Neutron-to-proton ratios of quasiprojectile and midrapidity emission in the ${}^{64}\mathrm{Zn}$ + ${}^{64}\mathrm{Zn}$ reaction at 45 MeV/nucleon

Simultaneous measurement of both neutrons and charged particles emitted in the reaction ${}^{64}\mathrm{Zn}$ + ${}^{64}\mathrm{Zn}$ at 45 MeV/nucleon allows comparison of the neutron to proton ratio at midrapidity with that at projectile rapidity. The evolution of $N/Z$ in both rapidity regimes with increasing centrality is examined. For the completely reconstructed midrapidity material one finds that the neutron to proton ratio is above that of the overall ${}^{64}\mathrm{Zn}$ + ${}^{64}\mathrm{Zn}$ system. In contrast, the reconstructed ratio for the quasiprojectile is below that of the overall system. This difference provides the most complete evidence to date of neutron enrichment of midrapidity nuclear matter at the expense of the quasiprojectile.

Figure 1

Total relative velocity between a cluster and the QP residue, $V_{\mathrm{rel}}$, backward distribution (clear) and forward $V_{\mathrm{rel}}$ distribution backward-reflected (shadowed) for $Z=1$ isotopes coupled with a QP residue of $Z=14$ (upper panel) and for $Z=2$ isotopes coupled with a QP residue of $Z=21$ (lower panel).

Figure 2

Multisource fit for free neutrons (upper panel) and free protons (lower panel) at selected angles. Selected centrality classes defined with the charge of the QP residue are shown. QP (dashed lines), QT (dotted lines), MRM (dotted-dashed lines), and total (full lines) contributions are illustrated.

Figure 3

Free-neutron (upper panel) and proton (lower panel) average multiplicities as a function of the projectile charge minus the average QP residue charge. QP (dots) and MRM (squares) multiplicities are illustrated. Solid lines are fitted to the obtained values and dashed lines to the upper and lower uncertainties.

Figure 4

Upper panel: Average charge of the QP and MRM as a function of the charge difference between the projectile and the residue. Lower panel: Average $N/Z$ ratios of QP (solid circles) and MRM (solid squares) as a function of the charge difference between the projectile and the residue. The open symbols correspond to the clusters. Thin dotted line: original $N/Z$ of the projectile and target. Errors reported reflect the multisource fit uncertainty for free nucleons.