Isotopic Transparency in Central Xe+Sn Collisions at 100 MeV/nucleon

Abstract

A new method, based on comparing isotopic yield ratios measured at forward and sideward polar angles and on cross-bombarding heavy nuclei with different neutron-to-proton ratios, is used to quantify the stopping power of nuclear matter in heavy-ion collisions. For central collisions of isotopically separated $^{124,129}$Xe+$^{112,124}$Sn at 100~MeV/nucleon bombarding energy, measured with the 4$π$ multidetector INDRA at GSI, a moderate transparency is deduced for hydrogen isotopes, whereas for heavier fragmentation products with atomic number $Z \ge 3$ a high transparency exceeding 50\% is observed. An anomalously large transparency is found for alpha particles, and possible explanations are presented.

Figures (5)

• Figure 1: Isotropy ratio $R_p$ as a function of the fragment atomic number $Z$ for the four combinations of $^{124,129}$Xe+$^{112,124}$Sn reactions at 100 MeV/nucleon incident energy in central collisions selected according to $M_{c}$ (left panel) and $E_{\perp 12}$ (right panel).
• Figure 2: Two-dimensional invariant distributions in the transverse versus longitudinal rapidity space of deuterons, $^4$He, and $^7$Li isotopes (top to bottom) for Xe+Sn central collisions at 100 MeV/nucleon incident energy. Dashed lines represent the forward and sideward acceptance cuts in the center-of-mass frame ($\theta_{\rm cm}\leq34^{\circ}$ and $80^{\circ}\leq\theta_{\rm cm}\leq100^{\circ}$, respectively). Full lines depict the geometrical acceptance of the 17 rings of the INDRA multidetector, including high and low energy thresholds. The arrows below the x-axes at $y_{\|} = 0.46$ and 0.23 mark the longitudinal rapidities of the projectiles and of the center-of-mass system assuming mass symmetry, respectively.
• Figure 3: Isotope yield ratios as functions of the total $N/Z$ of the system in central collisions, selected according to $M_c$, of $^{124,129}$Xe+$^{112,124}$Sn at 100 MeV/nucleon, for sideward angles (left panel, $80^{\circ}\leq\theta_{\rm cm}\leq100^{\circ}$) and forward angles (right panel, $\theta_{\rm cm}\leq 34^{\circ}$). To avoid overlaps of data symbols, the indicated factors were applied to the measured d/p and t/d ratios. Statistical errors range from below 1% for light particles up to $\approx 3$ or 4% for the Li and Be ratios and are smaller than the size of the symbols. The lines represent exponential fits to the ratios. In the right panel, to facilitate comparisons, fits are constrained to reproduce the slopes found in the left panel.
• Figure 4: Slopes $\alpha$ of exponential fits extracted from Fig. \ref{['fig:fig3']}, left panel (sideward emissions, $M_c$ selection), as a function of the difference $\Delta N$ in neutron number of the isotope pair. The t/$^3$He isobar ratio is not included. The displayed errors are those returned by the fit routine. The dotted line represents the weighted mean of the rise of $\alpha$ with $\Delta N$. The symbols are chosen as in Fig. \ref{['fig:fig3']}. For better visibility, the symbols related to the t/d and $^4$He/$^3$He ratios are slightly shifted horizontally.
• Figure 5: Transparency $\tau$ of forward emissions for the studied isotope pairs produced in central collisions of $^{124,129}$Xe+$^{112,124}$Sn at 100 MeV/nucleon selected according to $M_{c}$ (left panel) and $E_{\perp 12}$ (right panel). The symbols are chosen as in Figs. \ref{['fig:fig3']} and \ref{['fig:fig4']} and placed at the mean mass values of the isotope pairs. The numerical values are given in Table \ref{['tab:table3']}.