Influence of $\alpha$-clustering configurations in ${}^{16}\mathrm{O}+{}^{197}\mathrm{Au}$ collisions at Fermi energy

The $\alpha$-clustering configurations in light nuclei have attracted a great deal of attention over past years. Nuclear reactions serve as one possible way to study this topic. The aim of this paper was to discuss whether or not heavy-ion collisions at Fermi energy could be a good tool to investigate the cluster configurations in light nuclei. In particular, ${}^{16}\mathrm{O}+{}^{197}\mathrm{Au}$ collisions are simulated using a transport model to check if any observable change is sensitive to the cluster configurations. Within an extended quantum molecular dynamics model, two different $\alpha$-clustering configurations (chain and tetrahedron) of ${}^{16}\mathrm{O}$ were employed in the initialization. ${}^{16}\mathrm{O}+{}^{197}\mathrm{Au}$ collisions at a beam energy of 40 MeV/nucleon with an impact parameter of 3 fm were simulated; then the collective flow parameters ($v_1,v_2,v_3$, and $v_4$) of free protons were analyzed as a function of both the rapidity and the transverse momentum. It was found that flow parameters ($v_1,v_2,v_3$, and $v_4$) decrease with an increase of the flow order, and that the difference in the flow parameters of free protons caused by the initial $\alpha -$clustering configurations increases with transverse momentum. In the higher transverse momentum region (0.2–0.25 GeV/$c$), these flow patterns are quite sensitive to the initial $\alpha$-clustering configurations, and can be regarded as sufficiently sensitive probes that can be used to study the clustering configurations in light nuclei.

Figure 1

Charge multiplicity distributions in collisions of ${}^{12}\mathrm{C}+{}^{197}\mathrm{Au}$ (a), ${}^{14}\mathrm{N}+{}^{197}\mathrm{Au}$ (b), and ${}^{36}\mathrm{Ar}+{}^{197}\mathrm{Au}$ (c) around the Fermi energy. The experimental data from Refs. [34, 35, 36] are shown by stars, calculations with the EQMD model under the same reaction conditions as for these experimental data are shown by solid lines.

Figure 2

Contour plots of the nucleon density in the $x-z$ reaction plane produced from ${}^{16}\mathrm{O}+{}^{197}\mathrm{Au}$ collisions at 40 MeV/nucleon with impact parameters $b=3$ fm. Simulations with the chain [upper panels, (a)–(d)] and tetrahedron [lower panels, (e)–(h)] configurations at four different evolution times (120, 140, 160, and 180 fm/$c$) are shown. Here only one random event is displayed.

Figure 3

The directed flow of free protons produced in a ${}^{16}\mathrm{O}+{}^{197}\mathrm{Au}$ collision at 40 MeV/nucleon and $b=3$ fm. The results at different times are shown by different lines.

Figure 4

The flow parameters $v_1,v_2,v_3$, and $v_4$ of free protons as a function of rapidity $y_z$ for ${}^{16}\mathrm{O}+{}^{197}\mathrm{Au}$ collisions at 40 MeV/nucleon and $b=3$ fm. The calculations obtained with the chain and tetrahedron configurations of ${}^{16}\mathrm{O}$ are shown by solid circles and solid diamonds, respectively.

Figure 5

The same as Fig. 4, but for the transverse momentum $p_t$ dependence of the flow parameters $v_1,v_2,v_3$, and $v_4$.