Fission cross sections as a probe of fusion dynamics at high angular momentum

Abstract

Background: Fusion of heavy nuclei requires energy dissipation to trap the system inside the capture barrier. At high angular momentum, the centrifugal potential causes the barrier radius to reduce, which may lead to energy dissipation outside the barrier, affecting the fusion angular momentum distributions and thus the capture cross sections. Purpose: To investigate the sensitivity of fusion-fission cross sections as a probe of fusion dynamics at high angular momentum. Method: Fission of the compound nucleus Yb164 formed by three different fusion reactions, namely, O16+Sm148, Si28+Ba136, and Ca40+Sn124, was measured at four beam energies well above their respective capture barriers. Fission cross sections were extracted from the measured fission fragment angular distributions and compared with model calculations of fusion and subsequent fission. Fusion and evaporation residue cross sections available in the literature for the same or similar reactions were used to guide model calculations and obtain the fusion angular momentum distributions. Results: The measured fission characteristics were found to be consistent with fusion-fission, as expected, justifying the use of the statistical model to calculate fission cross sections for each reaction. Significantly different fission cross-section predictions were obtained from calculations using angular momentum distributions corresponding to different coupling schemes and different diffuseness parameter of the nuclear potential. A large diffuseness parameter (0.65 fm) of the nuclear potential was observed to give the best reproduction of both the experimental fusion as well as fission cross sections. Conclusions: Experimental fission cross sections provide a stringent constraint to the fusion model calculations and thus prove to be a sensitive probe for understanding fusion dynamics at high angular momentum. This is shown in the present work by a simultaneous analysis of the fusion and fission cross sections for systems where fission cross sections form a small fraction of the fusion cross section and where noncompound nuclear processes are not a dominant competing channel. Observations from this work also suggest the requirement of evaporation residue as well as fission cross sections of higher precision than those generally available for drawing quantitative conclusions.