Breakdown of the compound-nucleus model in the fusion-evaporation process for110Pd+110Pd

Abstract

The fusion of the massive systems 11~176 and 1 lopd + 1 lOpd was uniquely identified by observing the c~ decay of the evaporation residues. The observed distribution of the fusion cross section on the different evaporation-residue channels is in clear contradiction to calculations based on the compound-nucleus model. As a possible explanation the precompound evaporation of c~ particles is proposed. 25.70.Jj; 25.70.Gh; 27.90. + b During the last years we studied the fusion of heavy symmetric systems like 124Sn+96Zr [-1] and l~176 + t l~ [2]. The present work extends these studies to even heavier symmetric systems. In such systems, the ratio between the repelling Coulomb force and the attractive nuclear force at the contact point of the two colliding nuclei, which decides whether the system is able to fuse or not, is as large as in the fusion reactions used to synthesize the heaviest elements, e.g. 5SFe+2~ leading to element 109 [--33. However, compared to the reactions which synthesized the heaviest elements the symmetric systems show a much higher evaporation-residue cross section thus allowing a systematic investigation of the fusion process. PACS:In contrast to the light and medium-heavy systems more massive systems slightly heavier than 9~ + 9~ [-4] exhibit a considerable deficit of fusion above the expected fusion barrier [-1, 2, 4, 53. This deficit is conventionally parametrized by a shift of the fusion barrier called extra-push, which is predicted by macroscopic models. These models consider the development of the combined system in a multidimensional space including potential and inertial forces [63 as well as one-body dissipation [7,8]. According to the ideas of Swiatecki, the * Dedicated to Prof. Dr. P. Kienle on the occasion of his 60th birthday. This work is part of the PhD thesis of W. Morawek ** Present address: Hewtett-Packard GmbH, W-7030 B6blingen, Federal Republic of Germany extra-push is mainly determined by the location of the fission barrier relative to the fusion barrier in a twodimensional landscape. The two dimensions are the deformation or a distance parameter between the centers of the two nuclei, and a parameter representing the neck diameter of the amalgamating nuclei. For light systems, the distance parameter at the fission barrier is larger than that at the fusion barrier which approximately corresponds to the contact point. Therefore fusion may be achieved as soon as the fusion barrier is passed. On the other hand, for heavy systems, the situation will be reversed: The fission barrier corresponds to a distance parameter smaller than the fusion barrier. After contact, i.e. at the fusion barrier, when the system is forming a neck, it may evolve towards reseparation instead of passing the fission saddle point and forming a compound nucleus. An extra-push is necessary in order to drive the system to a deformation smaller than that of the fission saddle. In the case the system 11~176 an extra-push energy of 60 MeV is predicted by the macroscopic model of Blocki et al. [-8].In this work...

Investigation of the fusion of heavy nearly symmetric systems

“…In a recent investigation [45], indications for an evaporation of charged particles during the dynamic evolution towards amalgamation have been found for even more massive symmetric systems like 11~176 From these data it seems, however, to be still justified to describe the fusion reactions of the lighter systems discussed in the present work to a good approximation as a two-step process as formulated by (1).…”

Compound nucleus formation in reactions between massive nuclei: Fusion barrier

“…Pd+~~oPd the experimental data [9] on evaporation residue cross sections are compared with results calculated in the framework of the standard. models: the optical model [2], and the surface friction model [4], and the macroscopic dynamic model [7] (Sec.…”

“…In expression (9) the thermal energy U, and rotational energy E"' are connected at the saddle point by the relation U, = E* -E"'. This form of the width I'f takes into account the change of the fission barrier of the rotating nucleus as far as By(I) = By(0) -(E"-E"')(see details in [15]).…”

Evaporation residues from heavy (Z=90?93) compound nuclei at excitation energies of Ex ? (75?110) MeV

“…This discrepancy is still more significant for the x n channels, where HIVAP predicts a value ~,--100 nb, while our experimental result is a~,(exp)< 0,25 nb. Possibly we observe here a similar effect as done in the reaction ~l~ + 11~ [29], leading to the similar compound nucleus 220U. The breakdown of the x n cross section was tentatively explained by precompound emission of c~ particles, which should be essential to enable fusion for the strongly hindered system according to the parametrisation of [30].…”