New results in low-energy fusion of Ca40+Zr90,92
AbstractBackground: Near- and sub-barrier fusion of various Ca + Zr isotopic combinations have been widely investigated. A recent analysis of Ca40+Zr96 data has highlighted the importance of couplings to multiphonon excitations and to both neutron and proton transfer channels. Analogous studies of Ca40+Zr90 tend to exclude any role of transfer couplings. However, the lowest measured cross section for this system is rather high (840μb). A rather complete data set is available for Ca40+Zr94, while no measurement of Ca40+Zr92 fusion has been performed in the past.Purpose: Our aim is to measure the full excitation function of Ca40+Zr92 near the barrier and to extend downward the existing data on Ca40+Zr90, in order to estimate the transfer couplings that should be used in coupled-channels calculations of the fusion of these two systems and of Ca40+Zr94.Methods: Ca40 beams from the XTU Tandem accelerator of INFN–Laboratori Nazionali di Legnaro were used, bombarding thin metallic Zr90 (50μg/cm2) and ZrO922 targets (same thickness) enriched to 99.36% and 98.06% in masses 90 and 92, respectively. An electrostatic beam deflector allowed the detection of fusion evaporation residues (ER) at very forward angles, and angular distributions of ER were measured.Results: The excitation function of Ca40+Zr92 has been measured down to the level of ≃60μb. Coupled-channels (CC) calculations using a standard Woods-Saxon (WS) potential and following the line of a previous analysis of Ca40+Zr96 fusion data give a good account of the new data, as well as of the existing data for Ca40+Zr94. The previous excitation function of Ca40+Zr90 has been extended down to 40μb.Conclusions: Transfer couplings play an important role in explaining the fusion data for Ca40+Zr92 and Ca40+Zr94. The strength of the pair-transfer coupling is deduced by applying a simple recipe based on the value obtained for Ca40+Zr96. The logarithmic slopes and the S factors for fusion are reproduced fairly well for all three systems by the CC calculations, and there are no indications of a fusion hindrance at the lowest energies. In contrast, the new data for Ca40+Zr90 indicate the onset of a fusion hindrance at the lowest energies.