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Why is boron-10 effective at absorbing thermal neutrons?

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A color-coded periodic table of the elements displays each element's symbol and atomic number, with categories such as alkali metals, noble gases, halogens, transition metals, and actinoid elements distinguished by different background colors. — boron group element Modern version of the periodic table of the elements (printable).

Boron-10 is highly effective at absorbing thermal neutrons because it possesses an extremely large capture cross section—specifically 3,836 barns—which allows it to readily capture neutrons of low energy.1

When a boron-10 nucleus captures a thermal neutron, the reaction results in the expulsion of an alpha particle. Because these high-energy alpha particles do not travel far in normal matter, boron and its compounds are utilized in the fabrication of neutron shields and in Geiger counters to detect neutrons. Additionally, this affinity for neutrons is the foundation of boron neutron capture therapy (BNCT), a medical technique used to target and destroy brain tumours by releasing tissue-damaging alpha particles directly within the tumor site.1