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Caesium-137 (137 55 Cs), 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 ...
Caesium-137 is one such radionuclide. It has a half-life of 30 years, and decays by beta decay without gamma ray emission to a metastable state of barium-137 (137m Ba). Barium-137m has a half-life of a 2.6 minutes and is responsible for all of the gamma ray emission in this decay sequence. The ground state of barium-137 is stable.
The accompanying decay scheme diagram shows the beta decay of caesium-137. 137 Cs is noted for a characteristic gamma peak at 661 keV, but this is actually emitted by the daughter radionuclide 137m Ba. The diagram shows the type and energy of the emitted radiation, its relative abundance, and the daughter nuclides after decay.
The radioactive waste from spent fuel rods consists primarily of cesium-137 and strontium-90, but it may also include plutonium, which can be considered transuranic waste. [38] The half-lives of these radioactive elements can differ quite extremely. Some elements, such as cesium-137 and strontium-90 have half-lives of approximately 30 years.
In nuclear reactors both caesium-137 and strontium-90 are found in locations away from the fuel because they're formed by the beta decay of noble gases (xenon-137, with a 3.8-minute half-life, and krypton-90, with a 32-second half-life) which enable them to be deposited away from the fuel, e.g. on control rods.
It is also not produced by nuclear weapons because 135 Cs is created by beta decay of original fission products only long after the nuclear explosion is over. 136 Cs also captures neutrons with a cross section of 13.00 barns, becoming medium-lived radioactive 137 Cs. Caesium-136 undergoes beta decay (β−), producing 136 Ba directly.
As caesium 133, 135, and 137 are formed by the beta particle decay of the corresponding xenon isotopes, this causes the caesium to become physically separated from the bulk of the uranium oxide fuel. Because 135 Xe is a potent nuclear poison with the largest cross section for thermal neutron absorption, the buildup of 135 Xe in the fuel inside ...
Figure 1: Sodium iodide gamma spectrum of caesium-137 (137 Cs) An example of a NaI spectrum is the gamma spectrum of the caesium isotope 137 Cs —see Figure 1. 137 Cs emits a single gamma line of 662 keV. The 662 keV line shown is actually produced by 137m Ba, the decay product of 137 Cs, which is in secular equilibrium with 137 Cs.