Abstract
Strong coupling to a single channel with zero Q-value is predicted to produce a characteristic fusion barrier distribution with two peaks, one on each side of the original uncoupled Coulomb barrier. In practical cases, only coupling to an elastic transfer channel may produce such a distribution which, however, has never been observed sofar, probably because low-lying surface vibrations usually have a dominant role, and this may obscure the two-peak structure. The case of the two-neutron (2n) elastic transfer in 42 Ca + 40 Ca is particularly attractive, because of the relatively rigid nature of the two nuclei.We have measured the fusion excitation function of this system using the 42 Ca beam of the XTU Tandem of LNL on a thin 40 Ca target enriched to 99.96% in mass 40. Cross sections have been measured down to ≤ 1 mb. The extracted barrier distribution shows clearly two main peaks. We have performed preliminary CC calculations where the 2+ coupling strengths have been taken from the literature and the schematic 2n pair transfer form factor has been used, with a deformation length σt = 0.39 fm. The excitation function is well reproduced by the calculation including the 2n transfer channel. However, including the octupole excitations destroys the agreement.
Highlights
The fusion barrier distributionsIn the near- and sub-barrier fusion of heavy ions, several measurements exploited the concept of a fusion barrier distribution (BD) to identify the nature of couplings responsible for cross section enhancements
Y the calculation including the 2n transfer channel
In the near- and sub-barrier fusion of heavy ions, several measurements exploited the concept of a fusion barrier distribution (BD) to identify the nature of couplings responsible for cross section enhancements
Summary
In the near- and sub-barrier fusion of heavy ions, several measurements exploited the concept of a fusion barrier distribution (BD) to identify the nature of couplings responsible for cross section enhancements. Different shapes of barrier distribution are predicted for the coupling to low-energy surface vibrations and for the transfer of one or more nucleons (see Fig. 1, left panel for some well-known examples [1]). Only coupling to an elastic transfer channel (of one or two nucleons) may produce such a distribution which, has never been observed sofar, because low-lying surface vibrations usually have a dominant role. This complicates and may totally obscure the two-peak structure. A recent example is given by the measurement on 32S + 48Ca [8], whose BD is reported in Fig. 2 (right) in comparison with the case of 36S + 48Ca where transfer does not play any role
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