Fusion and breakup in reactions involving weakly bound nuclei

Abstract

Fusion and breakup in reactions involving weakly bound nuclei A. Szanto de Toledo”, F.A. Souza”, C. Beckb, S.J. Sandersc, M.G. Munhoza, J Takahashi”, N. Carlin”, A.A.P. Suaide”, M.M. de Moura” and E.M. Szanto” “Instituto de Fisica - Universidade de Slo Paulo, Deptartamento de Fisica Nuclear, Laboratorio Pelletron, CP 66318, 05315-970, Sao Paulo - SP, Brazil bCentre National de la Recherche Scientifique Universitd Louis Pasteur, 23 rue du Loess, B.P. 28, F-67037 Strasbourg Cedex 2, France ‘University of Kansas, Lawrence, KS 66045, USA The effect of collective degrees of freedom on the fusion process has been extensively investigated over the past few years [l]. A significant enhancement of the sub-barrier fu- sion cross section is often found as compared to the predictions of one-dimensional barrier penetration models. This enhancement is understood in terms of dynamical processes involving couplings to collective inelastic excitations of the target and/or projectile. A precise determination of the “barrier distributions” leading to the enhancement requires an understanding of the dominant channels that couple to the fusion channel[2]. However, in the case of reactions where at least one of the colliding nuclei has a sufficiently low binding energy so that breakup becomes an important process, conflicting experimental and theoretical results have been reported [3-111. The many questions regarding the influence of breakup become more relevant with the recent availability of radioactive beams and the renewed interest in super-heavy element formation. Radioactive ion beams are likely to produce intense breakup yields. In the fusion processes, and more specifically in the fusion of weakly bound nuclei, that can be used in super-heavy element studies, two different and independent processes can be distinguished both experimentally and theoretically. One, denoted as “Complete Fusion” (CF), is associated with the capture of all of the projectile constituents by the target. The other, denoted as “Incomplete Fusion” (ICF) or Partial Fusion, occurs when part of the projectile is captured by the target and the remaining part escapes. “Total Fusion” is understood as the sum of these two processes (CF + ICF). In order to avoid misinterpre- tations, a clear definition of the experimental and theoretical quantities being compared is essential. This is the main difficulty in comparing data and/or calculations from different authors. Depending on the theoretical approach, different results are achieved. If fusion occurs incoherently, i.e., with possible breakup of the incoming particle [4,5], the survival probability of this particle prior to fusion, is lower than unity, resulting in a decrease in the effective entrance-channel flux and a corresponding reduction of the fusion cross section. On the other hand, if the breakup channel is coupled coherently to the fusion channel, as expected in references [3,6], entrance barrier fluctuations will lead to an effective lowering