Critical behavior in light nuclear systems: Experimental aspects

Y.G. Ma , J.B. Natowitz , R. Wada , K. Hagel , J. Wang , T. Keutgen , Z. Majka , M. Murray , L. Qin , P. Smith , R. Alfaro , J. Cibor , M. Cinausero , Y. El Masri , D. Fabris , E. Fioretto , A. Keksis , M. Lunardon , A. Makeev , Nathalie Marie (1) , E. Martin , A. Martinez-Davalos , A. Menchaca-Rocha , G. Nebbia , G. Prete , V. Rizzi , A. Ruangma , D. V. Shetty , G. Souliotis , P. Staszel , M. Veselsky , G. Viesti , E.M. Winchester , S.J. Yennello

1 Cyclotron institute

An extensive experimental survey of the features of the disassembly of a small quasiprojectile system with A~36, produced in the reactions of 47 MeV/nucleon 40Ar + 27Al, 48Ti, and 58Ni, has been carried out. Nuclei in the excitation energy range of 1-9 MeV/nucleon have been investigated employing a new method to reconstruct the quasiprojectile source. At an excitation energy ∼5.6 MeV/nucleon many observables indicate the presence of maximal fluctuations in the deexcitation processes. These include the normalized second moments of the Campi plot and normalized variances of the distributions of order parameters such as the atomic number of the heaviest fragment Zmax and the total kinetic energy. The evolution of the correlation of the atomic number of the heaviest fragment with that of the second heaviest fragment and a bimodality test are also consistent with a transition in the same excitation energy region. The related phase separation parameter, Sp, shows a significant change of slope at the same excitation energy. In the same region a Δ-scaling analysis for of the heaviest fragments exhibits a transition to Δ = 1 scaling, which is predicted to characterize a disordered phase. The fragment topological structure shows that the rank-sorted fragments obey Zipf's law at the point of largest fluctuations, providing another indication of a liquid gas phase transition. The Fisher droplet model critical exponent τ ∼ 2.3 obtained from the charge distribution at the same excitation energy is close to the critical exponent of the liquid gas phase transition universality class. The caloric curve for this system shows a monotonic increase of temperature with excitation energy and no apparent plateau. The temperature at the point of maximal fluctuations is 8.3±0.5 MeV. Taking this temperature as the critical temperature and employing the caloric curve information we have extracted the critical exponents β,γ, and σ from the data. Their values are also consistent with the values of the universality class of the liquid gas phase transition. Taken together, this body of evidence strongly suggests a phase change in an equilibrated mesoscopic system at, or extremely close to, the critical point.

Domaines: Physique Nucléaire Expérimentale [nucl-ex]

Y.G. Ma, J.B. Natowitz, R. Wada, K. Hagel, J. Wang, et al.. Critical behavior in light nuclear systems: Experimental aspects. Physical Review C, 2005, 71, pp.054606. ⟨10.1103/PhysRevC.71.054606⟩. ⟨hal-00832105⟩