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

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$\pi$ 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.