Bulletin of the American Physical Society
2007 Annual Meeting of the Division of Nuclear Physics
Volume 52, Number 10
Wednesday–Saturday, October 10–13, 2007; Newport News, Virginia
Session BF: Nuclear Theory |
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Chair: D. J. Dean, Oak Ridge National Laboratory Room: Newport News Marriott at City Center Pearl Salon II |
Thursday, October 11, 2007 2:00PM - 2:12PM |
BF.00001: Defeating The Sign Problem in the Auxiliary-Field Monte Carlo Method for Nuclear Structure: The Shifted-Contour Method Gergana Stoitcheva, Erich Ormand Configuration interaction (CI) methods, which rely on diagonalizing the Hamiltonian within a basis, are often used to develop a fully microscopic description for quantum many-body systems. CI methods, however, are limited in their applicability because the basis dimension grows dramatically with particle number. Since the computational effort for the Auxiliary-field Monte Carlo (AFMC) method scales more gently with particle number, it is a natural approach for large-scale problems. But, AFMC is often crippled by the notorious sign problem, which essentially makes the Monte Carlo sampling impossible. The sign problem substantially limits the efficacy of the AFMC method, and has limited nuclear applications to only even-particle systems with simple schematic interactions or the extrapolation method. We report a novel, but straightforward, solution to the sign problem: the shifted-contour method. We show exact results for sd- and fp-shell nuclei with fully realistic interactions without reliance on extrapolations. [Preview Abstract] |
Thursday, October 11, 2007 2:12PM - 2:24PM |
BF.00002: Defeating The Sign Problem in the Auxiliary-Field Monte Carlo Method for Nuclear Structure: Level Densities and Other Properties Erich Ormand, Gergana Stoitcheva The auxiliary-field Monte Carlo method (AFMC) within the thermal formalism permits an opportunity to probe structure properties such as nuclear level densities and Gamow-Teller strength distributions. These properties play an essential role for estimates of reaction rates in various astrophysical nucleosynthesis processes. Most conventional calculations of the nuclear level density are based on the back-shifted Fermi gas model. These calculations involve empirical parameters that can lead to large uncertainties in determining the reaction rates. The AFMC is the only approach that allows the determination of nuclear properties at finite temperature that includes the full effect of correlations among the valence nucleons in large model spaces. Using the shifted-contour method to defeat the sign problem we report on the first results of various nuclear properties, including nuclear level densities, using fully realistic shell-model interactions. [Preview Abstract] |
Thursday, October 11, 2007 2:24PM - 2:36PM |
BF.00003: Shell-Model Effective Interactions from the No-Core Shell Model Bruce Barrett, Alexander Lisetskiy, Michael Kruse, Petr Navr\'atil, James Vary The extension of the No-Core Shell Model (NCSM) approach [1] to heavier nuclei (i.e., to the sd- and pf-shells) is a challenging problem. Standard shell-model calculations usually employ either empirical or theoretial effective 2-body matrix elements (TBME), which contain excluded many-body correlations. We show how a NCSM investigation in a 4$\hbar\Omega$ model space can yield TB sd-shell ME, which exactly reproduce the many-body correlations present in the original calculation. We demonstrate how the effective Hamiltonian derived in the 4$\hbar\Omega$ NCSM at the 2-body cluster level should be modified to properly account for the many-body correlations produced by truncating to a single major shell. The capability of these standard sd-shell TBME for $^{18}$F, obtained by direct projection, to reproduce the results of large scale NCSM calculations for F, Ne and Na isotopes will be shown.\\ 1. P.Navr\'atil, {\it et al.}, Phys. Rev. C {\bf 62}, 054331 (2000). [Preview Abstract] |
Thursday, October 11, 2007 2:36PM - 2:48PM |
BF.00004: Effective Interactions for pf Shell from the No Core Shell Model Erdal Dikmen, Alexander Lisetsky, Bruce Barrett, Petr Navr\'atil, James Vary Following the ideas of the Tucson nuclear theory group for the extension of the No-Core Shell Model (NCSM) approach to sd-shell nuclei [1], we show how a NCSM calculation for $^{42}$Ca in a 2$\hbar\Omega$ model space can yield two-body effective interactions for the pf-shell. We demonstrate how the effective Hamiltonian derived in the 2$\hbar\Omega$ NCSM at the 2-body cluster level should be modified to properly account for the many-body correlations produced by truncating to a single major shell. The pf-shell two-body effective interactions for $^{42}$Ca, obtained by direct projection, are used to reproduce the results of large scale NCSM for other Ca isotopes.\\ 1. B.R. Barrett et al, DNP 2007 abstract. [Preview Abstract] |
Thursday, October 11, 2007 2:48PM - 3:00PM |
BF.00005: Partial Dynamical Symmetries Larry Zamick Two examples of partial dynamical symmetries are presented. 1) The j=9/2 shell of identical particles e.g. neutrons affords the first shell where one can have seniority mixing via a two-body interaction.It was however noted by Escuderos and Zamick that even with a seniority violating interaction certain states remain pure. For 4 neutrons in the 9/2 shell for total ang. momentum I=4 there is one pure seniority v=4 state. This does not mix with the single seniority V=2 state or with the other v=4 state. A proof is presented showing that this special state does not mix with V=2. A similar scenario plays out for I=6. 2) In 44Ti if we sent the two=body T=0 matrix elements to zero, keeping only T=1, then in the single j shell model we get degeneracies of certain states e.g. a 3+,7+,9+, and 10+ state are all degenerate. The ``symmetry'' is partial because we do not get degeneracies for I=0,2,4,6,8. The explanation is that the symmetry only occurs if in 44T (2protons and 2 neutrons) the total angular momenta are ones that cannot occur for 4 identical particles i.e. 44Ca. Where the partial dynamical symmetry applies Jp and Jn are good ``dual'' quantum numbers for all the T=0 states. [Preview Abstract] |
Thursday, October 11, 2007 3:00PM - 3:12PM |
BF.00006: The effect of magnetic field to the pairing phase transition of mesoscopic system Tony Sumaryada, Alexander Volya In this presentation we discuss pairing phase transitions in mesoscopic system. We investigate the role played by the magnetic field which is equivalent to rotation within the cranking model. Using exact solution of pairing we examine spin fluctuations, magnetization, specific heat, energy, and entropy for several systems with various statistical approaches. We emphasize a resemblance between observed mesoscopic properties and those known in the macroscopic physics of superconductors. At low field the normal and superconducting phases are separated by the second order phase transition. In the next region of higher magnetic field the normal and superconducting phases are separated by the transition of a different nature associated with a simultaneous peak in spin susceptibility end enhanced spin fluctuations. Finally, at even higher fields a superconducting state is not supported at all. [Preview Abstract] |
Thursday, October 11, 2007 3:12PM - 3:24PM |
BF.00007: Band Terminations and Density Functional Theory: A Critical Analysis Anatoli Afanasjev It was recently suggested in Refs. [1,2] that the set of terminating states in the N$\sim $Z, A$\sim $44 mass region provides unique and reliable constraint on time-odd mean fields and the strength of the spin-orbit interaction in Skyrme and covariant (relativistic) density functionals. The authors of these references claim that their method based on the energies of terminating states is free from the drawbacks of standard approaches to define the strength of the spin-orbit interaction employing the single-particle energies of the spin-orbit partner orbitals in spherical nuclei. If that would be true the isoscalar and isovector channels of spin-orbit interaction could be defined very accurately. However, the detailed analysis performed shows that this is not a case. The results of this analysis will be presented in the talk. \newline [1] H.Zdunczuk et al, Phys. Rev. C71, 024305 \newline [2] A.Bhagwat et al, reprint nucl-th/0605009 [Preview Abstract] |
Thursday, October 11, 2007 3:24PM - 3:36PM |
BF.00008: Recent progress with {\it ab initio} calculations of the nuclear equation of state Francesca Sammarruca The properties of dense nuclear matter is a topic of current interest. More empirical information is becoming available through heavy-ion collision data and astrophysical observations, which can help set constraints on the equation of state (EOS) of nuclear matter in different density regions \footnote{T. Kl{\"a}hn {\it et al.}, Physical Review C {\bf 74}, 035802 (2006)}. In this contribution, I will present recent progress within the {\it ab initio} approach pursued by my group. Our goal is to gain a broad overview over nuclear matter properties, especially under the ``exotic" conditions of isospin and spin asymmetry. Moreover, the impact of non-nucleonic degrees of freedom on the EOS at higher densities will be discussed. [Preview Abstract] |
Thursday, October 11, 2007 3:36PM - 3:48PM |
BF.00009: Origin of Apparent Negative Heat Capacity in Constrained Microcanonical Modeling of Excited Nuclear Systems Michael Quinlan, Jan T\~oke, Iwona Pawelczak, Wolf-Udo Schr\"oder The origin of negative heat capacity in certain classes of microcanonical models of phase transitions in small systems is studied. It is demonstrated that the domain of negative heat capacity appears in such calculations as a result of an unphysical discontinuity in the model phase space and, specifically, the exclusion of energetically (microcanonically) allowed micro-states filling the space between the domains corresponding to different phases. It is also shown that already a crude filling of these unphysical gaps in the model phase space results in a restoration of the concavity of the entropic curve S(E*) and thus in an elimination of the faux negative heat capacity in the phase transition region. [Preview Abstract] |
Thursday, October 11, 2007 3:48PM - 4:00PM |
BF.00010: Nuclear Multifragmentation as Generalized Fission Jan T\~oke, Udo Schr\"oder An explanation for the phenomenon of statistical nuclear multifragmentation is presented within the framework of thermodynamical theory. Within this framework, multifragmentation occurs as a result of interplay between the disruptive Coulomb and/or centrifugal forces and cohesive surface free energy (surface tension) in a system undergoing fast statistical shape fluctuations -- a process similar to binary fission, except that generalized to multifragment decay channels. The process sets in at elevated excitation energies where it relies on thermal reduction of surface tension. It is gentle in that it does not involve violent collective flows of matter. Rather, the highly excited and shape-fluctuating system gets torn apart slowly by Coulomb forces as, aided by decreasing surface free energy, it reaches on fast time scales any of the very many possible multifragment saddle configurations.. [Preview Abstract] |
Thursday, October 11, 2007 4:00PM - 4:12PM |
BF.00011: Density-constrained TDHF calculation of fusion cross sections for neutron-rich nuclei Volker Oberacker, Sait Umar We have developed a new microscopic approach for calculating heavy-ion fusion cross sections. The method is based on the TDHF evolution of the nuclear system coupled with density-constrained Hartree-Fock calculations to obtain the heavy-ion interaction potential. This approach incorporates all of the dynamical entrance channel effects such as neck formation, particle transfer, internal excitations (including giant resonances), and dynamical deformation effects. In particular, we focus on systems involving one or two deformed nuclei ($^{64}$Ni, $\beta_2=-0.081$) in which case the dynamical nuclear alignment arising from multiple Coulomb excitation must be taken into account. Fusion cross sections below and above the barrier are calculated using the incoming wave boundary condition (IWBC) method. A recently completed analysis [Ref. 1] of the neutron-rich system $^{64}$Ni+$^{132}$Sn will be presented, and we will also discuss new preliminary results for the $^{64}$Ni+$^{64}$Ni system where experimental data show a hindrance of subbarrier fusion. \newline Ref. 1: A.S. Umar and V.E. Oberacker, Phys. Rev. C74, 061601(R) (2006) and Phys. Rev. C (2007), in print. [Preview Abstract] |
Thursday, October 11, 2007 4:12PM - 4:24PM |
BF.00012: Random Interactions on 2-Level Systems Declan Mulhall The systematics of the regularities of the spectra of systems of $N$ particles on 2 spin-$j$ levels interacting under random 2-body interactions with certain global symmetries exhibit regularities. A statistical theory is developed to explain these regularities, based on the statistical mechanics of a quantum gas. A comparison is made with the Interaction Boson Model. [Preview Abstract] |
Thursday, October 11, 2007 4:24PM - 4:36PM |
BF.00013: Relative elemental and meson production rates in the collision of space crafts with cosmic particles M.S. Sabra, F.B. Malik A modified statistical model that includes final state interaction between two binary fragments in the collision of a target nucleus with hydrogen and helium has been successful in explaining the existing data of alpha induced fragmentation of $^{28}$Si, a major component in semi-conductors instruments. Proton induced collision of $^{16}$O will be presented along with calculated production rates of all allowed elements and their isotopes. The observed data indicates that the fragments are pre-dominantly emitted in excited states, and have broad kinetic energy distributions, which are accounted for by this theory but not by the usual evaporation models. The nature of the potentials between two emerging fragments in the final state and their level density functions are the important factors in determining their kinetic energy spectra and degree of excitations. The collision between materials of space crafts and cosmic particles leads to copious emission of hot nuclei, a factor that must be added to the existing data base needed for developing protection against space radiations. A preliminary study of the possibility of meson productions and the emission of the residual hot-nuclei in the process that could subsequently emits further hot nuclei will also be discussed. [Preview Abstract] |
Thursday, October 11, 2007 4:36PM - 4:48PM |
BF.00014: The time-dependent, relativistic Aharonov-Bohm effect Zachary Kertzman, Athanasios Petridis It is known that the Aharonov-Bohm effect provides a definite proof that charged fermions couple directly to the 4-vector potential. Numerical, time-dependent solutions to the relativistic Dirac equation coupled with an external electromagnetic field are produced in order to study this phenomenon in detail. The staggered leap-frog method is used on a spatial lattice. The numerical stability of the method in two dimentions is a crucial issue. It is studied analytically and ensured by taking appropriately small time-steps. The action of the magnetic potential in the region of zero magnetic field is evaluated by means of the diffraction patterns it produces. [Preview Abstract] |
Thursday, October 11, 2007 4:48PM - 5:00PM |
BF.00015: The Explanation of the Photon's Electric and Magnetic Fields; and its Particle and Wave Characteristics Russell Moon, Victor Vasiliev Using the principles of the Vortex Theory, the creation of the photon's electric and magnetic components are explained: the condensed region of space is responsible for creating the photon's electric component and its particle effect; its expansion and contraction is responsible for its frequency; its motion through three dimensional space creates a wave in the surrounding space. This wave is responsible for the photon's magnetic component and wave characteristics. The simultaneous expansion and contraction of both the dense region of space that is the photon and the surrounding space it passes through explains why the electric and magnetic effects are at right angles to each other. Also the photon's particle and wave characteristics are explained. 1.Russell Moon, \textit{The Bases of the Vortex Theory of Space}. Publishing house of ZNAK, Moscow, Russia, 2002, 32 pp., (in Russian). 2 R.G. Moon, \textit{The Possible Existence of a New Particle: the Neutral Pentaquark}? Book of materials, The Research Centre of Ecological Safety of the Russian Academy of Sciences: Scientific Seminar 0f Ecology and Space 1, February 22, 2005, Saint-Petersburg, Russia, 2005, pp. 98-104. [Preview Abstract] |
Thursday, October 11, 2007 5:00PM - 5:12PM |
BF.00016: Variable Differential Cross Section Due to Nuclear Motion Stewart Brekke The standard differential cross section formula assumes a motionless massive point nucleus. However, nuclear oscillation is a reality, changing the position of the impact parameter in relation to the incoming beam of particles or ions thereby affecting the cross section. If the static differential cross section is $d\sigma/d\Omega = (Ze^2\csc^2 \theta/8\pi\epsilon_0T)^2$, an oscillator can be added to the formula describing the movement of the cross section so that the resulting formula for differential cross section is $(Ze^2\csc^2\theta/2/8\pi\epsilon_0T)^2 (A_x\cos\alpha i + A_y\cos\beta j +Az\cos\gamma k$. By taking in ot account nuclear motion there will be some reconciliation between experimental and theoretical values. [Preview Abstract] |
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