Isoscaling in spectator fragmentation at relativistic energies

The phenomenon of isoscaling is receiving considerable interest because of its relation with the symmetry term in statistical [1] and dynamical [2] descriptions of fragmentation reactions. As reported recently, the symmetry term coefficient γ in the liquid-drop description of the nascent fragments appears to be decreasing from the standard value γ = 25 MeV for isolated nuclei to about 15 MeV or lower for fragments in the hot environment of multifragmentation processes [3, 4]. Similar results were obtained by other groups [5] and interpreted as caused by fragment modifications [6] at the low-density chemical freeze-out [7]. The present study is based on the data collected recently with the ALADIN spectromer, using primary beams of Sn and neutron-poor secondary beams of Sn and La delivered by the FRS fragment separator, all with 600 A MeV incident energy, and natural Sn targets [8, 9].