arXiv subjects

J. Rayford Nix

Publications and source records attributed to J. Rayford Nix.

15 recordsLinked to original sources

arXiv1998

What Invariant One-Particle Multiplicity Distributions and Two-Particle Correlations Are Telling Us about Relativistic Heavy-Ion Collisions

We use a nine-parameter expanding source model that includes special relativity, quantum statistics, resonance decays, and freeze-out on a realistic hypersurface in spacetime to analyze invariant pi+, K+, and K- one-particle multiplicity distributions and pi+ and pi- two-particle correlations in nearly central collisions of Pb + Pb at a laboratory bombarding momentum per nucleon of 158 GeV/c. The adjustable parameters of the model are determined by minimizing chi-square with a total of 2137 data points for the five types of data considered, which leads to an acceptable value of 1.017 for chi-square per degree of freedom. Compared to earlier results obtained with this model for nearly central collisions of Si + Au at a laboratory bombarding momentum per nucleon of 14.6 GeV/c, the present results indicate that in Pb + Pb collisions the freeze-out density is somewhat lower, the freeze-out temperature is slightly higher, the source at freeze-out is somewhat larger, and the longitudinal collective velocity is very poorly determined (because of the limited experimental coverage in rapidity). For both reactions, the freeze-out temperature is less than 100 MeV and the longitudinal and transverse collective velocities-which are anti-correlated with the temperature-are substantial. We reconcile these results with those in several previous analyses that yielded a much higher freeze-out temperature of approximately 140 MeV, and finally perform two additional studies with our expanding source model.

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arXiv1998

Low Freeze-out Temperature and High Collective Velocities in Relativistic Heavy-Ion Collisions

On the basis of a nine-parameter expanding source model that includes special relativity, quantum statistics, resonance decays, and freeze-out on a realistic hypersurface in spacetime, we analyze in detail invariant pi+, pi-, K+, and K- one-particle multiplicity distributions and pi+ and K+ two-particle correlations in nearly central collisions of Si + Au at a laboratory bombarding energy per nucleon of 14.6 GeV/c. By considering separately the one-particle data and the correlation data, we find that the central baryon density, nuclear temperature, transverse collective velocity, longitudinal collective velocity, and source velocity are determined primarily by one-particle multiplicity distributions and that the transverse radius, longitudinal proper time, width in proper time, and pion incoherence fraction are determined primarily by two-particle correlations. By considering separately the pion data and the kaon data, we find that although the pion freeze-out occurs somewhat later than the kaon freeze-out, the 99% confidence-level error bars associated with the two freeze-outs overlap. These and other detailed studies confirm our earlier conclusion based on the simultaneous consideration of the pion and kaon one-particle and correlation data that the freeze-out temperature is less than 100 MeV and that both the longitudinal and transverse collective velocities--which are anti-correlated with the temperature--are substantial. We also discuss the flaws in several previous analyses that yielded a much higher freeze-out temperature of approximately 140 MeV for both this reaction and other reactions involving heavier projectiles and/or higher bombarding energies.

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arXiv1997

Masses and Deformations of Neutron-Rich Nuclei

We have calculated the masses, deformations, and other properties of 8979 nuclei ranging from oxygen-16 to Z = 136, A = 339 and extending from the proton drip line to the neutron drip line on the basis of the 1992 version of the finite-range droplet model. The predicted quantities include the ground-state mass, deformation, microscopic correction, odd-proton and odd-neutron spins and parities, proton and neutron pairing gaps, binding energy, one- and two-neutron separation energies, quantities related to beta-delayed one- and two-neutron emission probabilities, beta-decay energy release and half-life with respect to Gamow-Teller decay, one- and two-proton separation energies, and alpha-decay energy release and half-life. For 1654 nuclei heavier than oxygen-16 whose masses were known experimentally in 1989 and which were included in the adjustment of model constants, the theoretical error is 0.669 MeV. For 371 additional nuclei heavier than oxygen-16 whose masses have been measured between 1989 and 1996 and which were not used in the adjustment of the model constants, the theoretical error is 0.570 MeV.

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arXiv1997

Stability and Production of Superheavy Nuclei

Beyond uranium heavy elements rapidly become increasingly unstable with respect to spontaneous fission as the proton number Z increases, because of the disruptive effect of the long-range Coulomb force. However, in the region just beyond Z = 100 magic proton and neutron numbers and the associated shell structure enhances nuclear stability sufficiently to allow observation of additional nuclei. Some thirty years ago it was speculated that an island of spherical, relatively stable superheavy nuclei would exist near the next doubly magic proton-neutron combination beyond 208:Pb, that is, at proton number Z = 114 and neutron number N = 184. Theory and experiment now show that there also exists a rock of stability in the vicinity of Z = 110 and N = 162 between the actinide region, which previously was the end of the peninsula of known elements, and the predicted island of spherical superheavy nuclei slightly southwest of the magic numbers Z = 114 and N = 184. We review here the stability properties of the heavy region of nuclei. Just as the decay properties of nuclei in the heavy region depend strongly on shell structure, this structure also dramatically affects the fusion entrance channel. The six most recently discovered new elements were all formed in cold-fusion reactions. We discuss here the effect of the doubly magic structure of the target in cold-fusion reactions on the fusion barrier and on dissipation.

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arXiv1996

From Nuclear Astrophysics to Superheavy Elements: Predictions for Nuclei Beyond Our Shores

Properties of 8,979 nuclei ranging from oxygen-16 to Z = 136, A = 339 and extending from the proton drip line to the neutron drip line have been calculated by use of the 1992 version of the finite-range droplet model. The calculated quantities include the ground-state mass, deformation, microscopic correction, odd-proton and odd-neutron spins and parities, proton and neutron pairing gaps, binding energy, one- and two-neutron separation energies, quantities related to beta-delayed one- and two-neutron emission probabilities, beta-decay energy release and half-life with respect to Gamow-Teller decay, one- and two-proton separation energies, and alpha-decay energy release and half-life. For 1,654 nuclei heavier than oxygen-16 whose masses were known experimentally in 1989 and which were included in the adjustment of model constants, the theoretical error is 0.669 MeV. For 371 additional nuclei heavier than oxygen-16 whose masses have been measured between 1989 and 1996 and which were not used in the adjustment of the model constants, the theoretical error is 0.570 MeV. We also discuss the extrapolateability of two other recent global models of the macroscopic-microscopic type, and conclude with a brief discussion of the recently discovered rock of metastable superheavy nuclei near Z = 110, A = 272 that had been correctly predicted by macroscopic-microscopic models.

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arXiv1996

Realistic Expanding Source Model for Invariant One-Particle Multiplicity Distributions and Two-Particle Correlations in Relativistic Heavy-Ion Collisions

We present a realistic expanding source model with nine parameters that are necessary and sufficient to describe the main physics occuring during hydrodynamical freezeout of the excited hadronic matter produced in relativistic heavy-ion collisions. As a first test of the model, we compare it to data from central Si + Au collisions at p_lab/A = 14.6 GeV/c measured in experiment E-802 at the AGS. An overall chi-square per degree of freedom of 1.055 is achieved for a fit to 1416 data points involving invariant pi^+, pi^-, K^+, and K^- one-particle multiplicity distributions and pi^+ and K^+ two-particle correlations. The 99-percent-confidence region of parameter space is identified, leading to one-dimensional error estimates on the nine fitted parameters and other calculated physical quantities. Three of the most important results are the freezeout temperature, longitudinal proper time, and baryon density along the symmetry axis. For these we find values of 92.9 +/- 4.4 MeV, 8.2 +/- 2.2 fm/c, and 0.0222 + 0.0096 / - 0.0069 fm^-3, respectively.

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arXiv1996

Realistic Expanding Source Model for Relativistic Heavy-Ion Collisions

We introduce a new realistic expanding source model for invariant one-particle multiplicity distributions and two-particle correlations in nearly central relativistic heavy-ion collisions that contains nine adjustable parameters, which are necessary and sufficient to properly characterize the gross properties of the source during its freezeout from a hydrodynamical fluid into a collection of noninteracting, free-streaming hadrons. These nine physically relevant parameters fall into three categories of three parameters each, with the first category corresponding to the source's longitudinal motion, the second category corresponding to its transverse motion, and the third category corresponding to its intrinsic properties. As an initial application, we apply our model to the analysis of invariant pi^+, pi^-, K^+, and K^- one-particle multiplicity distributions and pi^+ and K^+ two-particle correlations for nearly central Si + Au collisions at p_lab/A = 14.6 GeV/c. In a minimization with 1416 data points, the resulting chi-square is 1484.6, which corresponds to an acceptable chi-square per degree of freedom of 1.055. We list the values of the nine parameters determined this way plus several additional calculated freezeout quantities of physical interest, along with their uncertainties at 99% confidence limits.

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arXiv1995

Collisions of Deformed Nuclei and Superheavy-Element Production

A detailed understanding of complete fusion cross sections in heavy-ion collisions requires a consideration of the effects of the deformation of the projectile and target. Our aim here is to show that deformation and orientation of the colliding nuclei have a very significant effect on the fusion-barrier height and on the compactness of the touching configuration. To facilitate discussions of fusion configurations of deformed nuclei, we develop a classification scheme and introduce a notation convention for these configurations. We discuss particular deformations and orientations that lead to compact touching configurations and to fusion-barrier heights that correspond to fairly low excitation energies of the compound systems. Such configurations should be the most favorable for producing superheavy elements. We analyse a few projectile-target combinations whose deformations allow favorable entrance-channel configurations and whose proton and neutron numbers lead to compound systems in a part of the superheavy region where alpha half-lives are calculated to be observable, that is, longer than 1 microsecond.

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arXiv1995

Collisions of Deformed Nuclei: A Path to the Far Side of the Superheavy Island

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arXiv1995

Extracting Source Parameters from Gaussian Fits to Two-Particle Correlations

Using a quadratic saddle-point approximation, we show how information about a particle-emitting source can be extracted from gaussian fits to two-particle correlation data. Although the formalism is completely general, extraction of the relevant parameters is much simpler for sources within an interesting class of azimuthally symmetric models. After discussing the standard fitting procedure, we introduce a new gaussian fitting procedure which is an azimuthally symmetric generalization of the Yano-Koonin formalism for spherically symmetric sources. This new fitting procedure has the advantage that in addition to being able to measure source parameters in a fixed frame or the longitudinally co-moving system, it can also measure these parameters in the local rest frame of the source.

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arXiv1995

Macroscopic-Microscopic Mass Models

We discuss recent developments in macroscopic-microscopic mass models, including the 1992 finite-range droplet model, the 1992 extended-Thomas-Fermi Strutinsky-integral model, and the 1994 Thomas-Fermi model, with particular emphasis on how well they extrapolate to new regions of nuclei. We also address what recent developments in macroscopic-microscopic mass models are teaching us about such physically relevant issues as the nuclear curvature energy, a new congruence energy arising from a greater-than-average overlap of neutron and proton wave functions, the nuclear incompressibility coefficient, and the Coulomb redistribution energy arising from a central density depression. We conclude with a brief discussion of the recently discovered rock of metastable superheavy nuclei near 272:110 that had been correctly predicted by macroscopic-microscopic models, along with a possible new tack for reaching an island near 290:110 beyond our present horizon.

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arXiv1994

Particle-Production Mechanism in Relativistic Heavy-Ion Collisions

We discuss the production of particles in relativistic heavy-ion collisions through the mechanism of massive bremsstrahlung, in which massive mesons are emitted during rapid nucleon acceleration. This mechanism is described within the framework of classical hadrodynamics for extended nucleons, corresponding to nucleons of finite size interacting with massive meson fields. This new theory provides a natural covariant microscopic approach to relativistic heavy-ion collisions that includes automatically spacetime nonlocality and retardation, nonequilibrium phenomena, interactions among all nucleons, and particle production. Inclusion of the finite nucleon size cures the difficulties with preacceleration and runaway solutions that have plagued the classical theory of self-interacting point particles. For the soft reactions that dominate nucleon-nucleon collisions, a significant fraction of the incident center-of-mass energy is radiated through massive bremsstrahlung. In the present version of the theory, this radiated energy is in the form of neutral scalar ($σ$) and neutral vector ($ω$) mesons, which subsequently decay primarily into pions with some photons also. Additional meson fields that are known to be important from nucleon-nucleon scattering experiments should be incorporated in the future, in which case the radiated energy would also contain isovector pseudoscalar ($π^+$, $π^-$, $π^0$), isovector scalar ($δ^+$, $δ^-$, $δ^0$), isovector vector ($ρ^+$, $ρ^-$, $ρ^0$), and neutral pseudoscalar ($η$) mesons.

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arXiv1994

Classical Hadrodynamics: A New Approach to Ultrarelativistic Heavy-Ion Collisions

We discuss a new approach to ultrarelativistic heavy-ion collisions based on classical hadrodynamics for extended nucleons, corresponding to nucleons of finite size interacting with massive meson fields. This new theory provides a natural covariant microscopic approach that includes automatically spacetime nonlocality and retardation, nonequilibrium phenomena, interactions among all nucleons and particle production. In the current version of our theory, we consider $N$ extended unexcited nucleons interacting with massive neutral scalar ($σ$) and neutral vector ($ω$) meson fields. The resulting classical relativistic many-body equations of motion are solved numerically without further approximation for soft nucleon-nucleon collisions at $p_{\rm lab}$ = 14.6, 30, 60, 100 and 200 GeV/$c$ to yield the transverse momentum imparted to the nucleons. For the future development of the theory, the isovector pseudoscalar ($π^+$, $π^-$, $π^0$), isovector scalar ($δ^+$, $δ^-$, $δ^0$), isovector vector ($ρ^+$, $ρ^-$, $ρ^0$) and neutral pseudoscalar ($η$) meson fields that are known to be important from nucleon-nucleon scattering experiments should be incorporated. In addition, the effects of quantum uncertainty on the equations of motion should be included by use of techniques analogous to those used by Moniz and Sharp for nonrelativistic quantum electrodynamics.

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arXiv1994

Quantum Size Effects in Classical Hadrodynamics

We discuss future directions in the development of classical hadrodynamics for extended nucleons, corresponding to nucleons of finite size interacting with massive meson fields. This new theory provides a natural covariant microscopic approach to relativistic nucleus-nucleus collisions that includes automatically spacetime nonlocality and retardation, nonequilibrium phenomena, interactions among all nucleons, and particle production. The present version of our theory includes only the neutral scalar ($σ$) and neutral vector ($ω$) meson fields. In the future, additional isovector pseudoscalar ($π^+$,~$π^-$,~$π^0$), isovector vector ($ρ^+$,~$ρ^-$,~$ρ^0$), and neutral pseudoscalar ($η$) meson fields should be incorporated. Quantum size effects should be included in the equations of motion by use of the spreading function of Moniz and Sharp, which generates an effective nucleon mass density smeared out over a Compton wavelength. However, unlike the situation in electrodynamics, the Compton wavelength of the nucleon is small compared to its radius, so that effects due to the intrinsic size of the nucleon dominate.

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arXiv1993

Applications of a Global Nuclear-Structure Model to Studies of the Heaviest Elements

We present some new results on heavy-element nuclear-structure properties calculated on the basis of the finite-range droplet model and folded-Yukawa single-particle potential. Specifically, we discuss calculations of nuclear ground-state masses and microscopic corrections, $α$-decay properties, $β$-decay properties, fission potential-energy surfaces, and spontaneous-fission half-lives. These results, obtained in a global nuclear-structure approach, are particularly reliable for describing the stability properties of the heaviest elements.

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