Caesium 137 is a radioactive element with a relatively long half-life of 30.15 years. This particular isotope of caesium is both a beta and gamma emitter. It is produced in some abundance by fission reactions. A lot of attention is focused on caesium 137 : it is the main source of long term contaminations after atmosphric tests of atomic bombs and nuclear accidents. Along with strontium 90 and a handful of plutonium isotopes, it will also be the principal source of radiation from radioactive waste for the next hundred or so years.
Strontium 90 and caesium 137 are, in fact, the two principal fission products that have medium-length half-lives. About 10 kilograms of the former, and 24kg of the latter, are produced every year in a conventional pressurized water reactor (PWR)
Caesium 137 is also one of the principal sources of radioactive contamination after nuclear reactor accidents. After Chernobyl, for instance, large quantities of caesium were released into the atmosphere, but by 1995 the levels of radiation had dropped below what they had been before the 1986 accident.
In the natural environment, caesium is far less mobile than iodine for instance. Attached to minerals, it tends to stay on the ground and is particularly abundant on the forest floor, after having been intercepted by the vegetation. The mycelium, the name given to the vegetative part of fungi and mushrooms that grow within a few centimeters of the ground, traps caesium in the same way as it (unintentionally) traps pesticides. The isotope can concentrate in the food chain: in the flesh of fish or game, for instance
Once absorbed by a human body, caesium spreads through the muscles until, after the 100 days of its biological lifespan, it disappears. This comparatively quick elimination means that only one nucleus out of 160 will decay inside the human body
In the world of industry, caesium 137 is used for its characteristic penetrative gamma ray signature
Spent reactor fuel contains other isotopes of caesium; particularly caesium 134, with a relatively short) half-life of two years and caesium 135 with a particularly long half-life of 2.3 million years.
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