3H/3He Squeeze-Out - A Signature of the Fireball’s Isospin Distribution?

The Phase I FOPI data evidence for highly central Au + Au collisions at 100, 150 and 250 A·MeV systematically larger mean kinetic energies for He fragments relative to the H values [1]. The EOS Collaboration find the same trend with somewhat larger size [2]. Microscopic transport models [3] and hybrid hydrodynamical models, including pure Coulomb repulsion at the freeze-out moment [4] do not succeed to explain quantitatively this difference. Similar findings are recently reported by the INDRA Collaboration [5] for Xe + Sn at 50 A·MeV. At a higher incident energy 1.15 A·GeV it seems that this difference disappears. Both types of fragments have the same mean kinetic energy, following a trend as a function of the fragment mass only, which is specific for the emission from an expanding source.