On the evolution of the mass asymmetry in damped heavy-ion collisions

Abstract

Total kinetic energy vs. fragment mass distributions have been measured in the reactions of 64Zn projectiles with ll4Sn, natAg and 1]gLa nuclei at incidence energies of 20 -50 ~ above the Coulomb barrier. Conclusions are drawn on the possible influence of nuclear structure effects on the energy dissipation and the mass drift behaviour of the present collision systems.Reactions between complex nuclei at incidence energies not far above the Coulomb barrier are well known to exhibit predominantly binary character with two heavy fragments in the exit channel. Depending on the entrance channel parameters, different degrees of kinetic-energy dissipation and mass relaxation have been observed. A considerable fraction of the reaction cross section is associated with the so-called damped and fusion-like processes /1/. The latter become especially important for heavier collision systems. In our previous work /2/, for example, we investigated the influence of nuclear shell effects on the dynamics of the fragmentation process for collision systems with total nuclear charge numbers Zp + Z t = 108.The present work focuses on the question: can nuclear structure effects of the colliding nuclei also be observed in the evolution of the massasymmetry degree of freedom of other collision systems?We addressed this question to the medium-heavy systems 64Zo + ll4Sn (Z=50, N=64), 64Zn + 139La (N=82) and 64Zn + natAg(non-magic).The experiments were performed at the heavyion cyclotron U-300 of the JINR Laboratory of Nuclear Reactions in Oubna. 8ased on the kinematic coincidence method, the binary reaction products were detected and identified using the double-arm time-of flight spectrometer OEMAS, which has extensively been described elsewhere /3/. 8y measuring the velocity vectors of both correlated final fragments, the pre-evaporative mass and total kinetic energy (TKE) distributions for two-body events were deduced /]/. Thereby, the two-body assumption was verified for every event by demanding collinearity of the two fragment velocity vectors in the centre-of-mass system.In the following, we restrict ourselves to the presentation of TKE vs. fragment mass distributions, which provide an important tool for investigating the microscopic origin of energy dissipation in damped nuclear reactions /i/.

Shell effects in the evolution of the mass asymmetry in heavy-ion collisions leading to composite systems withZ=108

“…The principle of this spectrometer, its mode of operation as well as data acquisition and analysis are extensively described in [9][10][11]. For a better understanding of the problems, which had to be solved in studying kinematic coincidences, we give here only a short description of the experimental equipment sketched in Fig.…”