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https://en.wikipedia.org/wiki/Caesium-137

Caesium-137 (137
55Cs), cesium-137 (US),[7] or radiocaesium, is a radioactive isotope of caesium that 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. Trace quantities also originate from spontaneous fission of uranium-238. It is among the most problematic of the short-to-medium-lifetime fission products. Caesium has a relatively low boiling point of 671 °C (1,240 °F) and easily becomes volatile when released suddenly at high temperature, as in the case of the Chernobyl nuclear accident and with atomic explosions, and can travel very long distances in the air. After being deposited onto the soil as radioactive fallout, it moves and spreads easily in the environment because of the high water solubility of caesium's most common chemical compounds, which are salts. Caesium-137 was discovered by Glenn T. Seaborg and Margaret Melhase.

Decay
Caesium-137 has a half-life of about 30.04 years, decaying by beta emission to stable 137Ba. About 94.6% of the decays go to a metastable nuclear isomer of barium: barium-137m (137mBa, Ba-137m) 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) emitting photons having energy 0.6617 MeV; this is responsible for all of the gamma ray emissions in samples of 137Cs.

Health risks
The biological behaviour of caesium is similar to that of potassium[14] and rubidium. After entering the body, caesium gets more or less uniformly distributed throughout the body, with the highest concentrations in soft tissue.[15] : 114  However, unlike group 2 radionuclides like radium and strontium-90, caesium does not bioaccumulate and is excreted relatively quickly. The biological half-life of caesium is about 70 days.[16]

A 1961 experiment showed that mice dosed with 21.5 μCi/g had a 50% fatality within 30 days (implying an LD50 of 245 μg/kg).[17] A similar experiment in 1972 showed that when dogs are subjected to a whole body burden of 3800 μCi/kg (140 MBq/kg, or approximately 44 μg/kg) of caesium-137 (and 950 to 1400 rads), they die within 33 days, while animals with half of that burden all survived for a year.[18]

Important researches have shown a remarkable concentration of 137Cs in the exocrine cells of the pancreas, which are those most affected by cancer.[19][20][21] In 2003, in autopsies performed on 6 children who died in the polluted area near Chernobyl (of reasons not directly linked to the Chernobyl disaster; mostly sepsis), where they also reported a higher incidence of pancreatic tumors, Bandazhevsky found a concentration of 137Cs 3.9 times higher than in their livers (1359 vs 347 Bq/kg, equivalent to 36 and 9.3 nCi/kg in these organs, 600 Bq/kg = 16 nCi/kg in the body according to measurements), thus demonstrating that pancreatic tissue is a strong accumulator and secretor in the intestine of radioactive caesium.[22] Accidental ingestion of caesium-137 can be treated with Prussian blue (FeIII4[FeII(CN)6] 3), which binds to it chemically and reduces the biological half-life to 30 days.[2