ELASTOPLASTICITY IN DISSIPATIVE HEAVY-ION COLLISIONS

The spectral distributions of delta electrons emitted in dissipative h eavy-ion collisions represent an established tool to study the time ev olution of the collision process. Especially, the yield of high-energy delta electrons, which are measured for the first time up to an energ y of 8 MeV, reflects short time scales and offers a unique possibility to examine the short deceleration phase at the beginning of the colli sion, which takes place within a few 10(-22) s. The delta-electron spe ctra measured in dissipative collisions of Pb+Pb at an incident energy of 12 MeV/nucleon are compared with theoretical predictions using a c oupled-channels formalism. The nuclear trajectories needed as input fo r the calculations are obtained from reaction models. Using nuclear tr ajectories calculated within the one-body dissipation model give a far too low yield of high-energy delta electrons indicating that the dece leration predicted by this model is too slow. The new dissipative diab atic model, which takes into account a non-Markovian dissipation at th e beginning of the collision and which ascribes elastoplastic properti es to nuclear matter, is able to reproduce both the fast deceleration and the long nuclear contact time deduced from the experimental data.