Nuclear reactions, that occur in a time comparable to the time of transit of an incident particle across the nucleus (~10-22 s), are called direct nuclear reactions. Interaction time is critical for defining the reaction mechanism. The very short interaction time allows for an interaction of a single nucleon only (in extreme cases). In fact, there is always some non-direct (a multiple internuclear interaction) component in all reactions, but the direct reactions have this component limited. To limit the time available for multiple internuclear interactions, the reaction have to occur at high energy.
Direct reactions have another property which is very important. Products of a direct reaction are not distributed isotropically in angle, but they are forward focused. This reflects the fact that the projectiles makes only one, or very few, collisions with nucleons in the target nucleus and its forward momentum is not transferred to an entire compound state.
The cross-sections for direct reactions vary smoothly and slowly with energy in contrast to the compound nucleus reactions and these cross-sections are comparable to the geometrical cross-sections of target nuclei. Types of direct reactions:
- Elastic scattering in which a passing particle and a targes stay in their ground states.
- Inelastic scattering in which a passing particle changes its energy state. For example the (p, p’) reaction.
- Transfer reactions in which one or more nucleons are transferred to the othes nucleus. These reactions are further classified to as:
- Stripping reaction in which one or more nucleons are transferred to a target nucleus from passing particle. For example the neutron stripping in the (d, p) reaction.
- Pick-up reaction in which one or more nucleons are transferred from a target nucleus to a passing particle. For example the neutron pick-up in the (p, d) reaction
- Break-up reaction in which a breakup of a projectile into two or more fragments occurs.
- Knock-out reaction in which a single nucleon or a light cluster is removed from the projectile by a collision with the target.
Example: This threshold reaction of fast neutron with an isotope 10B is one of the ways, how radioactive tritium in primary circuit of all PWRs is generated.