Does the break-up process influence the fusion cross section?

Abstract

We give an overall picture of our present understanding of the effect of the break-up of stable weakly bound nuclei on their fusion cross section with light, medium and heavy mass nuclei, at energies above the Coulomb barrier. The discussion is based mostly on recent data obtained by our group in collaborative experiments with ANU, USP and Tandar Laboratories. We conclude that there is complete fusion suppression for heavy targets, due to the loss of flux, corresponding to the occurrence of incomplete fusion of one of the break-up fragments. For medium and light mass targets, the incomplete fusion is negligible and therefore is no complete fusion suppression.Fusion of heavy ions is a subject that has been extensively studied in the last decades. For energies not too much above the Coulomb barrier (the so-called "region I"), the fusion process has the major contribution to the reaction cross section. As the bombarding energy increases, the competition with other reaction mechanisms decreases the fraction of the reaction cross section corresponding to the fusion cross section, that becomes saturated (the so-called "region II"). At energies close and below the Coulomb barrier, the study of the fusion mechanism is particularly interesting, due to its dependence on the nuclear structure of the colliding nuclei and to its strong couplings with elastic, inelastic and transfer channels. Simultaneous analyses of fusion and elastic scattering data [1,2] have shown that the fusion process is decided at the Coulomb barrier position, or even outside the barrier. Transfer channels that have sharp form factors, and consequently a relative small average transfer distance, act as important doorway to fusion and enhance the fusion cross section at this energy regime [3,4].Nuclei that have small separation energies have a large probability of breaking-up when the colliding nuclei approach each other and their interactions convert potential and kinetic energy into relative kinetic energy between the two fragments. An important question that has been raised in the last years is: What is the effect of the break-up process on the fusion cross section? Although there has been many theoretical and experimental works on this subject, it is still far from being full understood. There is a special interest on this field due to the recently available radioactive beams of very weakly bound nuclei. Reactions of astrophysical interest are induced by weakly bound nuclei and, if the coupling with the break-up channel leads to remarkable enhancement of the fusion cross section, super-heavy nuclei could be more easily produced. Due to the low intensities of the radioactive beams, it is very convenient to produce fusion reactions with the high intensity stable beams that are weakly bound, and consequently should have a reasonable break-up probability. Such reactions are important reference for the understanding of reactions induced by radioactive proton and neutron rich beams, althoght the reactions with these stable nuclei do not cover all the phy...

Effect of the breakup on the fusion and elastic scattering of weakly bound projectiles onZn64

“…A metallic 64 Zn target, with thickness of 50 µg/cm 2 , was deposited on a 10 µg/cm 2 carbon backing and used for runs collecting data for both measured systems. The fusion cross sections data were obtained using the time of flight method and complement previously reported results obtained by the same method, at higher energies, for the 6 Li + 64 Zn system [41] and by the gamma ray spectroscopy method for the 9 Be + 64 Zn system [38]. Further description of the detection system and additional experimental details can be found in Ref.…”

“…It is worthwhile to notice that the total fusion cross sections obtained for the 9 Be + 64 Zn system by the gamma ray method [38,44] and using the TOF technique agree very well, as can be seen in Figure 2.…”

“…Figure 3 shows that total fusion (σ T F ) and reaction cross sections (σ R ) are similar for the whole energy range. The dashed line is the result of CCFULL [19] calculations without any couplings, and the full line is the result including the couplings of the first two excited states of the target (2 Figure 4 shows that for the weakly bound projectile 9 Be, σ T F and the deduced σ R are also very similar for most of the studied energy range, but for energies close to the Coulomb barrier, σ T F becomes appreciably smaller than σ R . Here, σ T F corresponds to the sum of the fusion of 9 Be, 8 Be and one alpha fragment with the target.…”

“…The experiments to measure the elastic scattering for the 16 O+ 64 Zn, 9 Be + 64 Zn and reported [38,39].…”

“…The full understanding of the fusion and break-up processes induced by these beams is an important reference for similar studies involving radioactive proton and neutron rich projectiles [7,8,9].…”