Reaction mechanisms of the ${}^{16}\mathrm{O}+{}^{65}\mathrm{Cu}$ system

We have measured a precise quasielastic excitation function for the ${}^{16}\mathrm{O}+{}^{65}\mathrm{Cu}$ system, at ${\theta }_{\mathrm{LAB}}={161}^{\circ }$, and at bombarding energies near the Coulomb barrier. A quasielastic barrier distribution for this system was deduced from the experimental quasielastic excitation function. An $\alpha$-stripping excitation function has also been measured at the same experimental conditions. These new data have been used to investigate the relative importance of several reaction channels in the reaction mechanism of the ${}^{16}\mathrm{O}+{}^{65}\mathrm{Cu}$ system. Large-scale coupled-channel calculations and coupled-reaction-channel calculations have been performed. No imaginary potential was used at the barrier region because many channels have been explicitly included in the calculations. Only an inner short-range potential was used to account for the fusion process. We did not fit data by varying potential parameters, and our theoretical results were compared directly to data. Good agreement was found between data and calculations. Owing to the high sensitivity of the barrier distribution, important results have been obtained. The first excited state ($1/2^{-}$) of ${}^{65}\mathrm{Cu}$ has less influence in the reaction mechanism than the second ($5/2^{-}$) and third ($7/2^{-}$) states, which are the most relevant among all the investigated ones. We have also observed a striking influence of the reorientation of the ground-state spin of the ${}^{65}\mathrm{Cu}$ nucleus on the elastic scattering at backward angles. In addition, calculations have shown that the excitation of the states $3^{-},2^{+},1^{-}$, and $2^{-}$ of the projectile ${}^{16}\mathrm{O}$ are also important for excellent agreement obtained with both the excitation function and the distribution of barriers. The $\alpha$-stripping data have been compared to the results of coupled-reaction-channel calculations and good agreement was obtained with the inclusion of the first excited state of ${}^{12}\mathrm{C}$ in the coupling scheme. However, the $\alpha$-transfer process has a small influence on the reaction dynamics of this system at the investigated energies.

Figure 1

(a) $E-\mathrm{\Delta }E$ spectrum of the ${}^{16}\mathrm{O}+{}^{65}\mathrm{Cu}$ system taken at $E_{\mathrm{LAB}}=41\mathrm{MeV}$ and ${\theta }_{\mathrm{LAB}}={161}^{\circ }$ and (b) energy spectrum of the events with $Z=8$ of the $E-\mathrm{\Delta }E$ spectrum above.

Figure 2

Excitation functions measured at ${\theta }_{\mathrm{LAB}}={161}^{\circ }$ for the systems ${}^{16}\mathrm{O}+{}^{65}\mathrm{Cu}$ (present work) and ${}^{16,18}\mathrm{O}+{}^{63}\mathrm{Cu}$ from Refs. [22, 23].

Figure 3

Quasielastic barrier distributions measured at ${\theta }_{\mathrm{LAB}}={161}^{\circ }$ for the systems ${}^{16}\mathrm{O}+{}^{65}\mathrm{Cu}$ (present work) and ${}^{16,18}\mathrm{O}+{}^{63}\mathrm{Cu}$ from Refs. [22, 23].

Figure 4

(a) Experimental quasielastic excitation function, at ${\theta }_{\mathrm{LAB}}={161}^{\circ }$, of the ${}^{16}\mathrm{O}+{}^{65}\mathrm{Cu}$ system and (b) its corresponding quasielastic barrier distribution. The curves are the results of the coupled-channel calculations discussed in the text.

Figure 5

(a) Experimental quasielastic excitation function, at ${\theta }_{\mathrm{LAB}}={161}^{\circ }$, of the ${}^{16}\mathrm{O}+{}^{65}\mathrm{Cu}$ system and (b) its corresponding quasielastic barrier distribution. The curves are the results of the coupled-channel calculations discussed in the text.

Figure 6

Schematic representation of the CRC couplings. The lines represent the couplings between ground states of the initial partition (${}^{16}\mathrm{O}$ and ${}^{65}\mathrm{Cu}$) to the states of the final partition (${}^{12}\mathrm{C}$ and ${}^{69}\mathrm{Ga}$). The values of the excited-state energies are given in MeV.

Figure 7

Experimental data and different theoretical calculations of the excitation function for the ${}^{16}\mathrm{O}({}^{65}\mathrm{Cu},{}^{69}\mathrm{Ga}){}^{12}\mathrm{C}$ reaction. The data for the ${}^{16}\mathrm{O}({}^{63}\mathrm{Cu},{}^{67}\mathrm{Ga}){}^{12}\mathrm{C}$ reaction are plotted for comparison, and they were extracted from Ref. [22]. The lines are discussed in the text.

Figure 8

Comparison of the experimental data and different theoretical excitation functions for the ${}^{16}\mathrm{O}({}^{65}\mathrm{Cu},{}^{69}\mathrm{Ga}){}^{12}\mathrm{C}$ reaction. The green dashed line corresponds to the sum of the cross section corresponding to excited states of the ${}^{69}\mathrm{Ga}$ residual nucleus. The blue dotted line stands for the cross section when the ${}^{12}\mathrm{C}$ ejectile was excited. The summed total cross section is represented by the solid black line.

Figure 9

(a) Angular distribution measured at $E_{\mathrm{LAB}}=46.5\mathrm{MeV}$. The lines represent the different calculations (see text for further details). The vertical axis scale in logarithmic scale: (b) same as in panel (a), but the vertical axis is in linear scale.