Probing the fusion of ${}^7\mathrm{Li}$ with ${}^{64}\mathrm{Ni}$ at near-barrier energies

Background: The stable isotopes of Li, ${}^6\text{Li}$ and ${}^7\text{Li}$, have two-body cluster structures of $\alpha +d$ and $\alpha +t$ with $\alpha$-separation energies or breakup thresholds at 1.47 and 2.47 MeV, respectively. The weak binding of these projectiles introduces several new reaction channels not usually observed in the case of strongly bound projectiles. The impact of these breakup or breakup-like reaction channels on fusion, the dominant reaction process at near-barrier energies, with different target masses is of current interest.

Purpose: Our purpose is to explore the fusion, at above and below the Coulmb barrier, of ${}^7\text{Li}$ with ${}^{64}\text{Ni}$ target in order to understand the effect of breakup or breakup-like processes with medium-mass target in comparison with ${}^6\text{Li}$, which has a lower breakup threshold.

Measurement: The total fusion (TF) excitation of the weakly bound projectile ${}^7\text{Li}$ with the medium-mass target ${}^{64}\text{Ni}$ has been measured at the near-barrier energies (0.8 to $2 V_B$). The measurement was performed using the online characteristic $\gamma$-ray detection method. The complete fusion (CF) excitation function for the system was obtained using the $xn$-evaporation channels with the help of statistical model predictions.

Results: At the above barrier energies CF cross sections exhibit an average suppression of about 6.5% compared to the one-dimensional barrier penetration model (1DBPM) predictions, while the model describes the measured TF cross section well. But below the barrier, both TF and CF show enhancements compared to 1DBPM predictions. Unlike ${}^6\text{Li}$, enhancement of CF for ${}^7\text{Li}$ could not be explained by inelastic coupling alone.

Conclusion: Whereas the ${\sigma }_{\mathrm{TF}}$ cross sections are almost the same for both the systems in the above barrier region, the suppression of ${\sigma }_{\mathrm{CF}}$ at above the barrier is less for the ${}^7\text{Li}+{}^{64}\text{Ni}$ system than for the ${}^6\text{Li}+{}^{64}\text{Ni}$ system. Also direct cluster transfer has been identified as the probable source for producing large enhancement in TF cross sections.

Figure 1

Representative characteristic $\gamma$-ray spectrum from the collision of the system ${}^7\text{Li}+{}^{64}\text{Ni}$ at $E_{\mathrm{lab}}=24$ MeV. Some of the characteristic $\gamma$ transitions of the residues are marked.

Figure 2

Representative characteristic offline x-ray spectrum from the fusion of the system ${}^7\text{Li}+{}^{64}\text{Ni}$ at $E_{\mathrm{lab}}=26$ MeV. The characteristic $K$ x-rays of Zn have been marked.

Figure 3

The experimental data and the fitted growth curve, $Y\left(t\right)$, as a function of time of the daughter nuclei (Zn) of ${}^{68}\text{Ga}$ from offline measurement of the system ${}^7\text{Li}+{}^{64}\text{Ni}$.

Figure 4

The measured excitation functions of individual residue channels are shown along with the experimental TF excitation function for the system ${}^7\text{Li}+{}^{64}\text{Ni}$. The dotted line represents the 1DBPM prediction for fusion. The cross section of the residue ${}^{68}\text{Ga}$ for $E_{\mathrm{lab}}=26$ MeV from offline x-ray measurement is shown by a solid star.

Figure 5

The measured TF (open bullet), derived CF (solid bullet), and experimental ($2n+3n$) (open triangle) excitation functions for ${}^7\text{Li}+{}^{64}\text{Ni}$. The 1DBPM and CC predictions are shown by dotted and solid lines, respectively. The pace prediction for ${\sigma }_{2n+3n}^{\mathrm{stat}}$ is shown by the dashed line.

Figure 6

The enhancement factors of TF, CF, and CC cross sections relative to the 1DBPM cross sections are plotted as a function of $E_{\mathrm{c}.\mathrm{m}.}/V_B$, where $V_B$ is the height of the fusion barrier from Table 3. The same for the residue channels ${}^{65}\text{Cu}$ and ${}^{66}\text{Cu}$ are also shown by open boxes and solid triangles, respectively.

Figure 7

Ratio of the non-CF cross sections for the residues ${}^{65}\text{Cu}$ and ${}^{66}\text{Cu}$ for the system ${}^7\mathrm{Li}+{}^{64}\mathrm{Ni}$. The solid bullets represent the data and the solid line represents the calculation from the SM model. See text for details.

Figure 8

Ratio of ${}^6\text{Li}$ to ${}^7\text{Li}$ TF cross sections as a function of the reduced center-of-mass energy. The ratio with the ${}^{64}\text{Ni}$ target (solid bullets) is compared with the same ratio with other medium- and low-mass targets taken from Refs. [11, 14]. See text for details.

Figure 9

Experimental reduced TF cross section for the systems ${}^{6,7}\text{Li}+{}^{64}\text{Ni}$. The solid line represents the UFF. For details, see the text.

Figure 10

Experimental reduced CF cross section for the systems ${}^{6,7}\text{Li}+{}^{64}\text{Ni}$. The solid line represents the UFF. For details, see the text.