To understand the nature of 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 nuclear 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 nuclear 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.
What is the Compound Nucleus and the Nuclear Resonance?
There is no difference between the compound nucleus and the nuclear resonance.
The compound nucleus is the intermediate state formed in a compound nucleus reaction. It is normally one of the excited states of the nucleus formed by the combination of the incident particle and target nucleus. Suppose a target nucleus X is bombarded with particles a. In that case, it is sometimes observed that the ensuing nuclear reaction occurs with appreciable probability only if the particle’s energy is in the neighborhood of certain definite energy values. These energy values are referred to as resonance energies. The compound nuclei of these certain energies are referred to as nuclear resonances. Resonances are usually found only at relatively low energies of the projectile. The widths of the resonances increase in general with increasing energies. At higher energies, the widths may reach the order of the distances between resonances, and then no resonances can be observed. The narrowest resonances are usually the compound states of heavy nuclei (such as fissionable nuclei) and thermal neutrons (usually in (n,γ) capture reactions). The observation of resonances is by no means restricted to neutron nuclear reactions.
Danish physicist Niels Bohr introduced the compound nucleus model (the idea of compound nucleus formation) in 1936. This model assumes that the incident particle and the target nucleus become indistinguishable after the collision and constitute the nucleus’s particular excited state – the compound nucleus. The projectile has to suffer collisions with constituent nucleons of the target nucleus until it has lost its incident energy to become indistinguishable. Many so these collisions lead to a complete thermal equilibrium inside the compound nucleus. The compound nucleus is excited by both the kinetic energy of the projectile and by the binding nuclear energy.
This compound system is a relatively long-lived intermediate state of the particle-target composite system. From the definition, the compound nucleus must live for at least several times longer than is the time of transit of an incident particle across the nucleus (~10-22 s). The time scale of compound nucleus reactions is 10-18 s – 10-16 s, but lifetimes as long as 10-14 s have also been observed.
A very important feature and a direct consequence of the thermal equilibrium inside a compound nucleus is that the mode of decay of the compound nucleus does not depend on how the compound nucleus is formed. Many collisions between nucleons lead to the loss of information on the entrance channel from the system. The decay mechanism (exit channel) that dominates the decay of C* is determined by the excitation energy in C* and by the law of probability.
These reactions can be considered as two-stage processes.
- The first stage is the formation of a compound nucleus expressed by σa+X➝C*
- The second stage is the decay of a compound nucleus expressed by PC*➝b+Y
- The result cross-section of certain reaction a+X➝[C*]➝b+Y is given by σ(a,b)= σa+X➝C* . PC*➝b+Y
For the compound nucleus, peaks in the cross-section are typical. Each peak manifests a particular compound state of the nucleus. These peaks and the associated compound nuclei are usually called resonances. The behavior of the cross-section between two resonances is usually strongly affected by the effect of nearby resonances.
Resonances (particular compound states) are mostly created in neutron nuclear reactions, but it is by no means restricted to neutron nuclear reactions. The quantum nature of nuclear forces causes the formation of resonances. Each nuclear reaction is a transition between different quantum discrete states or energy levels. The discrete nature of energy transitions plays a key role. Suppose the energy of the projectile (the sum of the Q value and the kinetic energy of the projectile) and the energy of the target nucleus are equal to a compound nucleus at one of the excitation states. In that case, a resonance can be created, and a peak occurs in the cross-section. The allowable state density in this energy region is much lower for the light nucleus, and the “distance” between states is higher. For heavy nuclei, such as 238U, we can observe a large resonance region in the neutron absorption cross-section.
The compound states (resonances) are observed at low excitation energies. This is due to the fact, the energy gap between the states is large. At high excitation energy, the gap between two compound states is very small, and the widths of resonances may reach the order of the distances between resonances. Therefore, no resonances can be observed at high energies, and the cross-section in this energy region is continuous and smooth.
Region of resonances of 238U nuclei. Source: JANIS (Java-based Nuclear Data Information Software); The ENDF/B-VII.1 Nuclear Data Library
Direct Reactions vs. Compound Nucleus 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.
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 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.