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Which rare-earth elements are used in nuclear-reactor control rods?

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A line graph showing electron probability distributions for gadolinium, with separate curves for 4f, 5s, 5p, 5d, and 6s orbitals. The x-axis shows distance from the nucleus in atomic units and the y-axis shows probability P2(r). — rare-earth element Electron probabilities, P²*(r)*, for the 4f, 5s, 5p, 5d, and 6s electrons of gadolinium.

Several rare-earth elements are utilized in nuclear reactors due to their high capture cross sections for thermal neutrons, which allow them to effectively absorb neutrons.1

Europium and dysprosium are commonly incorporated into control rods used to regulate the operation of nuclear reactors.12
Gadolinium is specifically used in control rods designed to shut down reactors if they become unstable, as it possesses an exceptionally high neutron-absorption cross section.13

While other rare-earth elements like samarium also have high cross sections, europium and dysprosium are preferred for control rods because they can absorb more neutrons per atom before their effectiveness as absorbers is depleted. Additionally, certain lanthanides are used as burnable neutron absorbers to maintain nearly constant reactivity within a reactor as uranium undergoes fission.1