Transparency in Central Xe+Sn Collisions

The formation of longitudinally elongated composite systems is a general feature of central heavy-ion collisions [1]. It can be the result of a transparency, i.e. of an incomplete stopping of the incoming projectile by the target, as, e.g., observed by the FOPI collaboration for Zr+Ru collisions at 400 A.MeV [2]. Here, we report new data, measured with INDRA at GSI, for the Xe+Sn collisions at 100 A.MeV, obtained by cross-bombarding isotopically separated Sn targets with Xe projectiles. The total transverse energy of light charged particles (Z=1,2) has been used to select central collisions. We present in Fig. 1, left panel, the yield ratios obtained with various light isotopes emitted at sideward angles in the center-of-mass (80 ≤ θcm ≤ 100) as a function of the total N/Z of the system. We see in all cases a remarkable exponential dependence which reflects, as expected at midrapidity, a full mixing of the target and the projectile [3]. Slope parameters of the exponential fits are proportional to the difference in N between the isotopes forming the ratios (Fig. 2), suggesting a statistical type of emission in the grand-canonical description [4]. Ratios obtained at forward angles (θcm ≤ 34), presented in Fig. 1, right panel, deviate from the exponential law and exhibit a characteristic step pattern common to all the ratios. The difference in yield ratio is larger if the projectile is changed and smaller if the target is changed, indicating an incomplete isospin equilibration for the reaction products at forward angles. One can derive from these deviations a transparency coefficient τ , with regard to a given isotope ratio, using the following relation: τ = τP − τT τP + τT where τP and τT are the proportions of N/Z transfer at forward angles, from the projectile and the target, respectively. For example, τP is obtained by comparing the measured ratios from two reactions with different projectiles on the same target with the expectation for pure projectile matter according to: