Determination of critical exponents from the multifragmentation of gold nuclei

Nuclear multifragmentation time scale and fluctuations of the largest fragment size

Distributions of the largest fragment charge, Zmax, in multifragmentation reactions around the Fermi energy can be decomposed into a sum of a Gaussian and a Gumbel distribution, whereas at much higher or lower energies one or the other distribution is asymptotically dominant. We demonstrate the same generic behavior for the largest cluster size in critical aggregation models for small systems, in or out of equilibrium, around the critical point. By analogy with the time-dependent irreversible aggregation model, we infer that Zmax distributions are characteristic of the multifragmentation time scale, which is largely determined by the onset of radial expansion in this energy range.

Liquid to vapor phase transition in excited nuclei

The thermal component of the 8 GeV/c pi+ Au data of the ISiS Collaboration is shown to follow the scaling predicted by Fisher's model when Coulomb energy is taken into account. Critical exponents tau and sigma, the critical point (p(c),rho(c),T(c)), surface energy coefficient c(0), enthalpy of evaporation DeltaH, and critical compressibility factor C(F)(c) are determined. For the first time, the experimental phase diagrams, (p,T) and (T,rho), describing the liquid vapor coexistence of finite neutral nuclear matter have been constructed.

Event-by-event analysis of proton-induced nuclear multifragmentation: determination of the phase transition universality class in a system with extreme finite-size constraints

A percolation model of nuclear fragmentation is used to interpret 10.2 GeV/c p+197Au multifragmentation data. Emphasis is put on finding signatures of a continuous nuclear matter phase transition in finite nuclear systems. Based on model calculations, corrections accounting for physical constraints of the fragment detection and sequential decay processes are derived. Strong circumstantial evidence for a continuous phase transition is found, and the values of two critical exponents, sigma = 0.5+/-0.1 and tau = 2.35+/-0.05, are extracted from the data. A critical temperature of T(c) = 8.3+/-0.2 MeV is found.

Local (in time) maximal lyapunov exponents of fragmenting drops

We analyze the dynamics of fragment formation in simulations of exploding three-dimensional Lennard-Jones hot drops, using the maximum local (in time) Lyapunov exponent (MLLE). The dependence of this exponent on the excitation energy of the system displays two different behaviors according to the stage of the dynamical evolution: one related to the highly collisional stage of the evolution, at early times, and the other related to the asymptotic state. We show that in the early, highly collisional, stage of the evolution the MLLE is an increasing function of the energy, as in an infinite-size system. On the other hand, at long times, the MLLE displays a maximum, depending mainly on the size of the resulting biggest fragment. We compare the time scale at which the MLLE's reach their asymptotic values with the characteristic time of fragment formation in phase space. Moreover, upon calculation of the maximum Lyapunov exponent (MLE) of the resulting fragments, we show that their dependence with the mass can be traced to bulk effects plus surface corrections. Using this information the asymptotic behavior of the MLLE can be understood and the fluctuations of the MLE of the whole system can be easily calculated. These fluctuations display a sudden increase for that excitation energy which produces a power-law-like asymptotic distribution of fragments.

Nuclear multifragmentation, percolation, and the fisher droplet model: common features of reducibility and thermal scaling

It is shown that the Fisher droplet model, percolation, and nuclear multifragmentation share the common features of reducibility (stochasticity in multiplicity distributions) and thermal scaling (one-fragment production probabilities are Boltzmann factors). Barriers obtained, for cluster production on percolation lattices, from the Boltzmann factors show a power-law dependence on cluster size with an exponent of 0.42+/-0.02. The EOS Collaboration Au multifragmentation data yield barriers with a power-law exponent of 0.68+/-0.03. Values of the surface energy coefficient of a low density nuclear system are also extracted.

Signals for a transition from surface to bulk emission in thermal multifragmentation

Excitation-energy-gated two-fragment correlation functions have been studied between E(*)/A = (2-9)A MeV for equilibriumlike sources formed in 8-10 GeV/c pi(-) and p+197Au reactions. Comparison with an N-body Coulomb-trajectory code shows an order of magnitude decrease in the fragment emission time in the interval E(*)/A = (2-5)A MeV, followed by a nearly constant breakup time at higher excitation energy. The decrease in emission time is strongly correlated with the onset of multifragmentation and thermally induced radial expansion, consistent with a transition from surface-dominated to bulk emission expected for spinodal decomposition.