High energy gamma-rays as a probe for dynamical effects in fusion reactions

The importance of dynamical effects in the formation and decay of compound nuclei was investigated with the γ-ray decay of the giant dipole resonance built on highly excited states. These studies showed evidence for long formation times (∼10-20s) for medium mass nuclei (A∼160) when formed in almost symmetric reactions (64Ni + 96Zr). However, no evidence for dynamical effects was found in lighter systems (A∼100), neither in very asymmetric (18O + 92Mo) nor in almost symmetric reactions (50Ti + 60Ni). These observations are consistent with dissipative reaction dynamics calculations which predict long formation times only for heavy almost symmetric systems. A modified version of the statistical model code CASCADE, which included the possibility for particle/γ-ray emission during formation was able to describe the data qualitatively. These calculations also suggest that the γ-rays from the giant dipole resonance are the best probe to study dynamical effects