Nuclear Weapon

The nucleus of an atom can interact with a neutron that travels nearby in two basic ways. It can scatter the neutron - deflecting the neutron in a different direction while robbing it of some of its kinetic energy. Or it can capture the neutron, which in turn can affect the nucleus in several ways - absorption and fission being most important here. The probability that a particular nucleus will scatter or capture a neutron is measured by its scattering cross-section and capture cross-section respectively. The overall capture cross-section can be subdivided into other cross-sections - the absorption cross-section and the fission cross-section.

The stability of an atomic nucleus is determined by its binding energy - the amount of energy required to disrupt it. Any time a neutron or proton is captured by an atomic nucleus, the nucleus rearranges its structure. If energy is released by the rearrangement, the binding energy decreases. If energy is absorbed, the binding energy increases.

The isotopes important for the large scale release of energy through fission are uranium-235 (U-235), plutonium-239 (Pu-239), and uranium-233 (U-233). The binding energy of these three isotopes is so low that when a neutron is captured, the energy released by rearrangement exceeds it. The nucleus is then no longer stable and must either shed the excess energy, or split into two pieces. Since fission occurs regardless of the neutron's kinetic energy (i.e. no extra energy from its motion is needed to disrupt the nucleus), this is called "slow fission".

By contrast, when the abundant isotope uranium-238 captures a neutron it still has a binding energy deficit of 1 MeV after internal rearrangement. If it captures a neutron with a kinetic energy exceeding 1 MeV, then this energy plus the energy released by rearrangement can over come the binding energy and cause fission. Since a fast neutron with a large kinetic energy is required, this is called "fast fission".

The slow fissionable isotopes have high neutron fission cross-sections for neutrons of all energies, while having low cross-sections