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G. J. Wozniak

Publications and source records attributed to G. J. Wozniak.

At least 19 recordsLinked to original sources
arXiv2003

Precise Barriers and Shell Effects: a New Inroad to Fission Saddle Point Spectroscopy

Fission excitation functions have been measured for a chain of neighboring compound nuclei, from 207Po to 212Po. We present a new analysis which provides a determination of the fission barriers and ground state shell effects with nearly spectroscopic accuracy. The improved accuracy achieved in this analysis may lead to a future detailed exploration of the saddle mass surface and its spectroscopy. The sensitivity of the fission probabilities on shell effects extends to excitation energies of 150 MeV and negates recent claims for the disappearance of shell corrections due to collective effects.

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arXiv2002

Liquid-vapor phase transition in nuclei or compound nucleus decay?

Recent analyses of multifragmentation in terms of Fisher's model and the related construction of a phase diagram brings forth the problem of the true existence of the vapor phase and the meaning of its associated pressure. Our analysis shows that a thermal emission picture is equivalent to a Fisher-like equilibrium description which avoids the problem of the vapor and explains the recently observed Boltzmann-like distribution of the emission times. In this picture a simple Fermi gas thermometric relation is naturally justified. Low energy compound nucleus emission of intermediate mass fragments is shown to scale according to Fisher's formula and can be simultaneously fit with the much higher energy ISiS multifragmentation data.

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arXiv2002

Negative heat capacities and first order phase transitions in nuclei

Anomalous negative heat capacities have been claimed as indicators of first order phase transitions in finite systems in general, and fornuclear systems in particular. A thermodynamic approach allowing for all Q value terms is used to evaluate heat capacities in finitevan der Waals fluids and finite lattice systems in the coexistence region. Fictitious large effects and negative heat capacities are observed in lattice systems when periodic boundary conditions are introduced. Small anomalous effects are predicted for small drops and for finite lattice systems. A straightforward application of the analysis to nuclei shows that negative heat capacities cannot be observed for A>60.

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arXiv2002

Cnstructing the phase diagram of finite neutral nuclear matter

The fragment yields from the multifragmentation of gold, lanthanum and krypton nuclei obtained by the EOS Collaboration are examined in terms of Fisher's droplet formalism modifed to account for Coulomb energy. The critical exponents sigma and tau and the surface energy coefficient c0 are obtained. Estimates are made of the pressure-temperature and temperature-density coexistence curves of finite neutral nuclear matter as well as the location of the critical point.

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arXiv2001

Heavy Residue Formation in 20 MeV/nucleon 197Au + 90Zr collisions

The yields and velocity distributions of heavy residues and fission fragments from the reaction of 20 MeV/nucleon 197Au + 90Zr have been measured using the MSU A1200 fragment separator. A bimodal distribution of residues is observed, with one group, resulting from peripheral collisions, having fragment mass numbers A=160-200, while the other group, resulting from ``hard'' collisions, has A=120-160. This latter group of residues can be distinguished from fission fragments by their lower velocities. A model combining deep-inelastic transfer and incomplete fusion for the primary interaction stage and a statistical evaporation code for the deexcitation stage has been used to describe the properties of the product distributions.

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arXiv2001

The liquid to vapor phase transition in excited nuclei

For many years it has been speculated that excited nuclei would undergo a liquid to vapor phase transition. For even longer, it has been known that clusterization in a vapor carries direct information on the liquid- vapor equilibrium according to Fisher's droplet model. Now the thermal component of the 8 GeV/c pion + 197Au multifragmentation data of the ISiS Collaboration is shown to follow the scaling predicted by Fisher's model, thus providing the strongest evidence yet of the liquid to vapor phase transition.

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arXiv2001

The three-dimensional Ising model: A paradigm of liquid-vapor coexistence in nuclear multifragmentation

Clusters in the three-dimensional Ising model rigorously obey reducibility and thermal scaling up to the critical temperature. The barriers extracted from Arrhenius plots depend on the cluster size as $B \propto A^σ$ where $σ$ is a critical exponent relating the cluster size to the cluster surface. All the Arrhenius plots collapse into a single Fisher-like scaling function indicating liquid-vapor-like phase coexistence and the univariant equilibrium between percolating clusters and finite clusters. The compelling similarity with nuclear multifragmentation is discussed.

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arXiv2001

Negative heat capacities and first order phase transitions in nuclei and other mesoscopic systems

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 drop's radius, and surface, with evaporation.

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arXiv2000

Statistical Exploration of Fragmentation Phase Space Source Sizes in Nuclear Multifragmentation

The multiplicity distributions for individual fragment Z values in nuclear multifragmentation are binomial. The extracted maximum value of the multiplicity is found to depend on Z according to m=Z_0/Z, where Z_0 is the source size. This is shown to be a strong indication of statistical coverage of fragmentation phase space. The inferred source sizes coincide with those extracted from the analysis of fixed multiplicity charge distributions.

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arXiv2000

Nuclear multifragmentation, percolation and the Fisher Droplet Model: common features of reducibility and thermal scaling

It is shown that the Fisher Droplet Model (FDM), 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 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.

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arXiv1999

A statistical interpretation of the correlation between intermediate mass fragment multiplicity and transverse energy

Multifragment emission following Xe+Au collisions at 30, 40, 50 and 60 AMeV has been studied with multidetector systems covering nearly 4-pi in solid angle. The correlations of both the intermediate mass fragment and light charged particle multiplicities with the transverse energy are explored. A comparison is made with results from a similar system, Xe+Bi at 28 AMeV. The experimental trends are compared to statistical model predictions.

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arXiv1999

Resilient Reducibility in Nuclear Multifragmentation

The resilience to averaging over an initial energy distribution of reducibility and thermal scaling observed in nuclear multifragmentation is studied. Poissonian reducibility and the associated thermal scaling of the mean are shown to be robust. Binomial reducibility and thermal scaling of the elementary probability are robust under a broad range of conditions. The experimental data do not show any indication of deviation due to averaging.

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arXiv1998

Multifragmentation at Intermediate Energy: Dynamics or Statistics?

To make a statement about the nature and mechanism of fragmentation, it is necessary to probe directly any competition, or lack thereof, between the emission of various particle species as a function of excitation energy. The task is then to find a global observable that best follows the increase in excitation energy or dissipated energy. In the following, we will consider two contradictory claims that have been advanced recently: 1) the claim for a predominantly dynamical fragment production mechanism; and 2) the claim for a dominant statistical and thermal process. We will present a new analysis in terms of Poissonian reducibility and thermal scaling, which addresses some of the criticisms of the binomial analysis.

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arXiv1998

Z-dependent Barriers in Multifragmentation from Poissonian Reducibility and Thermal Scaling

We explore the natural limit of binomial reducibility in nuclear multifragmentation by constructing excitation functions for intermediate mass fragments (IMF) of a given element Z. The resulting multiplicity distributions for each window of transverse energy are Poissonian. Thermal scaling is observed in the linear Arrhenius plots made from the average multiplicity of each element. ``Emission barriers'' are extracted from the slopes of the Arrhenius plots and their possible origin is discussed.

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arXiv1997

Reduciblity, Arrhenius plots and the Uroboros Dragon, a reply to the preprint "Correlations in Nuclear Arrhenius-Type Plots" by M.B. Tsang and P. Danielewicz

In a recent preprint by Tsang and Danielewicz, the authors attempt to give alternative or trivial explanations for the reducible and "thermal" nature of the intermediate mass fragment excitation functions reported previously (Phys. Rev. Lett. 74, 1530 (1995), Phys. Lett B 361, 25 (1995), Phys. Rep. 287, 249 (1997)). We demonstrate that their proposed "self-correlation" explanation for linear Arrhenius plots is based upon a flawed autocorrelation analysis involving circular reasoning.

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arXiv1997

3He- and 4He-induced nuclear fission - a test of the transition state method

Fission in 3He and 4He induced reactions at excitation energies between the fission barrier and 140 MeV has been investigated. Twentythree fission excitation functions of various compound nuclei in different mass regions are shown to scale exactly according to the transition state prediction once the shell effects are accounted for. New precise measurements of excitation functions in a mass region where shell effects are very strong, allow one to test the predictions with an even higher accuracy. The fact that no deviations from the transition state method have been observed within the experimentally investigated excitation energy regime allows one to assign limits for the fission transient time. The precise measurement of fission excitation functions of neighboring isotopes enables us to experimentally estimate the first chance fission probability. Even if only first chance fission is investigated, no evidence for fission transient times larger than 30 zs can be found.

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arXiv1996

Stable Coulomb bubbles ?

Coulomb bubbles, though stable against monopole displacement, are unstable at least with respect to quadrupole and octupole distortions. We show that there exists a temperature at which the pressure of the vapor filling the bubble stabilizes all the radial modes. In extremely thin bubbles, the crispation modes become unstable due to the surface-surface interaction.

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