Effect of coupling in the Si 28 + Sm 154 reaction studied by quasi-elastic scattering

International audienceThe study of the coupling to collective states of the Si28 projectile and Sm154 target in fusion mechanism is reported. Understanding such couplings is important as they influence the barrier height and the formation probability of the compound nuclei, which in turn may be related to the synthesis of superheavy elements in heavier systems. In the present work, before performing the coupled-channel calculations, we wish to obtain an experimental signature of coupling to projectile and target excitation through barrier distribution (BD) study. To this end, the BDs of the Si28+Sm154 and O16+Sm154 systems have been compared using existing fusion data, scaled to compensate for the differences between the nominal Coulomb barriers and the respective coupling strengths. However, the large error bars on the high-energy side of the fusion BD prevent any definite identification of such signatures. We have, therefore, performed a quasi-elastic (QE) scattering experiment for the heavier Si28+Sm154 system and compared its results with existing QE data for the O16 projectile. Since QE BDs are precise at higher energies, the comparison has shown that the BD of Si28+Sm154 is similar to that of O16+Sm154 to a large extent except for a peaklike structure on the higher energy side. The similarity shows that the Sm154 deformation plays a major role in the fusion mechanism of Si28+Sm154 system. The peaklike structure is attributed to Si28 excitation. In contrast with previous studies, it is found that a coupled-channel calculation with vibrational coupling to the first 2+ state of Si28 reproduces this structure rather well. However, an almost identical result is found with the rotational coupling scheme if one considers the large positive hexadecapole deformation of the projectile. A value around that given by Möller and Nix (β4≈0.25) leads to a strong cancellation in the re-orientation term that couples the 2+ state back to itself, making that state look vibrational in this process. Thus, unlike the existing fusion data, our new QE results contain subtle details about the fusion mechanism of Si28+Sm154 system. They even show a sensitivity to the Si28 hexadecapole deformation and hence may be capable of giving a physically reasonable estimate for β4 in an indirect way