Critical analysis of data from peripheral Si+Sn reaction at 50 MeV/nucleon - probing N/Z degree of freedom

Evidence of nuclear liquid-gas phase transition in small mass systems A$\sim $36 has been reported previously [1], where the minimum of the critical topological exponent $\tau $ as a function of temperature of the multifragmenting source has been checked by two theoretical models which include liquid-gas phase transition, with good agreement. We will present an analysis of the fragmentation of quasiprojectiles obtained in the reactions $^{28}$Si+$^{124,112}$Sn at 30 and 50 MeV/nucleon [2]. We will show that apparent critical behavior is present also in smaller systems where the charge of the reconstructed quasiprojectiles is Z=12-15. We extracted the critical exponent $\tau $ as well as the second moment of charge distribution S$_{2}$. The minimum of $\tau $ and maximum of S$_{2}$ lie in the vicinity of excitation energy of qusiprojectiles E$^{\ast }\sim $5 MeV/nucleon. The dependence of $\tau $ and S$_{2}$ on N/Z degree of freedom of quasiprojectile will be discussed. [1] Y. G. Ma et al., Phys. Rev. \textbf{C69}, 031604(R) (2004) [2] R. Laforest et al., Phys. Rev. \textbf{C59}, 2567 (1999)