Radial flow in central Al+Al and Ni+Ni reactions at 2 AGeV

The question whether a global freeze-out occurs in heavy-ion collisions below and up to energies of a few AGeV and to which extent this phenomenon is embedded into a collective radial expansion has been studied by various experiments [1, 2, 3]. Measurements performed at BEVALAC [1] indicate that the emission of light particles (p, d, t, He, α) around mid-rapidity in Au+Au collisions at 1 AGeV is well described by a thermal source of 81 MeV with a radial expansion velocity of βr = 0.32. A similar investigation was done at SIS in the lighter Ni+Ni system at energies up to 2 AGeV [3]. The combined analysis of π−, p, d yielded values of T = 92 MeV and βr = 0.32 for the highest collision energy. A first attempt to incorporate strange particles into this framework was done in [2]. The study of Λ production in Ni+Cu reactions at 2 AGeV showed a temperature of T = 51 MeV and βr = 0.36 in the mid-rapidity region. However, the statistics was limited and the phase space coverage rather small. Recent advances in data processing now open up the possibility to collect statistics of several 100 million central (10-15%) heavy-ion collisions within a few weeks of beam time. FOPI was able to record about 110 million events of Ni+Ni collisions at 1.93 AGeV in 2003 and, recently, more than 300 million events of Al+Al reactions at 1.90 AGeV. By now huge data samples with 5-10·10 K S or Λ (depending on the selection criteria) are available.