Dominance of direct reaction channels at deep sub-barrier energies for weakly bound nuclei on heavy targets: The case 8B + 208Pb

Abstract

Wereport the first evidence of breakup dominance at deep sub-barrier energies for the proton halo nucleus 8B on a heavy target. Angular distribution measurements of the 8B breakup fragment, 7Be, on lead were performed at the TwinSol facility of the University of Notre Dame at a beam energy of 30 MeV, 58% of the Coulomb barrier and corresponding to a distance of closest approach of 20.5 fm. The 7Be yield was observed in two double sided silicon strip detector telescopes symmetrical to the radioactive beam and normalized using the 8B Rutherford scattering. The results are in excellent agreement with continuum discretized coupled channel calculations with a total breakup cross section (326 ± 84) mb. This is found to exhaust all of the total reaction cross section for the system 8B+208Pb, possibly prohibiting a fusion enhancement. This finding is expected to give more insight to the puzzle of fusion suppression at deep sub-barrier energies with possible major consequences on nuclear astrophysics.