Investigation of ${}^6\mathrm{Li}+{}^{64}\mathrm{Ni}$ fusion at near-barrier energies

The total fusion (TF) excitation function for a ${}^6\mathrm{Li}$ projectile with a ${}^{64}\mathrm{Ni}$ target has been measured using the online characteristic $\gamma$ ray detection method at energies around the Coulomb barrier. The complete fusion (CF) excitation function for the system is subsequently estimated from the dominating neutron evaporation channels with the help of statistical model predictions. The CF cross sections exhibit a suppression of about $13\%$ compared to the one-dimensional barrier penetration model (1DBPM) at above-barrier energies, but no suppression is observed for TF cross sections. The observation does corroborate the estimated suppression for ${}^6\mathrm{Li}$ on ${}^{59}\mathrm{Co}$ target, but does not corroborate the recently proposed universal suppression factor for the ${}^6\mathrm{Li}$ projectile. The result supports the conjecture of reduced suppression of CF cross sections with decreasing target mass. At energies below the barrier, both the TF and the CF cross sections are enhanced. The observed enhancement of the CF process can be explained by channel coupling (CC), but the enhancement in TF cross sections is significantly higher than the CC predictions.

Figure 1

Representative $\gamma$ spectrum for ${}^6\mathrm{Li}$+${}^{64}\mathrm{Ni}$ fusion at $E_{\mathrm{lab}}=28$ MeV. Some of the characteristic $\gamma$ transitions of the residues are marked.

Figure 2

The measured excitation functions of individual residue channels are shown along with the experimental TF excitation function for the system ${}^6\mathrm{Li}$+${}^{64}\mathrm{Ni}$. The solid line represents the 1DBPM prediction for fusion. The cross section of the residue ${}^{68}\mathrm{Ga}$ for $E_{\mathrm{lab}}=26$ MeV from off-line x-ray measurement is shown by a half-filled diamond at the corresponding relative energy value.

Figure 3

The measured TF (solid star), derived CF (solid bullet), experimental ($2n+3n$) (solid rectangle), and ($2n+3n+pn$) (solid triangle) excitation functions for ${}^6\mathrm{Li}$+${}^{64}\mathrm{Ni}$. The 1DBPM and CC predictions are shown by solid and dashed lines. The pace predictions for ${\sigma }_{2n+3n}^{\mathrm{stat}}$ and ${\sigma }_{2n+3n+pn}^{\mathrm{stat}}$ are shown with dot-dot-dashed and dotted lines respectively. See the text for details.

Figure 4

An expanded plot in linear scale, showing the comparison of ${\sigma }_{CF}^{2n+3n}$ and ${\sigma }_{CF}^{2n+3n+pn}$ with the corresponding 1DBPM prediction as a function of the ratio $E_{c.m.}/V_B$.

Figure 5

The suppression factor of ${\sigma }_{CF}$ with respect to the 1DBPM prediction ($F_{CF}$) in the above-barrier energy regime is plotted as a function of projectile-target charge product, $Z_P{Z}_T$. The data points represented by the solid circle are taken from Kumawat et al. [12] and the open circle from Palshetkar et al. [9].

Figure 6

The enhancement factor of TF, CF, and CC cross sections relative to the 1DBPM cross sections plotted as a function of $E_{c.m.}/V_B$ where $V_B$ is the height of the Coulomb barrier.