Fusionevaporation

Fusion-evaporation, also known as fusionevaporation, is a nuclear reaction mechanism in which two atomic nuclei fuse to form a highly excited compound nucleus that de-excites predominantly by evaporating light particles (neutrons, protons, and alpha particles) and, to a lesser extent, by gamma emission. The term is used especially in heavy-ion physics to describe the production of evaporation residues following a fusion event.

In a typical fusionevaporation reaction, a projectile nucleus is accelerated toward a target and collides at energies near or above the Coulomb barrier. The two nuclei fuse to form a compound nucleus with high excitation energy. The excited system then cools by sequential emission of light particles and gamma rays. The competition with fission is significant for heavy systems; if fission occurs before particle emission, the fusion-evaporation channel is suppressed. The resulting evaporation residues are the nuclei left after the emitted light particles have carried away excess energy.

Cross sections for fusion-evaporation are described by statistical models that treat the formation of the compound nucleus and its subsequent decay. The evaporation-residue cross section can be conceptually written as a sum over angular momenta of the entrance channel: sigma_ER ≈ sum_l (2l+1) T_in(l) P_sur(l), where T_in(l) is the transmission probability for capture and P_sur(l) is the survival probability against fission after particle emission. Practically, detailed calculations use Hauser-Feshbach theory and level-density inputs.

Applications and context: Fusion-evaporation reactions are central to the synthesis of heavy and superheavy elements and the study of nuclear structure in moderately heavy nuclei. They provide access to neutron-deficient isotopes and to nuclei near the proton drip line, depending on entrance-channel choices. The method competes with other reaction mechanisms such as deep-inelastic transfer and fusion-fission.