Neutron absorber

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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-135 (Xe-135, half life 9.1 hours), which absorbs a neutron to become Xe-136. Xe-135 is formed in nuclear reactors through the splitting of actinide metals indirectly as a decay product of iodine-135 (I-135), which also has a short half-life. Other isotopes that are major neutron absorbers include helium-3 (He-3), which becomes tritium and boron-11 (B-11) which becomes Li-7. 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.