Investigating the Dynamics of Fission at High Excitation Energy in Reactions Induced by Relativistic Protons and Deuterons on Lead
Résumé
Several observables such us total fission cross sections, pre-saddle neutron and gamma emission or the measurement of the fission time using crystal blocking techniques show clear indications for a delay of fission at high excitation energies when compared to other de-excitation channels, namely particle evaporation . This delay is explained as the manifestation of transient effects produced by the coupling between intrinsic and collective degrees of freedom in fission. This coupling can be macroscopically understood as a dissipative process. Moreover, the investigation of proton induced fission on lead is relevant for the radiological characterization of spallation targets. In this work we will present measurements of total fission cross sections and charge distributions of the fission fragments produced in proton and deuteron on lead reactions at relativistic energies. The experiment was performed in inverse kinematics at GSI Darmstadt. Beams of lead ions were accelerated by the SIS18 synchrotron at 500 A MeV impinging then onto a liquid hydrogen/deuterium target. Projectile reaction residues, in particular fission fragments flying forward, were identified using a set of plastic scintillators and ionization chambers. Measured data were used to benchmark model calculations describing residual nuclei production in spallation reactions. In particular we used several model calculations to prove the sensitivity of the width of the charge distributions of the fission fragments to the temperature reached by the fissioning system at saddle. The deduced temperatures are clearly influenced by transient effects which cool down the fissioning nuclei from ground to saddle.