Typical nuclear radii are of the order 10 m. Assuming spherical shape, we therefore expect the cross sections for nuclear reactions to be of the order of ⁠ π π r 2 ⁠ or 10 m (i.e., 1 barn). Observed cross sections vary enormously: for example, slow neutrons absorbed by the (n, γ γ ) reaction show a cross section much higher than 1,000 barns in some cases (boron-10, cadmium-113, and xenon-135), while the cross sections for transmutations by gamma-ray absorption are in the region of 0.001 barn.