compoundnucleus

In nuclear physics, a compound nucleus is a transient, highly excited intermediate system formed when a projectile is absorbed by a target nucleus during a reaction. The composite system rapidly shares the incoming energy among many nucleons, creating an equilibrated state that exists briefly before it decays.

Formation and equilibration: The absorption occurs via the strong interaction, and the system quickly equilibrates in energy, angular momentum, and parity. The compound nucleus loses memory of the entrance channel, and its subsequent decay depends only on its excitation energy and quantum numbers. This equilibration happens on an extremely short timescale, typically of the order of 10^-16 seconds.

Decay and statistical description: After equilibration, the nucleus decays by emission of neutrons, protons, alpha particles, or gamma rays, or may undergo fission. The probabilities are determined by transmission coefficients through the nuclear potential and the level density of available final states. The Hauser-Feshbach formalism provides a practical statistical framework for calculating reaction cross sections in this regime.

Bohr's hypothesis: The formation and decay processes are effectively independent; the decay probabilities depend only on the properties of the compound nucleus, not on the details of how it was formed, aside from overall energy, angular momentum, and parity constraints.

Limitations and scope: Not all reactions proceed through a fully equilibrated compound nucleus. Pre-equilibrium effects and direct reactions contribute in many cases, especially at higher energies or for light systems. Nevertheless, the compound-nucleus concept remains central in reaction theory and in applications such as nuclear astrophysics and reactor physics.