Investigation of Coulomb dipole polarization effects on reactions involving exotic nuclei

We have analyzed elastic scattering angular distributions and total reaction cross sections of the exotic nuclei ${}^{9,11}\text{Li}$ on ${}^{208}\text{Pb}$, at energies below and above the Coulomb barrier. For this purpose, we have used an optical potential with no adjustable parameters, composed by the nuclear São Paulo potential, derived from the nonlocal nature of the interaction, and the Coulomb dipole polarization potential, derived from the semiclassical theory of Coulomb excitation. Within this formalism, we identified an unusual long-range absorption for the ${}^{11}\text{Li}+{}^{208}\text{Pb}$ system, which is dominated by the Coulomb interaction. We compare it to the absorption mechanisms observed for ${}^6\text{He}+{}^{208}\text{Pb}$ which, unlike those of ${}^{11}\text{Li}+{}^{208}\text{Pb}$, take place at small interacting distances, where both Coulomb and nuclear interactions are important. The proposed approach shows to be a fundamental basis to study reactions involving exotic nuclei.

Figure 1

Matter distributions of He and Li isotopes.

Figure 2

${}^6\text{He}$ and ${}^{9,11}\text{Li}B\left(E1\right)$ theoretical distributions as functions of the excitation energy. The figure also presents the ${}^{11}\text{Li}B\left(E1\right)$ experimental values reported in [47].

Figure 3

Different components of the total optical potential for ${}^6\text{He}$, ${}^{11}\text{Li}+{}^{208}\text{Pb}$ at bombarding energies around the Coulomb barrier.

Figure 4

Elastic scattering angular distributions for ${}^9\text{Li}+{}^{208}\text{Pb}$ at 24.1 and 29.5 MeV (data from [7]). The lines represent the theoretical predictions obtained considering the OP of Eq. (5).

Figure 5

Elastic scattering angular distributions for ${}^{11}\text{Li}+{}^{208}\text{Pb}$ (data from [7]). The lines represent theoretical results obtained considering different optical potential components (see text for details).

Figure 6

The same of Fig. 5, but assuming in the calculations the ${}^{11}\text{Li}B\left(E1\right)$ experimental distribution reported in [47].

Figure 7

Elastic scattering angular distributions for ${}^6\text{He}+{}^{208}\text{Pb}$ from [4, 48, 49]. The elastic cross sections for $E_{\text{lab}}=16$ MeV are displaced by a constant value (0.2) to avoid superposition with the data of $E_{\text{lab}}=14$ MeV. The solid black lines represent the results obtained with the total optical potential of Eq. (5), while the dashed-dot-dot green lines are the results without the CDP potential. The dashed red lines correspond to the theoretical cross sections of Ref. [27].

Figure 8

(a) Reaction cross section density as a function of the interacting distance and (b) partial reaction cross section as a function of the angular momentum, for the ${}^{11}\text{Li}+{}^{208}\text{Pb}$ system at $E_{\text{lab}}=24.3$ MeV. The figure shows the contributions of the inner ($W_{\text{SPP}}$) and long-range CDP ($W_{\text{Pol}}$) imaginary potentials. The arrow indicates the approximate position of the $s$-wave barrier radius.

Figure 9

The same as Fig. 8 for $E_{\text{lab}}=29.8$ MeV.