Heavy residue excitation functions for the collisions ${}^{6,7}$Li+${}^{64}$Zn near the Coulomb barrier

Excitation functions for the production of heavy residues have been measured for the collisions ${}^{6,7}\mathrm{Li}+{}^{64}\mathrm{Zn}$ at energies around and below the Coulomb barrier. The cross sections for heavy residue production have been measured using an activation technique, detecting off-line the characteristic atomic x-rays emitted in the electron capture decay of the reaction products. The experimental relative yields of the residues have been compared with statistical model calculations performed by using the code cascade. Such a comparison suggests that heavy residue production is dominated by complete fusion at above-barrier energies, whereas different processes like incomplete fusion and/or transfer become dominant in the sub-barrier energy region. The heavy residue excitation function ratio between the ${}^6$Li- and ${}^7$Li-induced collisions shows an increasing trend as the energy decreases below the barrier.

Figure 1

Sketch of the experimental setup used for the activation of the targets. See text for details.

Figure 2

SEM plan view of the ${}^{64}$Zn target surface.

Figure 3

Beam current as function of time reconstructed for the activation performed using ${}^6$Li at 20 MeV.

Figure 4

Off-line x-ray energy spectra for the collision ${}^6\mathrm{Li}+{}^{64}\mathrm{Zn}$ at 20 MeV measured: (a) 7 h after the end of the activation; (b) 12 d after the end of the activation; (c) 82 d after the end of the activation.

Figure 5

Activity curve for the Ga isotopes relative to the collision ${}^6\mathrm{Li}+{}^{64}\mathrm{Zn}$ at 20 MeV. The bottom curve (b) is the same as the top one (a) but plotted on a longer time range. The time $t=0$ corresponds to the end of the activation. The continuous line is the result of the fit procedure, whereas the other lines represent the contribution of individual nuclides to the total activity. The presence of a contribution having an apparent half-life of 270 d is attributable to the decay of ${}^{68}$Ge ($T_{1/2}=270$ d) into ${}^{68}$Ga ($T_{1/2}=67$ min).

Figure 6

Same as Fig. 5 but for the Ge isotopes. Note that although plotted up to $t=5\times {10}^4$ min, the activity was measured and fitted up to about $t=2\times {10}^5$ min.

Figure 7

Fraction of stable ER predicted by CF calculations performed with the code cascade as a function of the center-of-mass energy for the collisions ${}^6\mathrm{Li}+{}^{64}\mathrm{Zn}$ (solid symbols) and ${}^7\mathrm{Li}+{}^{64}\mathrm{Zn}$ (open symbols).

Figure 8

Present TF excitation function for ${}^6\mathrm{Li}+{}^{64}\mathrm{Zn}$ (solid circles) compared with fusion data of Gomes et al.[19] (open circles) and total reaction data of Zadro et al.[12] (diamonds) for the same system. The continuous and dotted lines represent the results of single-barrier and CC calculations, respectively, for CF (see text for details).

Figure 9

Present TF excitation function for ${}^7\mathrm{Li}+{}^{64}\mathrm{Zn}$ (solid circles) compared with fusion data (open circles) and total reaction data (diamonds) of Gomes et al.[19] for the same system. The continuous and dotted lines represent the results of single-barrier and CC calculations, respectively, for CF (see text for details).

Figure 10

Ratio of the ${}^6$Li- to ${}^7$Li-induced TF excitation functions as a function of the reduced center-of-mass energy for the present data (solid red symbols), compared with same ratio on different targets taken from [22]) (open symbols). See text for details.

Figure 11

Experimental HR relative yield for ${}^6\mathrm{Li}+{}^{64}\mathrm{Zn}$ (black bars) compared with the predictions of statistical model calculations for CF (blue bars), $d$-ICF/DCT (gray bars), and $\alpha$-ICF/DCT (open bars) at different bombarding energies. Relative errors on the experimental data, not reported, are lower than 5$\%$ for the most populated residues. See text for details.

Figure 12

Experimental HR relative yield for ${}^7\mathrm{Li}+{}^{64}\mathrm{Zn}$ (black bars) compared with the predictions of statistical model calculations for CF (blue bars), $t$-ICF/DCT (gray bars), and $\alpha$-ICF/DCT (open bars) at different bombarding energies. Relative errors on the experimental data, not reported, are lower than 5$\%$ on the most populated residues. See text for details.