Bulletin of the American Physical Society
2008 Annual Meeting of the Division of Nuclear Physics
Volume 53, Number 12
Thursday–Sunday, October 23–26, 2008; Oakland, California
Session BF: Nuclear Reactions: Heavy-Ions |
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Chair: Jutta Escher, Lawrence Livermore National Laboratory Room: Simmons Ballroom 4 |
Friday, October 24, 2008 8:30AM - 8:42AM |
BF.00001: Semiclassical Calculations of Peripheral Heavy-Ion Collisions at Fermi Energies and the Nuclear Equation of State G.A. Souliotis, D.V. Shetty, S. Galanopoulos, S.J. Yennello A systematic study of quasi-elastic and deep-inelastic collisions at Fermi energies has been undertaken at Texas A\&M aiming at obtaining information on the mechanism of nucleon exchange and the course towards N/Z equilibration [1,2]. We expect to get insight in the dynamics and the nuclear equation of state by comparing our experimental heavy residue data to detailed calculations using microscopic models of quantum molecular dynamics (QMD) type. At present, we have performed detailed calculations using the code CoMD (Constrained Molecular Dynamics) of A. Bonasera and M. Papa [3]. The code implements an effective interaction with a nuclear-matter compressibility of K=200 (soft EOS) with several forms of the density dependence of the nucleon-nucleon symmetry potential. CoMD imposes a constraint in the phase space occupation for each nucleon, effectively restoring the Pauli principle at each time step of the collision. Results of the calculations and comparisons with our data will be presented and implications concerning the isospin part of the nuclear equation of state will be discussed. [1] G.A. Souliotis et al., Phys. Rev. Lett. 91, 022701 (2003). [2] G.A. Souliotis et al., Phys. Lett. B 588, 35 (2004). [3] M. Papa et al., Phys. Rev. C 64, 024612 (2001). [Preview Abstract] |
Friday, October 24, 2008 8:42AM - 8:54AM |
BF.00002: Fragment emission and production in Peripheral Collisions in the Intermediate Energy regime Sarah Soisson, B. Stein, G. Souliotis, D. Shetty, A. Keksis, S. Wuenschel, S.J. Yennello In recent years, examination of the kinetic energy spectra of emitted isotopes in multi-fragmentation reactions has shown that neutron-poor isotopes have larger mean kinetic energies than neutron-rich isotopes. Using the FAUST array, isotopically resolved fragments are detected from the reactions of A$\sim $20-40 projectiles with a heavy target, a reconstructed quasi-projectile can be determined. This reconstructed quasi-projectile allows for good source definition as well as good N/Z determination of the emitting source. By selecting on isotopically identified fragments emitted from well defined sources, it will be shown that the fragment contributions present in the Coulomb peak and the tail of the kinetic energy spectra evolve with the N/Z of the emitting source. Also, the fragment yield as a function of energy will be shown to be dependant of N/Z of the emitting source. [Preview Abstract] |
Friday, October 24, 2008 8:54AM - 9:06AM |
BF.00003: N/Z Equilibration in Peripheral Reactions on the FAUST Array Brian Stein, S.N. Soisson, G.A. Souliotis, D.V. Shetty, S. Galanopoulos, A.L. Keksis, S. Wuenschel, Z. Kohley, L. May, S.J. Yennello In recent years, nucleon transport in peripheral heavy ion collisions has been proposed as a probe of the density dependence of the nuclear symmetry energy. Recently a high statistics data set was taken of the systems $^{32, 36}$S +$^{112,124}$Sn at 45 MeV/A using the FAUST arrray. Quasi-projectiles have been reconstructed from isotopically resolved fragments (with mass identification up to Z=14) to be used as a probe of the N/Z equilibration. Initial results show that quasi-projectiles are produced with a wide range of N/Z for each reaction system. Also, the quasi-projectile distributions show sensitivity to the N/Z of both the projectile and target used. Experimental results will be presented with comparisons to theoretical models. [Preview Abstract] |
Friday, October 24, 2008 9:06AM - 9:18AM |
BF.00004: Density-constrained TDHF calculation of fusion cross sections for neutron-rich nuclei Volker Oberacker, Sait Umar The density-constrained TDHF method [Ref. 1] is a fully microscopic theory for calculating heavy-ion interaction potentials and fusion cross sections below and above the fusion barrier. The only input into the theory is the effective NN interaction (Skyrme force). There are no adjustable parameters. The method is based on the TDHF evolution of the nuclear system slighltly above the fusion barrier, coupled with density-constrained Hartree-Fock calculations. This approach incorporates all of the dynamical entrance channel effects such as neck formation, particle transfer, internal excitations and dynamical deformation effects. We will discuss applications to neutron-rich systems (64Ni+132Sn, 64Ni+64Ni, 40Ca+96Zr). Several prescriptions for calculating the coordinate-dependent mass parameters (effective mass, dynamical reduced mass) within the DC-TDHF method will be presented, and we examine their influence on the total fusion cross section. Ref. 1: A.S. Umar and V.E. Oberacker, PRC 77, 064605 (2008) [Preview Abstract] |
Friday, October 24, 2008 9:18AM - 9:30AM |
BF.00005: Isotopic width distributions and symmetry energy Sara Wuenschel, Stratos Galanopoulos, Kris Hagel, Zach Kohley, Dinesh Shetty, Sarah Soisson, George Souliotis, Brian Stein, Sherry Yennello Within the Microcanonical Multifragmentation Model (MMM), the bulk versus surface contribution to the symmetry energy is predicted to be distinguishable. If the symmetry energy is dominated by the surface, the symmetry energy coefficient should evolve with the size of the fragment studied. However, if the bulk term dominates, the symmetry energy coefficient should be constant across all fragment sizes. Symmetry energy can be accessed through knowledge of the parameter $\alpha $. This parameter may be obtained by isoscaling [1] or isotopic widths [2]. Differentiation between bulk and surface requires isotopic widths across a wide array of elements. Projectile fragmentation of $^{86}$Kr+$^{64}$Ni at 35MeV/u was taken on the NIMROD-ISiS detector. The wide range of isotopic resolution seen in NIMROD-ISiS data allows the isotopic widths for Z=3-15 to be extracted for this study. Events are characterized through reconstruction of the quasi-projectile. Isotopic width data will be presented. [Preview Abstract] |
Friday, October 24, 2008 9:30AM - 9:42AM |
BF.00006: Coupled Channel Calculations for Nucleon Induced Reactions Gustavo Nobre, Ian Thompson, Jutta Escher, Frank Dietrich, Marc Dupuis An {\em ab-initio} calculation of the optical potential for neutron-nucleus scattering has been performed by explicitly coupling the elastic channel to all the particle-hole (p-h) excitation states in the target. These p-h states may be regarded as {\em doorway states} through which the flux flows to more complicated configurations, and (in the end) to long-lived compound nucleus resonances. The random-phase approximation (RPA) provides the linear combinations of p-h states that include the residual interactions within the target, and we show preliminary results for elastic flux loss and total cross-section using both p-h and RPA descriptions of $^{90}$Zr and $^{208}$Pb target excitations for a wide projectile energy region. Within this procedure we were able to observe coupling and structure effects of the studied nuclei by comparing the different coupled channel calculations results with experimental data, and will soon be applied to other targets. [Preview Abstract] |
Friday, October 24, 2008 9:42AM - 9:54AM |
BF.00007: Fission-fragment properties in a microscopic approach Walid Younes The microscopic description of fission remains one the greatest challenges in nuclear physics. In particular, observed properties of the fission fragments (e.g., kinetic energies, emitted neutron multiplicities, etc.) are notoriously difficult to reproduce and provide a stringent test of the microscopic approach. In this talk, I will present fission-fragment properties extracted from Hartree-Fock-Bogoliubov calculations using the Gogny effective interaction for low-energy induced fission of $^{240}$Pu. This approach to fission provides a fully microscopic, self-consistent, quantum-mechanical framework where the only phenomenological input is the effective interaction between nucleons. I will discuss the formal identification of scission configurations and compare deduced fragment properties, such as excitation and kinetic energies, to experimental data. [Preview Abstract] |
Friday, October 24, 2008 9:54AM - 10:06AM |
BF.00008: Survival Probabilities in hot fusion reactions Walter Loveland, Donald Peterson The reported cross sections for the formation of superheavy elements in hot fusion reactions of $^{48}$Ca with actinide target nuclei decrease modestly in going from element 113 to element 118. This robust behavior is attributed to increasing survival probabilities of the product nuclei as one gets closer to Z=114 or N=184. The real situation is complicated with the fused systems starting at excitation energies of 30-50 MeV where shell effects on $\Gamma _{n}$/$\Gamma _{f}$ are not important but where dissipative effects may retard fission and ending at excitation energies where shell effects are very important. We demonstrate how these effects occur in the de-excitation of $^{258}$No excited to E*=61 MeV by combining measurements of the neutrons emitted in this reaction with evaporation residue measurements. [Preview Abstract] |
Friday, October 24, 2008 10:06AM - 10:18AM |
BF.00009: In-medium nucleon-nucleon scattering in reactions with rare isotopes Carlos Bertulani, K. Ogata Relativistic effects in the breakup of weakly-bound nuclei at intermediate energies are studied and compared with non-relativistic calculations. We show that relativistic corrections lead to larger breakup cross sections. Since many of these reactions can only be treated correctly if one accounts for the coupling between states in the continuum, we show that continuum-discretized coupled-channels calculations will also be strongly influenced by relativistic effects. [Preview Abstract] |
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