Energy evolution of channel coupling for the system ${}^6\mathrm{Li}+{}^{64}\mathrm{Ni}$ at near-barrier energies

Background: Different theoretical models have been used to understand the effects of coupling of direct reaction channels on reaction observables in recent years. However, very few attempts have been made to check the consistency of the model calculations in terms of comparing different experimental observables with the model predictions.

Purpose: In the present work, coupled channel calculations are performed with transfer and breakup coupling for collision of the weakly bound stable projectile ${}^6\mathrm{Li}$ with the medium-mass target ${}^{64}\mathrm{Ni}$ at near-barrier energies. The main goal is to find the consistency of the model calculation in simultaneous reproduction of reaction observables like the elastic angular distributions, back-angle elastic excitation function, and fusion excitation function to find the effective energy regions of dominance of these couplings.

Method: Both coupled reaction channel (CRC) and continuum discretized coupled channel (CDCC) calculations are performed using the code fresco. Transfer coupling is included through the CRC formalism. The CDCC scheme is used for breakup coupling and to distinguish the effect of resonant and nonresonant couplings.

Results: In the case of the CRC calculation, the full coupling including inelastic scattering, $1n$ stripping, and $1p$ stripping gives the best description of the experimental data and particularly reproduces the data in the subbarrier energy region. It is observed that the $1p$-transfer channel has a larger impact on reaction observables compared to $1n$ transfer. On the other hand, for breakup nonresonant breakup has the dominant effect on the reaction observables, although resonant breakup has a larger cross section. The full breakup coupling reproduces the elastic scattering angular distributions and excitation function in the above-barrier region. But the scheme overpredicts these observables in the subbarrier region.

Conclusion: CRC and CDCC calculations are complementary to each other. The energy dependencies of the relevant couplings for the ${}^6\mathrm{Li}+{}^{64}\mathrm{Ni}$ system around the barrier show that inelastic and transfer coupling are important at energies below the Coulomb barrier but continuum coupling is dominant in the above-barrier energy region.

Figure 1

Representative coupling schemes used in CRC calculation. Schemes CC I, CC II, and CC III are indicated and coupling scheme CC IV incorporates all the couplings simultaneously.

Figure 2

Theoretical elastic scattering angular distributions obtained using different coupling schemes in CRC calculations for the system ${}^6\mathrm{Li}+{}^{64}\mathrm{Ni}$ in comparison with the experimental angular distributions taken from Ref. [33]. See text for details.

Figure 3

Theoretical (a) back-angle elastic scattering and (b) fusion excitation functions obtained using different coupling schemes in CRC calculations for the system ${}^6\mathrm{Li}+{}^{64}\mathrm{Ni}$. Open triangles represent experimental quasielastic scattering cross sections from Ref. [25] and filled squares represent the corresponding elastic scattering cross sections. The short-dashed line is the theoretical prediction for quasielastic scattering from the CC IV coupling scheme of the CRC calculation. See text for details.

Figure 4

Calculated breakup cross section for different excitation energies (bin) from the continuum for both resonant and nonresonant states for the system ${}^6\mathrm{Li}+{}^{64}\mathrm{Ni}$ at energy 26 MeV. Bin numbers 29, 38, and 48 correspond to the resonant states 4.15, 2.88, and 0.716 MeV, respectively, in ${}^6\mathrm{Li}$, above the $\alpha +d$ threshold at 1.47 MeV with the widths 2.58, 1.85, and 0.025 MeV, respectively.

Figure 5

Theoretical elastic scattering angular distributions calculated from CDCC calculation for the system ${}^6\mathrm{Li}+{}^{64}\mathrm{Ni}$ for different coupling conditions at different energies near the barrier in comparison with the experimental angular distributions. See text for details.

Figure 6

Theoretical (a) back-angle elastic scattering and (b) fusion excitation functions obtained using different coupling schemes of CDCC calculations for the system ${}^6\mathrm{Li}+{}^{64}\mathrm{Ni}$ in comparison with the corresponding experimental data.

Figure 7

Comparative elastic scattering angular distributions calculated from CRC coupling scheme CC IV and the full CDCC coupling scheme for the system ${}^6\mathrm{Li}+{}^{64}\mathrm{Ni}$.

Figure 8

Comparative (a) back-angle elastic scattering and (b) fusion excitation functions calculated for the system ${}^6\mathrm{Li}+{}^{64}\mathrm{Ni}$. The dashed-dotted curve represents the normalized CDCC excitation function and the short-dashed curve represents the realistic fusion excitation function from the code fresco.