Knowing that the typical nuclear radius r is of the order of 10 cm, the expected nuclear cross section is of the order of π r or roughly 10 cm (thus justifying the definition of the barn). However, if measured experimentally ( σ = R / (Φ N)), the experimental cross sections vary enormously. As an example, for slow neutrons absorbed by the (n, γ) reaction the cross section in some cases (xenon-135) is as much as 2,650,000 barns, while the cross sections for transmutations by gamma-ray absorption are in the neighborhood of 0.001 barn (§ Typical cross sections has more examples).