Neutron-to-proton ratios of quasiprojectile and midrapidity emission in the 64Zn+64Zn reaction at 45 MeV/nucleon

Simultaneous measurement of both neutrons and charged particles emitted in the reaction Zn-64 + Zn-64 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 Zn-64 + Zn-64 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.