Various radiation types are produced in a nuclear environment. It could be alpha particles, beta particles, gamma rays, or neutrons. In this book, we are primarily concerned with the radiation damage and effects caused by neutrons. There could be several sources of neutrons, including alpha particle-induced fission, spontane­ous fission, neutron-induced fission, accelerator-based sources, spallation neutron source, photoneutron source, and nuclear fusion.

1.5

Interactions of Neutrons with Matter

Collision of neutrons with atom nuclei may lead to different scenarios — scattering of the neutrons and recoil of nuclei with conservation of momentum (elastic scat­tering) or loss of kinetic energy of the neutron resulting in gamma radiation (inelastic scattering). The capture of neutrons may result in the formation of new

nuclei (transmutation), or may lead to the fragmentation of the nucleus (fission) or

the emission of other nuclear particles from the nucleus. We shall discuss some of

the effects in more detail in Chapter 3.

a) Elastic Scattering

Elastic scattering refers to a neutron-nucleus event in which the kinetic energy and momentum are conserved.

b) Inelastic Scattering

This interaction refers to neutron-nuclide interaction event when the kinetic energy is not conserved, while momentum is conserved.

c) Transmutation

When a nuclide captures neutrons, one result could be the start of a sequence of events that could lead to the formation of new nuclide. The true examples of this type of reaction are (n, a), (n, p), (n, b+), (n, b), and (n, f). Reactions like (n, y) and (n, 2n) do not result in new elements, but only produce isotopes of the origi­nal nuclide.

d) Fission

Fission is a case of (n, f) reaction, a special case of transmutation reaction. Ura­nium is the most important nuclear fuel. The natural uranium contains about 0.7% U235, 99.3% U238, and a trace amount of U234. Here, we discuss the neu­tron-induced nuclear fission, which is perhaps the most significant nuclear reaction. When a slow (thermal) neutron gets absorbed by a U235 atom, it leads to the formation of an unstable radionuclide U236, which acts like an unstable oscillating droplet, immediately followed by the creation of two smaller atoms known as fission fragments (not necessarily of equal mass). About 2.5 neutrons on average are also released per fission reaction of U23 . An average energy of

193.5 MeV is liberated. A bulk of the energy (~160 MeV or ~83%) is carried out by the fission fragments, while the rest by the emitted neutrons, gamma rays, and eventual radioactive decay of fission products. Fission fragments rarely move more than 0.0127 mm from the fission point and most of the kinetic energy is transformed to heat in the process. As all of these newly formed parti­cles (mostly fission fragments) collide with the atoms in the surroundings, the kinetic energy is converted to heat. The fission reaction of U235 can occur in 30 different ways leading to the possibility of 60 different kinds of fission frag­ments. A generally accepted equation for a fission reaction is given below:

U235 + nj! Kr36 + Ba562 + 2n1 + Energy, (1.1)

which represents the fission of one U235 atom by a thermal neutron resulting into the fission products (Kr and Ba) with an average release of two neutrons and an average amount of energy (see above). It is clear from the atomic masses of the reactant and products, that a small amount of mass is converted into an equivalent energy following Einstein’s famous equation E = mc2.

U235 is the one and only naturally occurring radioisotope (fissile atom) in which fission can be induced by thermal neutrons. There are two other fissile atoms (Pu239 and U233) that are not naturally occurring. They are created during

the neutron absorption reactions of U238 and Th232, respectively. Each event con­sists of (n, c) reactions followed by beta decays. Examples are shown below:

The concept of the “breeder” reactors is based on the preceding nuclear reactions, and U238 and Th232 are known as “fertile” atoms. Heavy radioisotopes such as Th232, U238, and Np237 can also undergo neutron-induced fission, how­ever, only by fast neutrons with energy in excess of 1 MeV. That is why these radionuclides are sometimes referred to as “fissionable.”

1.5.1