The study of neutron nuclear reactions and nuclear reactions, in general, is of paramount importance in the physics of nuclear reactors. Progress in the understanding of nuclear reactions generally has occurred faster than similar studies of chemical reactions. Generally, a higher level of sophistication has been achieved.
Shortly after the neutron was discovered in 1932, it was quickly realized that neutrons might act to form a nuclear chain reaction. When nuclear fission was discovered in 1938, it became clear that if a neutron-induced fission reaction produces new free neutrons, each of these neutrons might cause further fission reaction in a cascade known as a chain reaction. Therefore the calculations of nuclear reactors are determined by the transport of neutrons, their interaction with matter, and their multiplication within a nuclear reactor.
Definition
A neutron nuclear reaction is considered when a neutron interacts with a nuclear particle to produce two or more nuclear particles or ˠ-rays (gamma rays). Thus, a neutron nuclear reaction must cause a transformation of the target nuclide to another nuclide. Sometimes if a nucleus interacts with another nucleus or particle without changing the nature of the nuclide, the process is referred to as a nuclear scattering rather than a neutron nuclear reaction.
To understand the nature of neutron nuclear reactions, the classification according to the time scale of these reactions has to be introduced. Interaction time is critical for defining the reaction mechanism.
There are two extreme scenarios for nuclear reactions (not only neutron reactions):
- A projectile and a target nucleus are within the range of nuclear forces for a very short time allowing for an interaction of a single nucleon only. These types of reactions are called direct reactions.
- A projectile and a target nucleus are within the range of nuclear forces, allowing for a large number of interactions between nucleons. These types of reactions are called the compound nucleus reactions.
There is always some non-direct (multiple internuclear interactions) component in all reactions, but the direct reactions have this component limited.
Basic characteristics of direct reactions:
- The direct reactions are fast and involve a single-nucleon interaction.
- The interaction time must be very short (~10-22 s).
- The direct reactions require incident particle energy larger than ∼ 5 MeV/Ap. (Ap is the atomic mass number of a projectile)
- Incident particles interact on the surface of a target nucleus rather than in the volume of a target nucleus.
- Products of the direct reactions are not distributed isotropically in angle, but they are forward-focused.
- Direct reactions are of importance in measurements of nuclear structure.
Basic characteristics of compound nucleus reactions:
- The compound nucleus is a relatively long-lived intermediate state of the particle-target composite system.
- The compound nucleus reactions involve many nucleon-nucleon interactions.
- A large number of collisions between the nucleons leads to a thermal equilibrium inside the compound nucleus.
- The time scale of compound nucleus reactions is of the order of 10-18 s – 10-16 s.
- The compound nucleus reactions are usually created if the projectile has low energy.
- Incident particles interact in the volume of a target nucleus.
- Products of the compound nucleus reactions are distributed near isotropically in angle (the nucleus loses memory of how it was created – Bohr’s hypothesis of independence).
- The decay mode of the compound nucleus does not depend on how the compound nucleus is formed.
- Resonances in the cross-section are typical for the compound nucleus reaction.