Slow moving neutrons (thermal neutrons, those with low energies of the order of 0.05eV) are easily captured by nuclei, which may then become neutron rich and decay by beta decay (electron emission), or they may remain stable. The thermal neutron capture cross sectional area (expressed in barns) is a measure of how easily neutrons are absorbed by nuclei, the larger the csa, the greater the probability of absorption.

In nuclear reactors, zirconium, which has a low neutron absorption, is used to contain the fuel rods, whereas uranium-238 absorbs neutrons and is either induced into nuclear fission, or goes on absorbing more neutrons and eventually becomes plutonium-239.

Those nuclides with a magic number of neutrons, N magic, have very low neutron absorptions.

Xenon-135 has the highest thermal neutron absorption csa of any known nuclide (2.65 Million barns); it is produced by the fission of uranium as a fission product in nuclear reactors, where it presents a problem being the most serious reactor poison, and may delay the restart of a reactor after a period of shutdown. By absorbing the neutrons produced by the spontaneous fission of uranium in a nuclear reactor, xenon-135 prevents the neutrons from inducing stimulated fission in the uranium, and hence quenches the nuclear chain reaction. Of the elements (with mixtures of isotopes in their normal natural abundance ratios) gadolinium has the highest average neutron capture cross sectional area of 48,000 barns.

The absorption of neutrons by nuclides is strongly dependant upon their energy, the absorption spectrum shows many peaks and troughs. High energy neutrons, those travelling fast, are unlikely to be absorbed, as are ultra-cold neutrons, those with a velocity less than 10-7m/s.