Obviously the cross section is of dimension of area, m 2 . This is a very large cross section. In nuclear reactors it is important to slow neutrons down to thermal velocities, because the capture cross section initiating fission has a large cross section at thermal velocities only. Usually boron, graphite, heavy water, or light water are used. These slow down neutrons by multiple elastic scattering.
Lighter nuclei have smaller binding energies because of surface effects.
Their surface to volume ratio is much larger, consequently a larger fraction of the nucleons are near the surface and have smaller binding energies. Nuclei heavier than those in the above range also have smaller binding energies because the positive Coulomb repulsion energy. There is another difference between heavy nuclei and medium range nuclei, besides the differing binding energy per nucleon. The easiest way to split a heavy nucleus is by neutrons. Neutrons are not repulsed by the positively charged nucleus and as such they penetrate the nucleus with ease.