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Why are europium and dysprosium useful for absorbing neutrons in nuclear reactors?

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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.

Europium and dysprosium are effective for use in nuclear reactor control rods because they possess large capture cross sections for thermal neutrons, meaning they are highly efficient at absorbing neutrons per unit area.1

The utility of these specific elements is determined by their capacity to remain effective absorbers over time. Naturally occurring europium can absorb 4.0 neutrons per atom before it becomes ineffective, while dysprosium can absorb 2.4 neutrons per atom. In contrast, other elements with high neutron-absorption cross sections, such as gadolinium and samarium, have much lower capacities—absorbing only 0.3 and 0.4 neutrons per atom, respectively—before they are depleted. Consequently, europium and dysprosium are preferred for regulating reactor operations because they provide a more durable and reliable means of neutron absorption compared to alternatives that become "worthless" more quickly.1