G. J. Wozniak

The 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 Fishers 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.

Time-scale and Branching Ratios in Sequential Multifragmentation

Abstract Experimental intermediate-mass-fragment multiplicity distributions are shown to be binomial at all excitation energies. From these distributions a single binary event probability can be extracted that has the thermal dependence p = exp [ -B T ] . Thus, it is inferred that multifragmentation is a sequence of thermal binary events. The increase of p with excitation energy implies a corresponding contraction of the time-scale and explains recently observed fragment-fragment Coulomb correlations.

Calibration of the response function of CsI(Tl) scintillators to intermediate-energy heavy ions

Abstract The response function of 2-cm-thick CsI(Tl) scintillators with photodiode readouts were studied by directly exposing the detectors to beams of heavy ions (2≤ Z ≤36) with energy up to 25 MeV/u. The dependence of the light output on the energy ( E ) as well as on the atomic number and the mass of the ion is analyzed and discussed, and a parameterization of the light output as a function of Z and E is proposed.

Sources of complex fragment emission in lanthanum-induced reactions at E/A = 14.7 and 18.0 MeV

Abstract Complex fragments with 4 ⩽ Z ≲ 50 have been detected in the reactions of 139 La + 12 C and 27 Al at E / A = 14.7 and 18.0 MeV. From the measured angular distributions, the cross sections for the isotropic, target-like and projectile-like components were extracted. The roles of deep-inelastic, fast-fission, and incomplete fusion processes, and the statistical, compound nucleus emission of complex fragments are discussed.

Constructing the phase diagram of finite neutral nuclear matter

Author(s): Elliott, J.B.; Moretto, L.G.; Phair, L.; Wozniak, G.L.; Albergo, S.; Bieser, F.; Brady, F.P.; Caccia, Z.; Cebra, D.A.; Chacon, A.D.; Chance, J.L.; Choi, Y.; Costa, S.; Gilkes, M.L.; Hauger, J.A.; Hirsch, A.S.; Hjort, E.L.; Insolia, A.; Justice, M.; Keane, D.; Kintner, J.C.; Lindenstruth, V.; Lisa, M.A.; Matis, H.S.; McMahan, M.; McParland, C.; Muller, W.F.J.; Olson, D.L.; Partlan, M.D.; Porile, N.T.; Potenza, R.; Rai, G.; Rasmussen, J.; Ritter, H.G.; Romanski, J.; Romero, J.L.; Russo, G.V.; Sann, H.; Scharenberg, R.P.; Scott, A.; Shao, Y.; Srivastava, B.K.; Symons, T.J.M.; Tincknell, M.; Tuve, C.; Wang, S.; Warren, P.; Wieman, H.H.; Wienold, T.; Wolf, K.

Reducibility and Thermal Scaling in Nuclear Multifragmentation

Abstract Recent studies have revealed the existence of a number of reducibility and thermal scaling properties in nuclear multifragmentation. The probability of emitting n -fragments is found to be reducible to the probability of emitting a single fragment through the binomial expression. The resulting one fragment probability shows thermal scaling by producing linear Arrhenius plots. Similarly, the charge distributions associated with n -fragment emission are reducible to the one-fragment charge distribution. Thermal scaling is also observed. The reducibility equation contains a constant whose value, zero or positive, can be related to a univariant (two phase) or bivariant (one-phase) regime. The light fragment particle-particle angular correlations also show reducibility to the single-particle angular distributions as well as thermal scaling . A mass scaling associated with the angular correlations suggests emission from several small sources ( A ≈ 20). The limits of applicability of scaling and reducibility are discussed as well as their implications for the mechanism of multifragmentation.

Transition state rates and mass asymmetric fission barriers of compound nuclei 90,94,98Mo

Abstract Excitation functions were measured for complex fragments with atomic number Z = 5–25 emitted from the compound nuclei 90,94,98Mo produced in the reactions 78,82,86 Kr + 12 C . Mass-asymmetric fission barriers were extracted by fitting the excitation functions with a transition state formalism. The extracted barriers are several MeV higher on average than the calculations of the Rotating Finite-Range Model and substantially lower than predicted by the Rotating Liquid Drop Model. The symmetric fission barriers measured support the hypothesis of a congruence term that doubles for the fission of strongly indented saddle-point shapes. The excitation functions were analyzed to search for atomic number Z- and energy E-dependent deviations from transition-state-method predictions. All of the measured excitation functions can be scaled onto a single universal straight line according to the transition-state predictions. No Z- and/or E-dependent effects that could be attributed to transient effects are visible.

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.

Negative heat capacities and first order phase transitions in nuclei

The origin of predicted and observed anomalies in caloric curves of nuclei and other mesoscopic systems is investigated. It is shown that a straightforward thermodynamical treatment of an evaporating liquid drop leads to a backbending in the caloric curve and to negative specific heats in the two phase coexistence region. The cause is found not in the generation of additional surface, but in the progressive reduction of the drops radius, and surface, with evaporation.

Dynamics and statistics in multifragment production

Abstract Complex fragment production in intermediate-energy heavy-ion reactions is described by coupling the dynamics of the collision stage with the subsequent statistical deexcitation stage. The simulations are compared to experimental data from the reaction 139La+27Al at 55 MeV/u. Many of the features observed in the inclusive charge distribution, as well as in the multifragment coincidence events are reproduced.