Neutron absorber

Neutron absorbers are isotopes of certain elements that absorb free neutrons creating heavier isotopes of the same element. The most prolific neutron absorbers are elements that become stable by absorbing a neutron such as xenon-133 (Xe-133), which absorbs a neutron to become Xe-134. Xe-133 has a half-life of 5.2 days while Xe-134 is stable. Xe-133 is formed in nuclear reactors through the splitting of actinide metals indirectly as a decay product of iodine-133 (I-133), which also has a short half-life. Other isotopes that are major neutron absorbers include helium-3 (He-3), which becomes He-4 and boron-11 (B-11) which becomes B-12, which has a half-life of only 0.2 seconds and decays into carbon-12 (C-12), the most common isotope in nature. Samarium-149 formed during the fission process is also a highly effective neutron absorber, with its very long half life it last effectively forever in the fuel until it absorbs a neutron and transmutes into Sm-150, which is stable. Other neutron absorbers used in nuclear power plants include cadmium and gadolinium, both of which consist of mixed isotopes some of which are voracious neutron absorbers.