Compound Nucleus

a nuclear system formed in the course of a nuclear reaction by the combination of the incident particle with the target nucleus. A compound nucleus is unstable and decays after a short period into final reaction products. The energy brought in by the incident particle is shared among the degrees of freedom of the compound nucleus, as is the case when bodies are heated. Because of statistical fluctuations, one or more nuclear particles may acquire an energy that is greater than the average energy value and that permits such particles to leave the “heated” nucleus. This process is analogous to the evaporation of a liquid and results in the decay of the compound nucleus. The mean life of a compound nucleus (10^–22–10^–21 sec) is many times greater than the time required by a particle to traverse the region of space occupied by the nucleus.

Nuclear reactions that proceed through the intermediate stages of the formation and decay of a compound nucleus have the following characteristics: the angular distribution of the final products is symmetric (forward-backward with respect to the direction of the incident particles in the center-of-mass system), the energy spectrum of the emitted particles is Maxwellian (seeMAXWELLIAN DISTRIBUTION), and the relative probabilities of the final channels of various reactions involving the same compound nucleus have identical values.

The notion of the compound nucleus was first advanced by N. Bohr in 1936. Ia. I. Frenkel’ suggested that a compound nucleus is analogous to a heated fluid. On the basis of these contributions, a quantitative thermodynamic theory of the compound nucleus was developed in 1936 and 1937 by H. A. Bethe, L. D. Landau, and V. F. Weisskopf.