lowboiling point of 671 °C (1,240 °F) and easily becomesvolatile when released suddenly at high temperature, as in the case of theChernobyl nuclear accident and withnuclear explosions, and can travel very long distances in the air. After being deposited onto the soil asradioactive fallout, it moves and spreads easily in the environment because of the highwater solubility ofcaesium's most commonchemical compounds, which aresalts. Caesium-137 was discovered byGlenn T. Seaborg andMargaret Melhase.
Caesium-137 has ahalf-life of about 30.04 years, decaying bybeta emission to stablebarium-137. About 94.6% of the decays go to ametastablenuclear isomer of barium:barium-137m (137m
Ba) and the remainder directly to the ground state. Barium-137m has a half-life of about 153 seconds, its dropping to the ground state usually (85.1% of all Cs-137 decays) emittingphotons having energy0.6617 MeV. This is responsible for all of thegamma ray emissions in samples of137
Cs.
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Caesium-137 has a number of practical uses. In small amounts, it is used to calibrate radiation-detection equipment.[8] In medicine, it is used inradiation therapy.[8] In industry, it is used inflow meters,thickness gauges,[8] moisture-density gauges (for density readings, withamericium-241/beryllium providing the moisture reading),[9] and inborehole logging devices.[10]
Caesium-137 is not widely used forindustrial radiography because it is hard to obtain a very highspecific activity material with a well defined (and small) shape, as caesium fromused nuclear fuel contains stablecaesium-133 and also long-livedcaesium-135.Isotope separation is too costly compared to cheaper alternatives. Also, the higher specific activity caesium sources tend to be made from highly solublecaesium chloride (CsCl); as a result, if a radiography source were to be damaged, the risk of radioactive contamination is high. It is possible to make water-insoluble caesium sources (withferrocyanides, for example) but their specific activity will be lower. Other chemically inert caesium compounds include caesium-aluminosilicate-glasses akin to the natural mineralpollucite. The latter has been used in demonstrations of chemically stable water-insoluble forms of nuclear waste for disposal indeep geological repositories. A large emitting volume will harm the image quality in radiography. The isotopes192
Ir and60
Co are preferred for radiography, since iridium and cobalt are chemically non-reactive metals and can be obtained with much higher specific activities by the activation of stable191
Ir and59
Co inhigh-flux reactors. However, while137
Cs is a waste product produced in great quantities in nuclear fission reactors,192
Ir and60
Co are specifically produced in commercial andresearch reactors and their life cycle entails the destruction of the involved high-value elements. Cobalt-60 decays to stablenickel, whereas iridium-192 can decay to either stable osmium or platinum. Due to the residual radioactivity and legal hurdles, the resulting material is not commonly recovered even from "spent" radioactive sources, meaning in essence that the entire mass is "lost" for non-radioactive uses.
As an almost purelysynthetic isotope not existing in the environment before 1945, caesium-137 has been used to date wine and detect counterfeits[11] and as a relative-dating material for assessing the age of sedimentation occurring after 1945.[12]
Caesium-137 is also used as aradioactive tracer in geologic research to measuresoil erosion and deposition; its affinity for fine sediments is useful in this application.[13]
The biological behaviour of caesium is similar to that ofpotassium[14] andrubidium. After entering the body, caesium gets more or less uniformly distributed throughout the body, with the highest concentrations insoft tissue.[15] However, unlikegroup 2 radionuclides likeradium andstrontium-90, caesium does notbioaccumulate and is excreted relatively quickly. Thebiological half-life of caesium is about 70 days.[16] It has been demonstrated that pancreatic tissue is a strong accumulator and secretor in the intestine of radioactive cesium (Cs-137).[17][18]A 1961 experiment showed that mice dosed with21.5 μCi/g of 137
Cs had a 50% fatality rate within 30 days, implying anLD50 of245 μg/kg.[19] A similar experiment in 1972 showed that when dogs are subjected to awhole body burden of3800 μCi/g (140 MBq/kg, or approximately44 μg/kg) of caesium-137 (and950 to 1400 rad), they die within 33 days, while animals with half of that burden all survived for a year.[20] A 1960 mouse study found there were high levels of Cs-137 for the first day after exposure in the mucus glands of the colon, the pancreas, cartilage, tendons, and skeletal muscle. After 24 hours, cartilage and skeletal muscle showed the highest activity.[21]
In 2003, a study found that children from the Cs-137-polluted area in Belarus nearChernobyl suffered from chronic diseases rarely found in children in other areas of Belarus. Measurements of Cs-137 exposure from autopsies performed on 52 children who died of various causes found that the concentration of Cs-137 was highest in the thyroid (2054±288 Bq/kg), adrenals (1576±290 Bq/kg), and pancreas (1359±350 Bq/kg), and lowest in the brain (385±72 Bq/kg) and liver (347±61 Bq/kg).[22]
Accidental ingestion of caesium-137 can be treated withPrussian blue (FeIII
4[FeII
(CN)
6]
3), which binds to it chemically and reduces itsbiological half-life to 30 days.[23]
| Nuclide | t1⁄2 | Yield | Q[a 1] | βγ |
|---|---|---|---|---|
| (a) | (%)[a 2] | (keV) | ||
| 155Eu | 4.74 | 0.0803[a 3] | 252 | βγ |
| 85Kr | 10.73 | 0.2180[a 4] | 687 | βγ |
| 113mCd | 13.9 | 0.0008[a 3] | 316 | β |
| 90Sr | 28.91 | 4.505 | 2826[a 5] | β |
| 137Cs | 30.04 | 6.337 | 1176 | βγ |
| 121mSn | 43.9 | 0.00005 | 390 | βγ |
| 151Sm | 94.6 | 0.5314[a 3] | 77 | β |
| ||||
Caesium-137, along with other radioactive isotopescaesium-134,iodine-131,xenon-133, andstrontium-90, were released into the environment during nearly allatmospheric nuclear weapon tests, and more recently somenuclear accidents, most notably theChernobyl disaster, theGoiânia Accident and theFukushima Daiichi disaster.
Caesium-137 is produced from thenuclear fission ofplutonium anduranium,[24] and by observing the characteristic gamma rays emitted by this isotope, one can determine whether the contents of a given sealed container were made before or after the first atomic bomb explosion (Trinity test, 16 July 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".[25] Surface soils and sediments are also dated by measuring the activity of137
Cs.
Bombs in the Arctic area ofNovaja Zemlja and bombs detonated in or near thestratosphere released caesium-137 that landed in upperLapland, Finland. Measurements of caesium-137 in the region in the 1960s were reportedly 45,000 becquerels. Figures from 2011 have a midrange of about 1,100 becquerels, but no increase in cancer cases has been identified.[26][27][28]
As of today and for the next few hundred years or so, caesium-137 andstrontium-90 continue to be the principal source of radiation in thezone of alienation around theChernobyl nuclear power plant, and pose the greatest risk to health, owing to their approximately 30-year half-life and biological uptake. Themean contamination of caesium-137 in Germany following theChernobyl disaster in 1986 was2000 to 4000 Bq/m2.[citation needed] 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 afterChernobyl.[29] TheChernobyl caesium-137 has now decayed by more than half, but could have been locally concentrated by much larger factors.
 | This section needs to beupdated. Please help update this article to reflect recent events or newly available information.(August 2025) |
In April 2011, elevated levels of caesium-137 were also being found in the environment after theFukushima Daiichi nuclear disasters in Japan. In July 2011, meat from 11 cows shipped toTokyo fromFukushima Prefecture was found to have1530 to 3200 becquerels per kilogram of137
Cs, considerably exceeding the Japanese legal limit of 500 becquerels per kilogram at that time.[30] In March 2013, a fish caught near the plant had a record 740,000 becquerels per kilogram of radioactive caesium, above the 100 becquerels per kilogram government limit.[31] A 2013 paper inScientific Reports found that for a forest site 50 kilometres (30 miles) from the stricken plant,137
Cs concentrations were high inleaf litter, fungi anddetritivores, but low in herbivores.[32] By the end of 2014, "Fukushima-derived radiocaesium had spread into the whole westernNorth Pacific Ocean", transported by theNorth Pacific current from Japan to theGulf of Alaska. It has been measured in theocean surface layer down to 200 meters (660 feet) and south of the current area down to 400 m (1,300 ft).[33]
Caesium-137 is reported to be the major health concern inFukushima. A number of techniques are being considered that will be able to strip out80% to 95% of the caesium from contaminated soil and other materials efficiently and without destroying the organic material in the soil. These includehydrothermal blasting.[further explanation needed] The caesium, precipitated with ferricferrocyanide (Prussian blue) would be the only waste requiring special burial sites.[34] The aim is to get annual exposure from the contaminated environment down to1 millisievert (mSv) abovebackground levels. The most contaminated area where radiation doses are greater than50 mSv/year must remain off-limits, but some areas that are currently less than5 mSv/year may be decontaminated, allowing 22,000 residents to return.[citation needed]
Caesium-137gamma sources have been involved in severalradiological accidents and incidents.
In theGoiânia accident of 1987, an improperly disposed ofradiation therapy system from an abandoned clinic inGoiânia, Brazil, was removed, then cracked to be sold injunkyards. The glowingcaesium salt was then sold to curious, unaware buyers.[35] This led to four confirmed deaths and several serious injuries from radiation contamination.[36][37]
TheKramatorsk incident happened in 1989 when a small capsule 8 by 4 millimetres (0.3 by 0.2 inches) in size of caesium-137 was found inside the concrete wall of an apartment building inKramatorsk,Ukrainian SSR. 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. Over 9 years, two families had lived in the apartment. By the time the capsule was discovered, 6 residents of the building had died, 4 fromleukemia and 17 more receiving varying doses of radiation.[38]
The1994Tammiku incident involved the theft ofradioactive material from anuclear waste storage facility inMänniku, Saku Parish,Harju County,Estonia. Three brothers, unaware of the facility's nature, broke into a shed while scavenging forscrap metal. One of the brothers received a4000 rad whole-body dose from a caesium-137 source that had been released from a damaged container, succumbing toradiation poisoning 12 days later.[citation needed]
In 1997, severalGeorgian soldiers suffered radiation poisoning and burns. They were eventually traced back to training sources left abandoned, forgotten, and unlabeled after thedissolution of the Soviet Union. One was a caesium-137 pellet in a pocket of a shared jacket that released about 130,000 times the level ofbackground radiation at a 1-metre (3-foot) distance.[39]
In theAcerinox accident of 1998, the Spanish recycling companyAcerinox accidentally melted down a mass of radioactive caesium-137 that came from agamma-ray generator.[40]
In 2009, a Chinese cement company inTongchuan,Shaanxi Province was demolishing an old, unusedcement 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 ofscrap metal on its way to asteel mill, where the radioactive caesium was melted down into the steel.[41]
In March 2015, the NorwegianUniversity ofTromsø lost 8 radioactive samples, including samples of caesium-137,americium-241, andstrontium-90. The samples were moved out of a secure location to be used for education. When the samples were supposed to be returned, the university was unable to find them. As of 4 November 2015[update], the samples are still missing.[42][43]
On3 and 4 March 2016, unusually high levels of caesium-137 were detected in the air inHelsinki, Finland. According to Finland'sRadiation and Nuclear Safety Authority (STUK), measurements showed4000 μBq/m3 –approximately 1,000 times the usualbackground level. A STUK investigation traced the source to a building from which STUK itself and a radioactive waste treatment company operate.[44][45]
Thirteen people were exposed to caesium-137 in May 2019 at the Research and Training building in theHarborview Medical Center complex inSeattle, Washington. A contract crew was transferring the caesium from the lab to a truck when the powder was spilled. Five people were decontaminated and released, but 8 who were more directly exposed were taken to the hospital while the research building was evacuated.[46]
Public health authorities inWestern Australia issued an emergency alert for a stretch of road measuring about 1,400 kilometres (870 miles) after a capsule containing caesium-137 was lost in transport on 25 January 2023. The 8 millimetres (0.3 inches) capsule contained a small quantity of the radioactive material when it disappeared from a truck. The State Government immediately launched a search, with the WA Department of Health's chief health officerAndrew Robertson warning an exposed person could expect to receive the equivalent of "about 10 X-rays an hour". Experts warned, if the capsule were found, the public should stay at least 5 metres (16 feet) away.[47] The capsule was found on 1 February 2023.[48]
A caesium-137 capsule went missing from asteam power plant inPrachinburi province, Thailand on 23 February 2023, triggering a search by officials from Thailand'sOffice of Atoms for Peace (OAP) and thePrachinburi provincial administration. However, the Thai public was not notified until 14 March.[49]
On 20 March, the Secretary-General of the OAP and the governor ofPrachinburi held a press conference stating that they had found caesium-137 contaminated furnace dust at a steel melting plant inKabin Buri district.[50]
On 5 April 2024, an emergency regime was introduced in the Russian city ofKhabarovsk after a local resident accidentally discovered that radiation levels had jumped sharply in one of the industrial areas of the city. According to volunteers of the dosimetric control group, the dosimeter at the NP site showed up to800 microsieverts, which is 1600 times the safe value.
Employees of the Ministry of Emergency Situations fenced off the area to 30 by 30 meters (100 by 100 ft), where they found a capsule with caesium from adefectoscope. This was placed in a protective container and taken away for disposal. The incident was first reported by theNovaya Gazeta.[51]
On 18 August 2025, the United StatesFood and Drug Administration (FDA) issued a statement indicating that shipments ofshrimp fromIndonesia had been contaminated. This was only the first of severalmajor recalls of irradiated shrimp processed by the Indonesian company Bahari Makmur Sejati (BMS Foods).[52]The source of the material was traced to a metal scrapyard in the Modern Cikande industrial estate near Jakarta, in which the company supplying the shipping containers was also based.[53]
| International | |
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| Other | |
