Exploring Fusion at Extreme Sub-Barrier Energies with Weakly Bound Nuclei

Results of measurement of residues formed in fusion of ${}^6\mathrm{Li}$ with ${}^{198}\mathrm{Pt}$ in the energy range of $0.68<E/V_b<1.3$ using a new sensitive off-beam technique are reported. The fusion excitation function and the derived average angular momenta do not indicate a change of slope at deep sub-barrier energies, contrary to recent observations. The present results for a system with weakly bound projectile confront the current understanding of the fusion hindrance at these low energies, underlying the role of internal reorganization on the dynamical path towards fusion.

Figure 1

Excitation functions for ${}^6\mathrm{Li}+{}^{198}\mathrm{Pt}$ system. (a) Evaporation residues from compound nuclear fusion. Dashed curves are the results of statistical model calculations (see text). (b) Cross sections of residues arising from $d$-capture (${}^{198–200}\mathrm{Au}$), $1n$-pickup (${}^{197}\mathrm{Pt}$), and $1n$-stripping (${}^{199}\mathrm{Pt}$) reactions. The dashed lines are to guide the eye.

Figure 2

Fusion excitation function and derived observables for ${}^6\mathrm{Li}+{}^{198}\mathrm{Pt}$ system. (a) Cross sections for compound-nucleus formation and direct processes obtained from a sum of the partial cross sections shown in Figs. 1a, 1b respectively. The arrow indicates the value of the Coulomb barrier ($V_B$). (b) Average angular momentum and (c) logarithmic derivative of the fusion excitation function. The results of the coupled-channels calculations using the WS potential (solid line), along with single-channel calculations using the WS potential (dashed line) and the M3Y potential with a repulsive core (dot-dashed line), are shown in panels (a)–(c) (see text).

Figure 3

Internuclear potentials for ${}^6\mathrm{Li}+{}^{198}\mathrm{Pt}$ using the WS (long dashed line), the M3Y-double folding (dotted line), and the M3Y with a repulsive core (dash-dotted line). The adiabatic potential is shown as a solid curve up to the formation of a neck configuration. The arrow indicates the lowest center-of-mass energy where the fusion cross sections were measured.