Effects of neutron richness on the behavior of nuclear systems at intermediate energies

We discuss results concerning the behavior of hot nuclear sources formed in ${}^{40,48}$Ca+${}^{40,48}$Ca reactions at 25 MeV/nucleon. A correlation between the neutron-to-proton ratio of the total systems and heavy residue production is found. This correlation underlines the strong role played by the neutron-to-proton ratio degree of freedom on the behavior of hot nuclei at intermediate energies. Comparisons of our data with Constrained Molecular Dynamics calculations confirm that a moderately stiff behavior of the symmetry potential must be used to reproduce experimental data. These findings open the way for future investigations with exotic beams at radioactive ion beam facilities.

Figure 1

(a) Mass distribution of the biggest emitted fragment (BEF) in the selected class of semicentral events. Black shaded histogram: ${}^{40}$Ca+${}^{40}$Ca. Red solid histogram: ${}^{40}$Ca+${}^{48}$Ca. Blue dots: ${}^{48}$Ca+${}^{48}$Ca. (b) Distributions of normalized mass $m_1/m_{\mathrm{tot}}$ of the BEF in the studied collisions for the same class of events. In both panels, $y$ axis have been normalized to the total number of counts and to the width of the bins. Mass evaluation is affected by 5$\%$ error, as discussed in the text.

Figure 2

(a,b,c,d) Mass distributions of the BEF in the selected semicentral events. Black dots: experimental data. Blue dashed line: binary-like and multifragmentation contribution. Shaded Gaussian: heavy-residue production. Red solid line: two Gaussian fit. (a) ${}^{40}$Ca+${}^{40}$Ca, (b) ${}^{40}$Ca+${}^{48}$Ca, (c) ${}^{48}$Ca+${}^{40}$Ca, and (d) ${}^{48}$Ca+${}^{48}$Ca cases. (e) Heavy-residue yields normalized to the total yield of selected events and obtained from the fit procedure, as a function of the $N/Z$ of analyzed systems.

Figure 3

Comparison of experimental $\Delta M_{\mathrm{nor}}$ distributions for selected semicentral events of ${}^{40}$Ca+${}^{40}$Ca (black dots), ${}^{40}$Ca+${}^{48}$Ca (red dashes), and ${}^{48}$Ca+${}^{40}$Ca (blue solid line) reactions. In the insert, we show the probability of populating the $\Delta M_{\mathrm{nor}}\ge 0.4$ region (typical of HR events) as a function of the $N/Z$ of the total system. Solid stars are experimental data, while lines are the predictions of CoMD-II model calculations performed by using different options for the stiffness of the symmetry potential: Stiff1 (red dotted line), Stiff2 (blue dashed line), and Soft (green dash-dotted line).

Figure 4

Comparison of experimental mass distributions $m_1/m_{\mathrm{tot}}$ and $\Delta M_{\mathrm{nor}}$ (black dots) with results obtained from CoMD-II+gemini [45] calculations (red line) for the case of ${}^{48}$Ca+${}^{48}$Ca reaction. Stiff2 parametrization is used to describe density dependence of symmetry potential.

Figure 5

Theoretical $\Delta M_{\mathrm{nor}}$ distributions, for the ${}^{48}$Ca+${}^{48}$Ca reaction. Stiff1, Stiff2, and Soft options are shown (blue, red, and green thick lines, respectively), together with interpolations corresponding to stiffness parameters $0.6\le \gamma \le 1.4$ (filled and empty symbols).

Figure 6

Deviation between theoretical and experimental $\Delta M_{\mathrm{nor}}$ distributions, for different choices of the symmetry potential stiffness. Dashed-dotted fuchsia histogram: Stiff2 option is used. Red dashed histogram: Stiff1 option. Blue dots: soft option. Filled area histogram: $\gamma =1.1$.