A series of collisions then finally a capture …

The neutrons resulting from a fission have an energy of the order of 1 or 2MeV (million electronvolts). They are said to be fast neutrons. They must be slowed down in most reactors and become slow neutrons, to be able to cause a second fission and sustain the chain reaction.

Neutrons slow down when colliding with nuclei that ionize the surrounding matter while recoiling. The slowdown is fast in a moderator environment rich in hydrogen where the neutron can happen to lose all its energy in a single collision. The result of this series of collisions is an erratic path that can be long in air.

Once captured, the neutron usually does not emerge from the nucleus. The nucleus reacts by emitting a radiative gamma. The capture often leaves the nucleus in an unstable therefore radioactive state.

The probability – called cross section – of this capture varies greatly with the nucleus and the neutron energy. At characteristic energies (which corresponds to energy levels of the nucleus) the neutron resonates as it passes nearby and finds itself absorbed. It is as if the nucleus became suddenly very big. The phenomenon reminds of what happens in a musical instrument that resonates at certain frequencies. The neutron energy plays here the role of frequency.

In a conventional nuclear reactor, one must not lose inadvertently neutrons. During the slowdown, the resonant energies should be avoided together with nuclei which have too much appetite for neutrons. The choice of materials becomes critical.

The neutron flux is abundant and dangerous near the core of nuclear reactors. The phenomenon of resonant capture is then welcomed for radioprotection. Nuclei, such as, boron-10 or cadmium fond of slow neutrons are intentionally included in control rods and shielding materials. Similarly, one controls the reactor reactivity when the nuclear fuel is new by adding boron in the moderator water. This “boric” water tempers the fission reactions. The amount of boron-10 decreases during operation, together with the reactor, whose fuel becomes depleted, has less and less need to be bridled.

Neutron radiation effects remain after a capture. An isotope that contains an extra neutron is, moreover, generally unstable. Irradiated support structures in reactors become radioactive. Neutron captures in the fuel generate also radioactive nuclei heavier than uranium, such as plutonium isotopes and minor actinides. Radioisotopes useful for applications are also extracted from reactors.