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Physical Review C

(Nuclear Physics)

October 2006

Volume 74, Number 4 , Articles (04xxxx)

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RAPID COMMUNICATIONS
Rapid

Shell corrections at the saddle point for mass ~200
K. Mahata, S. Kailas, and S. S. Kapoor
Published 26 October 2006 (5 pages)
041301(R)  Full Text: PDF (202 kB)  | Buy Article
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Statistical model analysis of the measured evaporation residue and fission excitation functions indicates the presence of shell corrections to the liquid drop energy at the saddle point deformations for nuclei of mass ~200, if one attempts to fit simultaneously the measured prefission neutron multiplicities.
Rapid

Charged- and neutral-pion production in the S-matrix approach
V. Malafaia, M. T. Peña, Ch. Elster, and J. Adam, Jr.
Published 31 October 2006 (5 pages)
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The S-matrix approach is used to calculate both charged- and neutral-pion production in nucleon-nucleon (NN) scattering near threshold. The irreducible pion-rescattering diagram, direct production mechanism, Delta isobars in intermediate states, and Z diagrams mediated by heavy isoscalar mesons are included in the calculation. For the NN distortions, we considered a realistic interaction, within the Bonn family of potentials, which describes the nucleonic inelasticities above the pion production energy threshold.

ARTICLES

Nucleon-Nucleon Interaction, Few-Body Systems

Incoherent single pion electroproduction on the deuteron with polarization effects
M. Tammam, A. Fix, and H. Arenhövel
Published 9 October 2006 (17 pages)
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Incoherent pion electroproduction on the deuteron is studied from threshold up to the second resonance region with special emphasis on the influence of the final-state interaction, in particular on polarization observables. The elementary gammaN-->piN amplitude is taken from the MAID-2003 model. The final-state interaction is included by considering complete rescattering in the final NN and piN subsystems. Investigated in detail is their influence on the structure functions governing the semi-exclusive differential cross section, where besides the scattered electron only the produced pion is detected. For charged pion-production the effect of NN rescattering is moderate whereas piN rescattering is almost negligible, except very close to threshold. NN rescattering appears much stronger in neutral pion production for which the primary mechanism is the elimination of a significant spurious coherent contribution in the impulse approximation. Sizeable effects are also found in some of the polarization structure functions for beam and/or target polarizations.

Nucleon-nucleon potential: Drift effects
M. R. Robilotta
Published 17 October 2006 (7 pages)
044002  Full Text: PDF (172 kB)  | Buy Article
See Also: Publisher's Note
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In the rest frame of a many-body system, used in the calculation of its static and scattering properties, the center of mass of a two-body subsystem is allowed to drift. We show, in a model-independent way, that drift corrections to the nucleon-nucleon potential are relatively large and arise from both one- and two-pion exchange processes. As far as chiral symmetry is concerned, corrections to these processes begin, respectively, at [script O](q2) and [script O](q4). The two-pion exchange interaction also yields a new spin structure, which promotes the presence of P waves in trinuclei and is associated with profile functions that do not coincide with either central or spin-orbit ones. In principle, the new spin terms should be smaller than the [script O](q3) spin-orbit components. However, in the isospin-even channel, a large contribution defies this expectation and gives rise to the prediction of important drift effects.

Measurement of the absolute differential cross section for np elastic scattering at 194 MeV
M. Sarsour, T. Peterson, M. Planinic, S. E. Vigdor, C. Allgower, B. Bergenwall, J. Blomgren, T. Hossbach, W. W. Jacobs, C. Johansson, J. Klug, A. V. Klyachko, P. Nadel-Turonski, L. Nilsson, N. Olsson, S. Pomp, J. Rapaport, T. Rinckel, E. J. Stephenson, U. Tippawan, S. W. Wissink, and Y. Zhou
Published 25 October 2006 (22 pages)
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A tagged medium-energy neutron beam was used in a precise measurement of the absolute differential cross section for np backscattering. The results resolve significant discrepancies within the np database concerning the angular dependence in this regime. The experiment has determined the absolute normalization with ±1.5% uncertainty, suitable to verify constraints of supposedly comparable precision that arise from the rest of the database in partial wave analyses. The analysis procedures, especially those associated with the evaluation of systematic errors in the experiment, are described in detail so that systematic uncertainties may be included in a reasonable way in subsequent partial wave analysis fits incorporating the present results.

K0 photoproduction on the deuteron and the extraction of the elementary amplitude
A. Salam, K. Miyagawa, T. Mart, C. Bennhold, and W. Glöckle
Published 26 October 2006 (8 pages)
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The photoproduction of K0 on the deuteron has been investigated with the inclusion of the YN and KN final-state interaction (FSI), as well as the pion-mediated process gammad-->piNN-->KYN. The YN rescattering effects for the inclusive cross section are found to be large in the threshold regions. Polarization observables show sizable FSI effects at larger kaon and hyperon angles. It is shown that the extraction of the elementary gammaN-->KY amplitude is possible in the quasifree scattering region where FSI effects are negligible. Furthermore, the cross sections in this region are large, indicating that measurements in this kinematic region are favored.

Nuclear Structure

Self-consistent description of multipole strength: Systematic calculations
J. Terasaki and J. Engel
Published 3 October 2006 (19 pages)
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We use the quasiparticle random-phase approximation with a few Skyrme density functionals to calculate strength functions in the Jpi=0+,1-, and 2+ channels for even Ca, Ni, and Sn isotopes from the proton drip line to the neutron drip line. We show where and how low-lying strength begins to appear as N increases. We also exhibit partial energy-weighted sums of the transition strength as functions of N for all nuclei calculated and transition densities for many of the interesting peaks. We find that low-energy strength increases with N in all multipoles, but with distinctive features in each. The low-lying 0+ strength at large N barely involves protons at all, with the strength coming primarily from a single two-quasineutron configuration with very large spatial extent. The low-lying 1- strength is different, with protons contributing to the transition density in the nuclear interior together with neutrons at large radii and with somewhat more collectivity in the heavier isotopes. The low-lying 2+ transition strength goes largely to more localized and frequently more collective states. In all channels the correlation between strength and collectivity is much weaker than in stable nuclei. The three Skyrme interactions we test produce similar results, differing most significantly in their predictions for the location of the neutron drip line, the boundaries of deformed regions, energies of and transition strengths to the lowest 2+ states between closed shells, and isovector energy-weighted sum rules.

Neutron capture cross sections of 148Gd and the decay of 149Gd
M. G. Rios, R. J. Casperson, K. S. Krane, and E. B. Norman
Published 4 October 2006 (9 pages)
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The thermal cross section and resonance integral were measured for radiative neutron capture by radioactive 148Gd. The deduced values are sigma=9600±900 b and I=28,200±2300. We also deduced upper limits for the n,p and n, alpha cross sections, respectively, 0.25 b and 13 b. The gamma-ray spectrum from the decay of 149Gd was studied in singles mode at high resolution to verify the previously determined energies and intensities. From the latter measurements, new transitions are proposed and upper limits are deduced for previously reported transitions.

Detailed spectroscopy of 249Fm
A. Lopez-Martens, K. Hauschild, A. V. Yeremin, A. V. Belozerov, Ch. Briançon, M. L. Chelnokov, V. I. Chepigin, D. Curien, O. Dorvaux, B. Gall, V. A. Gorshkov, M. Guttormsen, F. Hanappe, A. P. Kabachenko, F. Khalfallah, A. Korichi, A. C. Larsen, O. N. Malyshev, A. Minkova, Yu. Ts. Oganessian, A. G. Popeko, M. Rousseau, N. Rowley, R. N. Sagaidak, S. Sharo, A. V. Shutov, S. Siem, A. I. Svirikhin, N. U. H. Syed, and Ch. Theisen
Published 4 October 2006 (9 pages)
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Excited states in 249Fm were populated via the alpha decay of 253No and the subsequent decay was observed with the GABRIELA detection system installed at the focal plane of the VASSILISSA recoil separator. The energies, spins, and parities of these states could be established through combined alpha,gamma, and conversion-electron spectroscopy. The first members of the ground-state rotational band were identified. Their excitation energies as well as the observation of a cross-over E2 transition confirm the assignment of 7/2+[624] for the ground state of 249Fm. Two excited states were also observed and their decay properties suggest that they correspond to the particle excitation 9/2-[734] and hole excitation 5/2+[622]. The analysis suggests that the 279-keV transition de-exciting the 9/2- state has anomalous E1 conversion coefficients.

Comparison of local, semi-microscopic, and microscopic three-cluster models
M. Theeten, D. Baye, and P. Descouvemont
Published 10 October 2006 (14 pages)
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Two different three-body models are compared with a fully antisymmetrized microscopic three-cluster model. The local model makes use of local effective interactions involving forbidden states among the three particles. In the semi-microscopic model, nonlocal two-body interactions are derived within the resonating-group method from the same nucleon-nucleon effective forces as in the microscopic model. In both cases, calculations are performed in hyperspherical coordinates with the Lagrange-mesh method. The role of forbidden states and their elimination are discussed. The models are applied to an alphaalphan description of 9Be and an alphann description of 6He. The local model results are affected by almost forbidden states and may be unrealistic for 9Be. A comparison of the microscopic and semi-microscopic models shows that the effect of exchanges involving the three clusters is weak. An overbinding of 9Be cannot be avoided with nucleon-nucleon forces reproducing alphan and alphaalpha scattering properties. On the contrary, 6He is underbound under the same conditions. This can probably be attributed to a lack of three-nucleon forces.

Multiple octupole-type band structures in 220Th: Reflection-asymmetric tidal waves?
W. Reviol, C. J. Chiara, M. Montero, D. G. Sarantites, O. L. Pechenaya, M. P. Carpenter, R. V. F. Janssens, T. L. Khoo, T. Lauritsen, C. J. Lister, D. Seweryniak, S. Zhu, and S. G. Frauendorf
Published 12 October 2006 (8 pages)
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The 220Th level scheme has been considerably extended from an experiment using the 26Mg+198Pt reaction at 128 MeV. The evaporation residues from this very fissile system were selected with the HERCULES detector system and residue-gated gamma rays were measured with Gammasphere. The simplex feature (alternating-parity levels) persists up to the highest spins observed (23[h-bar]), but the nucleus exhibits a more vibrational-like behavior than the heavier Th isotopes. In addition, a doubling of the negative-parity, odd-spin states is seen as well as a staggering of the B(E1)/B(E2) ratios. A new interpretation based on a picture of tidal waves on a reflection-asymmetric nuclear surface is proposed.

gamma-soft analog of the confined beta-soft rotor model
Dennis Bonatsos, D. Lenis, N. Pietralla, and P. A. Terziev
Published 13 October 2006 (5 pages)
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A gamma-soft analog of the confined beta-soft rotor model is developed by using a gamma-independent displaced infinite-well beta potential in the Bohr Hamiltonian, for which exact separation of variables is possible. Level schemes interpolating between the E(5) critical point symmetry (with R4/2=E(41<sup>+</sup>)/E(21<sup>+</sup>)=2.20) and the O(5) gamma-soft rotor (with R4/2=2.50) are obtained; these exhibit a crossover of excited 0+ bandheads that lead to agreement with the general trends of 02<sup>+</sup> states in this region and is observed experimentally in 128,130Xe.

Intrinsic quadrupole tensor in the geometric interpretation of the interacting boson approximation (IBA-1)
Eugene R. Marshalek
Published 16 October 2006 (7 pages)
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For the first time, complete quantal expressions are provided for the intrinsic components of the quadrupole tensor in the geometric Holstein-Primakoff representation of the IBA-1. These expressions are valid throughout the Casten triangle.

Giant resonances in 46,48Ti
Y. Tokimoto, Y.-W. Lui, H. L. Clark, B. John, X. Chen, and D. H. Youngblood
Published 24 October 2006 (9 pages)
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The giant resonance region from 9 MeV <Ex<55 MeV in 46Ti and 48Ti has been studied with inelastic scattering of 240 MeV alpha particles at small angles including 0°. Isoscalar monopole strength in 46Ti (48Ti) was found corresponding to have 71+15/-12% (96+14/-12%) of the E0 energy weighted sum rule (EWSR) with a centroid of 18.66+0.65/-0.25% MeV (18.80+0.45/-0.18% MeV), respectively. In 46Ti (48Ti), 46±12% (56±12%) of the E1, and 60±11% (87±11%) of the E2 EWSR were identified.

Extensive investigation of 0+ states in rare earth region nuclei
D. A. Meyer, V. Wood, R. F. Casten, C. R. Fitzpatrick, G. Graw, D. Bucurescu, J. Jolie, P. von Brentano, R. Hertenberger, H.-F. Wirth, N. Braun, T. Faestermann, S. Heinze, J. L. Jerke, R. Krücken, M. Mahgoub, O. Möller, D. Mücher, and C. Scholl
Published 24 October 2006 (12 pages)
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The nature of 0+ excitations, especially in transitional and deformed nuclei, has attracted new attention. Following a recent experiment studying 158Gd, we investigated a large group of nuclei in the rare-earth region with the (p,t) pickup reaction using the Q3D magnetic spectrograph at the University of Munich MP tandem accelerator laboratory. Outgoing tritons were recorded at various lab angles, and their angular distributions are compared to those calculated using the distorted-wave Born approximation. Using the unique shape of the L=0 angular distribution, more than double the number of 0+ states than were previously known are identified. The distribution of 0+ energies and cross sections is discussed in terms of collective and noncollective degrees of freedom, and the density of low-lying 0+ states is discussed as a corroboration of a characteristic feature of phase transition regions. The degree of level mixing, as extracted from Brody distribution fits to the energy spacings of adjacent 0+ levels, is also explored.

Many-particles–plus–rotor description of magnetic bands at high spin
B. G. Carlsson and I. Ragnarsson
Published 25 October 2006 (14 pages)
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The high-spin parts of shears bands observed in 198,199Pb are described by coupling several valence particles to a deformed rotor core. To give the model enough freedom so that both the proton and the neutron spin vectors can find their preferred direction, the use of five to six valence particles and holes is found to be necessary. Effective parameters characterizing the rotor are deduced from the cranking model. The method is described and results are presented concerning transition probabilities and energies in the high-spin regime for several rotational bands in 199Pb. The possible termination of the rotational bands is also discussed.

alpha-cluster structure and exotic states in a self-consistent model for light nuclei
J. A. Maruhn, Masaaki Kimura, S. Schramm, P.-G. Reinhard, H. Horiuchi, and A. Tohsaki
Published 26 October 2006 (12 pages)
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In this article we examine to what extent traces of alpha clustering can be found in mean-field ground states of nalpha nuclei from 8Be through 36Ar as well as in some superdeformed states in 32S, 36Ar, and 40Ca. For this purpose we calculate the overlap of the mean-field Slater determinant with one containing pure Gaussians and perfect spin and isospin symmetry, optimizing the overlap by varying the alpha-particle positions and radii. In some cases a coherent sum over different configurations is also employed. We find quite large overlaps for some of the lighter systems that diminish for nuclei above 20Ne but again strong clustering in 36Ar.

Simultaneous description of four positive parity bands and four negative parity bands
A. A. Raduta, Al. H. Raduta, and C. M. Raduta
Published 26 October 2006 (24 pages)
044312  Full Text: PDF (866 kB)  | Buy Article
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The extended coherent state model is further extended to describe two dipole bands of different parities. The formalism provides a consistent description of eight rotational bands. A unified description for spherical, transitional, and deformed nuclei is possible. Projecting out the angular momentum and parity from a sole state, the Kpi=1+ band acquires a magnetic character, whereas the electric properties prevail for the other band. Signatures for a static octupole deformation in some states of the dipole bands are pointed out. Some properties that distinguish between the dipole band states and states of the same parity but belonging to other bands are mentioned. Interesting features concerning the decay properties of the two bands are found. Numerical applications are made for 158Gd, 172Yb, 228,232Th, 226Ra, 238U, and 238Pu, and the results are compared with the available data.

Nuclear spin-lattice relaxation of 62Cu at low temperatures in iron
V. V. Golovko, I. S. Kraev, T. Phalet, N. Severijns, D. Vénos, D. Zákoucký, P. Herzog, C. Tramm, U. Köster, D. Srnka, M. Honusek, B. Delauré, M. Beck, V. Yu. Kozlov, and A. Lindroth
Published 30 October 2006 (5 pages)
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The nuclear spin-lattice relaxation of 62Cu in iron has been studied with the low temperature nuclear orientation method. At temperatures ranging from 6.5 mK to about 100 mK and a magnetic field of 0.1 T the relaxation constant for 62Cu in Fe was found to be CK[62Cu]=4.34(25)sK.

Double beta decay of 100Mo to excited final states
M. J. Hornish, L. De Braeckeleer, A. S. Barabash, and V. I. Umatov
Published 30 October 2006 (9 pages)
044314  Full Text: PDF (236 kB)  | Buy Article
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A systematic study of the inclusive (0nu+2nu) double beta (betabeta) decay of 100Mo to various excited final states of 100Ru was performed. Utilizing two large HPGe detectors operated in coincidence, a search for the subsequent deexcitation gammagamma cascades was conducted. A 1.05-kg sample of isotopically enriched (98.4%)100Mo was investigated for 455 days, yielding an unambiguous observation of the betabeta decay of 100Mo to the 01<sup>+</sup> state (1130.3 keV) of 100Ru. This excited final state decays via the 01<sup>+</sup>-->21<sup>+</sup>-->0gs<sup>+</sup> sequence (with Egamma1=590.8 keV and Egamma2=539.5 keV), and 22 such coincidence events were detected, with a continuous background estimated to be 2.5 events. This counting rate corresponds to a decay half-life for the betabeta(0+-->01<sup>+</sup>) transition of T1/2<sup>(0 nu + 2 nu)</sup>=[6.0-1.1<sup>+1.9</sup>(stat)±0.6(syst)]×1020 years. Lower limits on decay half-lives were achieved for higher excited final states.

Isovector splitting of nucleon effective masses, ab initio benchmarks and extended stability criteria for Skyrme energy functionals
T. Lesinski, K. Bennaceur, T. Duguet, and J. Meyer
Published 31 October 2006 (15 pages)
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We study the effect of the splitting of neutron and proton effective masses with isospin asymmetry on the properties of the Skyrme energy density functional. We discuss the ability of the latter to predict observables of infinite matter and finite nuclei, paying particular attention to controlling the agreement with ab initio predictions of the spin-isospin content of the nuclear equation of state, as well as diagnosing the onset of finite size instabilities, which we find to be of critical importance. We show that these various constraints cannot be simultaneously fulfilled by the standard Skyrme force, calling at least for an extension of its P-wave part.

Nuclear Reactions

Dynamic nuclear Stark shift in superintense laser fields
Thomas J. Bürvenich, Jörg Evers, and Christoph H. Keitel
Published 3 October 2006 (9 pages)
044601  Full Text: PDF (286 kB)  | Buy Article
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The direct interaction of superintense laser fields in the optical frequency domain with nuclei is studied. As main observable, we consider the nuclear dynamic (AC) Stark shift of low-lying nuclear states due to the off-resonant excitation by the laser field. We include the case of accelerated nuclei to be able to control the frequency and the intensity of the laser field in the nuclear rest frame over a wide range of parameters. We find that AC-Stark shifts of the same order as in typical quantum optical systems relative to the respective transition frequencies are feasible with state-of-the-art or near-future laser field intensities and moderate acceleration of the target nuclei. Along with this shift, but only at intensities above the critical field strength, we find laser-induced modifications to the proton root-mean-square radii and to the proton density distribution. We thus expect direct laser-nucleus interaction to become of relevance together with other super-intense light-matter interaction processes such as pair creation.

Synthesis of the isotopes of elements 118 and 116 in the 249Cf and 245Cm+48Ca fusion reactions
Yu. Ts. Oganessian, V. K. Utyonkov, Yu. V. Lobanov, F. Sh. Abdullin, A. N. Polyakov, R. N. Sagaidak, I. V. Shirokovsky, Yu. S. Tsyganov, A. A. Voinov, G. G. Gulbekian, S. L. Bogomolov, B. N. Gikal, A. N. Mezentsev, S. Iliev, V. G. Subbotin, A. M. Sukhov, K. Subotic, V. I. Zagrebaev, G. K. Vostokin, M. G. Itkis, K. J. Moody, J. B. Patin, D. A. Shaughnessy, M. A. Stoyer, N. J. Stoyer, P. A. Wilk, J. M. Kenneally, J. H. Landrum, J. F. Wild, and R. W. Lougheed
Published 9 October 2006 (9 pages)
044602  Full Text: PDF (487 kB)  | Buy Article
See Also: Physics News Update
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The decay properties of 290116 and 291116, and the dependence of their production cross sections on the excitation energies of the compound nucleus, 293116, have been measured in the 245Cm (48Ca, xn)293-x116 reaction. These isotopes of element 116 are the decay daughters of element 118 isotopes, which are produced via the 249Cf+48Ca reaction. We performed the element 118 experiment at two projectile energies, corresponding to 297118 compound nucleus excitation energies of E*=29.2±2.5 and 34.4±2.3 MeV. During an irradiation with a total beam dose of 4.1×1019 48Ca projectiles, three similar decay chains consisting of two or three consecutive alpha decays and terminated by a spontaneous fission (SF) with high total kinetic energy of about 230 MeV were observed. The three decay chains originated from the even-even isotope 294118 (Ealpha=11.65±0.06 MeV, Talpha=0.89-0.31<sup>+1.07</sup> ms) produced in the 3n-evaporation channel of the 249Cf+48Ca reaction with a maximum cross section of 0.5-0.3<sup>+1.6</sup> pb.

Proton emission induced by polarized photons
M. Anguiano, G. Co', and A. M. Lallena
Published 13 October 2006 (9 pages)
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The proton emission induced by polarized photons is studied in the energy range above the giant resonance region and below the pion emission threshold. Results for the 12C, 16O, and 40Ca nuclei are presented. The sensitivity of various observables to final-state interaction, meson exchange currents, and short-range correlations is analyzed. We found relevant effects due to the virtual excitation of the Delta resonance.

Applications of Skyrme energy-density functional to fusion reactions for synthesis of superheavy nuclei
Ning Wang, Xizhen Wu, Zhuxia Li, Min Liu, and Werner Scheid
Published 16 October 2006 (10 pages)
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The Skyrme energy-density functional approach has been extended to study massive heavy-ion fusion reactions. Based on the potential barrier obtained and the parametrized barrier distribution the fusion (capture) excitation functions of a lot of heavy-ion fusion reactions are studied systematically. The average deviations of fusion cross sections at energies near and above the barriers from experimental data are less than 0.05 for 92% of 76 fusion reactions with Z1Z2<1200. For the massive fusion reactions, for example, the 238U-induced reactions and 48Ca+208Pb, the capture excitation functions have been reproduced remarkably well. The influence of structure effects in the reaction partners on the capture cross sections is studied with our parametrized barrier distribution. By comparing the reactions induced by double-magic nucleus 48Ca and by 32S and 35Cl, the "threshold-like" behavior in the capture excitation function for 48Ca-induced reactions is explored and an optimal balance between the capture cross section and the excitation energy of the compound nucleus is studied. Finally, the fusion reactions with 36S, 37Cl, 48Ca, and 50Ti bombarding 248Cm, 247,249Bk, 250,252,254Cf, and 252,254Es, as well as the reactions leading to the same compound nucleus with Z=120 and N=182, are studied further. The calculation results for these reactions are useful for searching for the optimal fusion configuration and suitable incident energy in the synthesis of superheavy nuclei.

Evidence of large nuclear deformation of 32S* formed in the 20Ne+12C reaction
Aparajita Dey, S. Bhattacharya, C. Bhattacharya, K. Banerjee, T. K. Rana, S. Kundu, S. Mukhopadhyay, D. Gupta, and R. Saha
Published 19 October 2006 (6 pages)
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Deformations of hot composite 32S* formed in the reaction 20Ne(~7–10 MeV/nucleon) + 12C have been estimated from the respective inclusive alpha-particle evaporation spectra. The estimated deformations for 32S* have been found to be much larger than the "normal" deformations of hot, rotating composites at similar excitations. This further confirms the formation of a highly deformed long-lived configuration of 20Ne+12C at high excitations (~70–100 MeV)—which was recently indicated from analysis of complex fragment emission data for the same system. Exclusive alpha-particle evaporation spectra from the decay of hot composite 32S* also show similar behavior.

Fusion and direct reactions around the barrier for the systems 7,9Be,7Li+238U
R. Raabe, C. Angulo, J. L. Charvet, C. Jouanne, L. Nalpas, P. Figuera, D. Pierroutsakou, M. Romoli, and J. L. Sida
Published 19 October 2006 (6 pages)
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We present new cross section data for the complete fusion of the weakly bound systems 7,9Be and 7Li on 238U at energies around the Coulomb barrier. In the same measurement, yields for direct processes and incomplete fusion are detected. For all systems, a suppression of the complete fusion cross section around and above the barrier is observed. At energies below the barrier, the fusion of the 7Be+238U system shows no enhancement with respect to simple model predictions.

Fusion enhancement in the 38S+208Pb reaction
W. Loveland, D. Peterson, A. M. Vinodkumar, P. H. Sprunger, D. Shapira, J. F. Liang, G. A. Souliotis, D. J. Morrissey, and P. Lofy
Published 19 October 2006 (7 pages)
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The capture-fission cross section was measured for the reaction of 38S with 208Pb for center-of-mass projectile energies Ecm of 160–265 MeV. The 38S beam was prepared by projectile fragmentation at the NSCL at higher energies and degraded to 4–7 MeV/nucleon. The time of flight (energy) of each interacting beam particle was measured along with the fission fragments. The data were compared to previous measurements of the capture-fission excitation function for the 32S+208Pb reaction. The interaction barrier for the 38S-induced reaction is 16.1±10.1 MeV lower than the 32S-induced reaction whereas the reduced excitation functions for the two reactions are similar. A discussion of the systematics of barrier shifts in the fusion of n-rich nuclei is given and the implications of this shift for the synthesis of heavy nuclei with radioactive beams are discussed.

Production cross sections of isotopes formed by fragmentation of ~1A GeV 80Kr beam
T. Yamaguchi, T. Ohnishi, T. Suzuki, F. Becker, M. Fukuda, H. Geissel, M. Hosoi, R. Janik, A. Kelic, K. Kimura, S. Mandel, G. Münzenberg, S. Nakajima, T. Ohtsubo, A. Ozawa, A. Prochazka, M. Shindo, B. Sitár, P. Strmen, T. Suda, K. Sümmerer, K. Sugawara, I. Szarka, M. Takechi, A. Takisawa, and K. Tanaka
Published 19 October 2006 (5 pages)
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Production cross sections for the projectile fragmentation of a 1.05A GeV 80Kr beam on a Be target were measured at the projectile fragment separator FRS at GSI. Cross sections were obtained for isotopes of the elements Ge to Kr close to the proton drip line. These data are compared to the results of the empirical parametrization EPAX and to abrasion-ablation calculations. We also compare the results to predictions of intranuclear-cascade calculations. The intranuclear-cascade calculations allow us to also compare well measured one-proton pickup cross sections.

Modification of surface energy in nuclear multifragmentation
A. S. Botvina, N. Buyukcizmeci, M. Erdogan, J. Lukasik, I. N. Mishustin, R. Ogul, and W. Trautmann
Published 20 October 2006 (10 pages)
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Within the statistical multifragmentation model we study modifications of the surface and symmetry energy of primary fragments in the freeze-out volume. The ALADIN experimental data on multifragmentation obtained in reactions induced by high-energy projectiles with different neutron richness are analyzed. We have extracted the isospin dependence of the surface energy coefficient at different degrees of fragmentation. We conclude that the surface energy of hot fragments produced in multifragmentation reactions differs from the values extracted for isolated nuclei at low excitation. At high fragment multiplicity, it becomes nearly independent of the neutron content of the fragments.

Pion-induced double-charge exchange reactions in the Delta resonance region
O. Buss, L. Alvarez-Ruso, A. B. Larionov, and U. Mosel
Published 20 October 2006 (9 pages)
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We have applied the Giessen BUU (GiBUU) transport model to the description of the double-charge exchange (DCX) reaction of pions with different nuclear targets at incident kinetic energies of 120–180 MeV. The DCX process is highly sensitive to details of the interactions of pions with the nuclear medium and, therefore, represents a major benchmark for any model of pion scattering off nuclei at low and intermediate energies. The impact of surface effects, such as the neutron skins of heavy nuclei, is investigated. The dependence of the total cross section on the nuclear mass number is also discussed. We achieve a good quantitative agreement with the extensive data set obtained at LAMPF. Furthermore, we compare the solutions of the transport equations obtained in the test-particle ansatz using two different schemes: the full and the parallel ensemble methods.

New isotope 264Sg and decay properties of 262-264Sg
K. E. Gregorich, J. M. Gates, Ch. E. Düllmann, R. Sudowe, S. L. Nelson, M. A. Garcia, I. Dragojevic, C. M. Folden III, S. H. Neumann, D. C. Hoffman, and H. Nitsche
Published 23 October 2006 (4 pages)
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New isotope, 264Sg, was identified using the 238U(30Si, xn)268-xSg reaction and excitation functions for 262-264Sg were measured. 264Sg decays by spontaneous fission with a half-life of 37-11<sup>+27</sup> ms. The spontaneous fission branch for 0.9-s 263Sg was measured for the first time and found to be (13±8)%. 262Sg decays by spontaneous fission with a 15-3<sup>+5</sup> ms half-life. Spontaneous fission partial half-life systematics are evaluated for even-even Sg isotopes from 258Sg through 266Sg, spanning the transition region between the N=152,Z=100 and N=162,Z=108 deformed shells.

Isotopic production cross sections of spallation-evaporation residues from reactions of 238U(1A GeV) with deuterium
E. Casarejos, J. Benlliure, J. Pereira, P. Armbruster, M. Bernas, A. Boudard, S. Czajkowski, T. Enqvist, R. Legrain, S. Leray, B. Mustapha, M. Pravikoff, F. Rejmund, K.-H. Schmidt, C. Stéphan, J. Taieb, L. Tassan-Got, C. Volant, and W. Wlazlo
Published 23 October 2006 (23 pages)
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Isotopic production cross sections and momentum distributions of 602 residual nuclei produced in the collision of 238U(1A GeV) with deuterium have been measured. These data are relevant for a better understanding of spallation reactions for use as neutron sources for accelerator-driven systems or to produce radioactive nuclear beams. Access to primary residue production makes it possible to study the main reaction mechanisms involved: intranuclear cascade, particle evaporation, and fission. The characteristics of the reaction investigated and the high fissility of the 238U and the dinucleon projectile system are discussed and compared with other available experimental data.

Nuclear symmetry potential in the relativistic impulse approximation
Zeng-Hua Li, Lie-Wen Chen, Che Ming Ko, Bao-An Li, and Hong-Ru Ma
Published 26 October 2006 (7 pages)
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Using the relativistic impulse approximation with the Love-Franey NN scattering amplitude developed by Murdock and Horowitz, we investigate the low-energy (100<=Ekin<=400 MeV) behavior of the nucleon Dirac optical potential, the Schrödinger-equivalent potential, and the nuclear symmetry potential in isospin asymmetric nuclear matter. We find that the nuclear symmetry potential at fixed baryon density decreases with increasing nucleon energy. In particular, the nuclear symmetry potential at saturation density changes from positive to negative values at nucleon kinetic energy of about 200 MeV. Furthermore, the obtained energy and density dependence of the nuclear symmetry potential is consistent with those of the isospin- and momentum-dependent MDI interaction with x=0, which has been found to describe reasonably well both the isospin diffusion data from heavy-ion collisions and the empirical neutron-skin thickness of 208Pb.

High-resolution transmission measurements for 54Cr+n
R. F. Carlton, C. Baker, and J. A. Harvey
Published 30 October 2006 (8 pages)
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Neutron total cross-section measurements on 54Cr up to a neutron energy of 2 MeV have been analyzed to deduce resonance parameters as well as spins and parities. Neutron strength functions have been deduced, based on data in the energy range 20–1000 keV, for s-, p-, and d-wave interactions and, in units of (10-4), are 2.8±0.9, 0.23±0.06, and 4.4±0.7, respectively. Corresponding level densities in units of keV-1are 0.025±0.003, 0.032±0.003, and 0.107±0.006, respectively. The distribution of nearest-neighbor spacings for s waves has been compared to Poisson and Wigner predictions for both 54Cr and 52Cr. Hints are seen of greater chaotic behavior in the case of the closed-neutron-shell 52Cr and of regular dynamics for 54Cr, suggestive of changes in chaoticity of nuclear dynamics with shell closure.

Deuteron global optical model potential for energies up to 200 MeV
Yinlu Han, Yuyang Shi, and Qingbiao Shen
Published 31 October 2006 (11 pages)
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We present a set of new deuteron global phenomenological optical model potential parameters for nuclides in the mass range 12<=A<=209 with incident energies from threshold up to 200 MeV, based on the experimental data of deuteron total reaction cross sections and elastic scattering angular distributions. The extracted deuteron global optical model potential parameters are compared to existing deuteron global optical potential.

Excitation modes of 6He from proton collisions
R. Crespo, I. J. Thompson, and A. M. Moro
Published 31 October 2006 (7 pages)
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The cross section for inelastic scattering of Borromean halo nuclei from protons is evaluated using the Multiple Scattering expansion of the Total transition amplitude (MST) formalism. Differential cross sections and energy spectra are shown for p-6He at 700 MeV/nucleon. The role of spin flip excitations is analyzed.

Relativistic Nuclear Collisions

Azimuthal flow of decay photons in relativistic nuclear collisions
Biswanath Layek, Rupa Chatterjee, and Dinesh K. Srivastava
Published 4 October 2006 (5 pages)
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An overwhelming fraction of photons from relativistic heavy-ion collisions has its origin in the decay of pi0 and eta mesons. We calculate the azimuthal asymmetry of the decay photons for several azimuthally asymmetric pion distributions. We find that the kT dependence of the elliptic flow parameter v2 for the decay photons closely follows the elliptic flow parameter v2<sup>pi[sup 0]</sup> evaluated at pT[approximate]kT+delta, where delta[approximate]0.1-0.2 GeV, for typical pion distributions measured in nucleus-nucleus collisions at relativistic energies. Similar results are obtained for photons from the 2-gamma decay of eta mesons. Assuming that the flow of pi0 is similar to those for pi+ and pi- for which independent measurements would be generally available, this ansatz can help in identifying additional sources for photons. Taken along with quark number scaling suggested by the recombination model, it may help to estimate v2 of the parton distributions in terms of azimuthal asymmetry of the decay photons at large kT.

Parton ladder splitting and the rapidity dependence of transverse momentum spectra in deuteron-gold collisions at the BNL Relativistic Heavy Ion Collider
Klaus Werner, Fu-Ming Liu, and Tanguy Pierog
Published 5 October 2006 (11 pages)
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We present a phenomenological approach (EPOS), based on the parton model, but going much beyond by incorporating elastic and inelastic parton ladder splitting. Based on this model, we try to understand proton-proton and deuteron-gold (dAu) collisions, in particular the rapidity dependence of transverse momentum results in dAu from all four experiments conducted on the BNL Relativistic Heavy Ion Collider.

Multiplicity fluctuations in hadron-resonance gas
V. V. Begun, M. I. Gorenstein, M. Hauer, V. P. Konchakovski, and O. S. Zozulya
Published 5 October 2006 (10 pages)
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The charged hadron multiplicity fluctuations are considered in the canonical ensemble. The microscopic correlator method is extended to include three conserved charges: baryon number, electric charge, and strangeness. The analytical formulas are presented that allow us to include resonance decay contributions to correlations and fluctuations. We make the predictions for the scaled variances of negative, positive, and all charged hadrons in the most central Pb+Pb (Au+Au) collisions for different collision energies from SIS and AGS to SPS and RHIC.

Dissipative hydrodynamics in 2+1 dimensions
A. K. Chaudhuri
Published 5 October 2006 (9 pages)
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In 2+1 dimensions, we simulated the hydrodynamic evolution of quark-gluon plasma (QGP) fluid with dissipation due to shear viscosity. Comparison of the evolution of ideal and viscous fluids, both initialized under the same conditions, e.g., same equilibration time, energy density and velocity profile, reveals that the dissipative fluid evolves slowly, cooling at a slower rate. Cooling slows even more at higher viscosities. The fluid velocities, however, evolve faster in a dissipative fluid than in an ideal fluid. The transverse expansion is also enhanced in dissipative evolution. For the same decoupling temperature, the freeze-out surface for a dissipative fluid is more extended than that for an ideal fluid. Dissipation produces entropy as a result of which particle production is increased. Particle production is increased as a result of the (i) the extension of the freeze-out surface and (ii) the change of the equilibrium distribution function to a nonequilibrium one, the latter effect being prominent at large transverse momentum. Compared to ideal fluid, transverse momentum distribution of pion production is considerably enhanced. Enhancement is greater at high pT than at low pT. Pion production also increases with viscosity; the greater the viscosity, the greater the pion production. Dissipation also modifies the elliptic flow, which is reduced in viscous dynamics. Also, contrary to ideal dynamics where elliptic flow continues to increase with transverse momentum, in viscous dynamics elliptic flow tends to saturate at large transverse momentum. The analysis suggests that the initial conditions of the hot, dense matter produced in Au+Au collisions at the Relativistic Heavy Ion Collider (RHIC), as extracted from ideal fluid analysis, can be changed significantly if the QGP fluid is viscous.

Eccentricity in heavy-ion collisions from color glass condensate initial conditions
Azfar Adil, Hans-Joachim Drescher, Adrian Dumitru, Arata Hayashigaki, and Yasushi Nara
Published 19 October 2006 (6 pages)
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The eccentricity in coordinate space at midrapidity of the overlap zone in high-energy heavy-ion collisions predicted by the k[perpendicular]-factorization formalism is generically larger than expected from scaling with the number of participants. We provide a simple qualitative explanation of the effect which shows that it is not caused predominantly by edge effects. We also show that it is quite insensitive to "details" of the unintegrated gluon distribution functions such as the presence of leading-twist shadowing and of an extended geometric scaling window. The larger eccentricity increases the azimuthal asymmetry of high transverse momentum particles. Finally, we point out that the longitudinal structure of the color glass condensate initial condition for hydrodynamics away from midrapidity is nontrivial but requires understanding of large-x effects.

Effect of an equilibrium phase transition on multiphase transport in relativistic heavy ion collisions
Yu Meiling, Du Jiaxin, and Liu Lianshou
Published 30 October 2006 (6 pages)
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The hadronization scheme for parton transport in relativistic heavy ion collisions is considered in detail. It is pointed out that the traditional scheme for particles being freezed out one by one leads to serious problem on unreasonable long lifetime of partons. A collective phase transition following a supercooling is implemented in a simple way. It turns out that the modified model with a sudden phase transition is able to reproduce the experimental longitudinal distributions of final state particles better than the original one does. The encouraging results indicate that equilibrium phase transition should be taken into proper account in parton transport models for relativistic heavy ion collisions.

J/psi production in Au+Au collisions at sqrt(s[sub NN])=200 GeV and the nuclear absorption
A. K. Chaudhuri
Published 31 October 2006 (6 pages)
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It is shown that a quantum chromodynamics-based nuclear absorption model, with few parameters fixed to reproduce experimental J/psi yield in 200 GeV proton-proton/proton-nucleon and 450 GeV proton-nucleon collisions, can explain the preliminary PHENIX data on the centrality dependence of J/psi suppression in Cu+Cu collisions at Relativistic Heavy Ion Collider (RHIC) energy, sqrt(s[sub NN])=200 GeV. However, the model does not give a satisfactory description for the preliminary PHENIX data on the centrality dependence of J/psi suppression in Au+Au collisions. The analysis suggests that in Au+Au collisions, J/psi are suppressed in a medium unlike the medium produced in CERN Super Proton Synchrotron energy nuclear collisions or in RHIC energy Cu+Cu collisions.

Classical strongly coupled quark-gluon plasma. I. Model and molecular dynamics simulations
Boris A. Gelman, Edward V. Shuryak, and Ismail Zahed
Published 31 October 2006 (9 pages)
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We propose a model for the description of strongly interacting quarks and gluon quasiparticles at T=(1-3)Tc as a classical and nonrelativistic colored Coulomb gas. The sign and strength of the interparticle interactions are fixed by the scalar product of their classical color vectors subject to Wong's equations. The model displays a number of phases as the Coulomb coupling is increased ranging from a gas, to a liquid, to a crystal with antiferromagnetic-like color ordering. We analyze the model using molecular dynamics simulations and discuss the density-density correlator in real time. We extract pertinent decorrelation times, diffusion, and viscosity constants for all phases. The classical results when extrapolated to the strongly coupled quark-gluon plasma suggest that the phase is liquid-like, with a diffusion constant D[approximate]0.1/T and a shear viscosity to entropy density ratio eta/s[approximate]1/3.

Classical strongly coupled quark-gluon plasma. II. Screening and equation of state
Boris A. Gelman, Edward V. Shuryak, and Ismail Zahed
Published 31 October 2006 (8 pages)
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We analyze the screening and bulk energy of a classical and strongly interacting plasma of color charges, a model we recently introduced for the description of a quark-gluon plasma at T=(1-3)Tc. The partition function is organized around the Debye-Hückel limit. The linear Debye-Hückel limit is corrected by a virial expansion. For the pressure, the expansion is badly convergent even in the dilute limit. The nonlinear Debye-Hückel theory is studied numerically as an alternative for moderately strong plasmas. We use the Debye theory of solid to extend the analysis to the crystal phase at very strong coupling. The analytical results for the bulk energy per particle compare well with the numerical results from molecular dynamics simulations for all couplings.

Hadronic Physics and QCD

Vector-meson–baryon coupling constants in QCD sum rules
G. Erkol, R. G. E. Timmermans, and Th. A. Rijken
Published 2 October 2006 (7 pages)
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The external-field quantum chromodynamics (QCD) sum rules method is used to evaluate the coupling constants of the vector mesons rho and omega to the nucleon and the Lambda,Sigma, and Xi baryons. It is shown that these coupling constants as calculated from QCD sum rules are consistent with SU(3)-flavor relations. By assuming ideal mixing, this leads to a determination of the F/(F+D) ratio of the vector-meson octet: we find alphav=1 and alpham=0.18 for the vector and the magnetic F/(F+D) ratios, respectively. The sensitivity of the results to the unknown vacuum susceptibility zeta is discussed. The coupling constants with SU(3)-breaking effects taken into account are also calculated.

Gauge-invariant approach to meson photoproduction including the final-state interaction
H. Haberzettl, K. Nakayama, and S. Krewald
Published 6 October 2006 (13 pages)
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A fully gauge-invariant (pseudoscalar) meson photoproduction amplitude off a nucleon including the final-state interaction is derived. The approach based on a comprehensive field-theoretical formalism developed earlier by one of the authors replaces certain dynamical features of the full interaction current by phenomenological auxiliary contact currents. A procedure is outlined that allows for a systematic improvement of this approximation. The feasibility of the approach is illustrated by applying it to both the neutral and charged pion photoproductions.

eta- and eta'-mesic nuclei and UA(1) anomaly at finite density
Hideko Nagahiro, Makoto Takizawa, and Satoru Hirenzaki
Published 20 October 2006 (9 pages)
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We discuss theoretically the possibility of observing the bound states of the eta and eta'(958) mesons in nuclei. We apply the NJL model to study the eta and eta' meson properties at finite density and calculate the formation cross sections of the eta and eta' bound states with the Green function method for (gamma,p) reaction. We also discuss the experimental feasibility at photon facilities like SPring-8. The contributions due to the omega meson production are also included to obtain the realistic (gamma,p) spectra. We conclude that we can expect to observe resonance peaks in (gamma,p) spectra for the formation of meson bound states and we can deduce new information on eta and eta' properties at finite density. These observations are believed to be essential to know the possible mass shift of eta' and deduce new information on the effective restoration of the UA(1) anomaly in the nuclear medium.

Measurement of the helicity dependence for the gammap-->npi+ channel in the second resonance region
J. Ahrens et al. (GDH- and A-2 Collaborations)
Published 20 October 2006 (10 pages)
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The helicity dependence of the gammap-->npi+ reaction has been measured for the first time in the photon energy range from 450 to 790 MeV. The experiment, performed at the Mainz Microtron (MAMI), used a 4pi-detector system, a circularly polarized, tagged photon beam, and a longitudinally polarized frozen-spin target. Although these polarized data are mainly sensitive to mechanisms involving the D13(1520) resonance, a cusp structure at the eta production threshold can be clearly observed. These data are significantly different from the predictions of the existing multipole analyses and are used to determine the helicity amplitudes of the D13(1520) and S11(1535) resonances with improved accuracy.

Extended partial-wave analysis of piN scattering data
R. A. Arndt, W. J. Briscoe, I. I. Strakovsky, and R. L. Workman
Published 23 October 2006 (14 pages)
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We present results from a comprehensive partial-wave analysis of pi±p elastic scattering and charge-exchange data, covering the region from threshold to 2.6 GeV in the lab pion kinetic energy, employing a coupled-channel formalism to simultaneously fit pi-p-->etan data to 0.8 GeV. Our main result, solution SP06, utilizes a complete set of forward and fixed-t dispersion relation constraints applied to the piN elastic amplitude. The results of these analyses are compared with previous solutions in terms of their resonance spectra and preferred values for couplings and low-energy parameters.

Pentaquark state in pole-dominated QCD sum rules
Toru Kojo, Arata Hayashigaki, and Daisuke Jido
Published 30 October 2006 (8 pages)
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We propose a new approach in QCD sum rules applied for exotic hadrons with a number of quarks, exemplifying the pentaquark Theta+(I=0,J=1/2) in the Borel sum rule. Our approach enables reliable extraction of the pentaquark properties from the sum rule with good stability in a remarkably wide Borel window. The appearance of its valid window originates from a favorable setup of the correlation functions with the aid of chirality of the interpolating fields on the analogy of the Weinberg sum rule for the vector currents. Our setup leads to large suppression of the continuum contributions which have spoiled the Borel stability in the previous analyses, and consequently enhances importance of the higher-dimensional contributions of the OPE, which are indispensable for investigating the pentaquark properties. Implementing the OPE analysis up to dimension 15, we find that the sum rules for the chiral-even and odd parts independently give the Theta+ mass of 1.68±0.22 GeV with uncertainties of the condensate values. Our sum rule indeed gives rather flat Borel curves almost independent of the continuum thresholds both for the mass and pole residue. Finally, we also discuss possible isolation of the observed states from the KN scattering state on view of chiral symmetry.

Nuclear Astrophysics

Unstable modes in relativistic neutron-electron-proton (npe) matter at finite temperature
L. Brito, C. Providência, A. M. Santos, S. S. Avancini, D. P. Menezes, and Ph. Chomaz
Published 4 October 2006 (9 pages)
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Isospin and density waves in neutral neutron-proton-electron (npe) matter are studied within a relativistic mean-field hadron model at finite temperature with the inclusion of the electromagnetic field. The dispersion relation is calculated and the collective modes are obtained. The unstable modes are discussed and the spinodals, which separate the stable from the unstable regions, are shown for different values of the momentum transfer at various temperatures. The critical temperatures are compared with the ones obtained in a system without electrons. The largest critical temperature, 12.39 MeV, occurs for a proton fraction yp=0.47. For yp=0.3 we get Tcr=5 MeV and for yp>0.495 Tcr<~8 MeV. It is shown that at finite temperature the distillation effect in asymmetric matter is not so efficient and that electron effects are particularly important for small momentum transfers.

Coupling of nuclear and electron modes in relativistic stellar matter
C. Providência, L. Brito, A. M. S. Santos, D. P. Menezes, and S. S. Avancini
Published 6 October 2006 (6 pages)
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The conditions under which nuclear collective modes couple to plasmon modes in asymmetric nuclear matter (ANM) neutralized by electrons, which is of interest for the study of neutron stars and supernovae, are investigated. The calculations were performed within a relativistic mean-field approach to nuclear matter, and the Coulomb field was included. We show that the coupling may be so strong that it affects the onset of the nuclear mode at low densities and may also affect its isovector/isoscalar character.

Lifetime of 19Ne*(4.03 MeV)
R. Kanungo, T. K. Alexander, A. N. Andreyev, G. C. Ball, R. S. Chakrawarthy, M. Chicoine, R. Churchman, B. Davids, J. S. Forster, S. Gujrathi, G. Hackman, D. Howell, J. R. Leslie, A. C. Morton, S. Mythili, C. J. Pearson, J. J. Ressler, C. Ruiz, H. Savajols, M. A. Schumaker, I. Tanihata, P. Walden, and S. Yen
Published 10 October 2006 (6 pages)
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The Doppler-shift attenuation method was applied to measure the lifetime of the 4.03 MeV state in 19Ne. By utilizing a 3He-implanted Au foil as a target, the state was populated using the 20Ne(3He, alpha)19Ne reaction in inverse kinematics at a 20Ne beam energy of 34 MeV. De-excitation gamma rays were detected in coincidence with alpha particles. At the 1sigma level, the lifetime was determined to be 11-3<sup>+4</sup> fs and at the 95.45% confidence level the lifetime is 11-7<sup>+8</sup> fs.

Astrophysically important 26Si states studied with the 28Si(p,t)26Si reaction. II. Spin of the 5.914-MeV 26Si level and galactic 26Al production
D. W. Bardayan, J. A. Howard, J. C. Blackmon, C. R. Brune, K. Y. Chae, W. R. Hix, M. S. Johnson, K. L. Jones, R. L. Kozub, J. F. Liang, E. J. Lingerfelt, R. J. Livesay, S. D. Pain, J. P. Scott, M. S. Smith, J. S. Thomas, and D. W. Visser
Published 11 October 2006 (6 pages)
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The 28Si(p,t)26Si reaction has been studied to resolve a controversy surrounding the properties of the 26Si level at 5.914 MeV and its contribution to the 25Al(p,gamma)26Si reaction rate in novae, which affects interpretations of galactic 26Al observations. Recent studies have come to contradictory conclusions regarding the spin of this level (0+ or 3+), with a 3+ assignment implying a large contribution by this level to the 25Al(p,gamma)26Si reaction rate. We have extended our previous study [Bardayan et al., Phys. Rev. C 65, 032801(R) (2002)] to smaller angles and find the angular distribution of tritons populating the 5.914-MeV level in the 28Si(p,t)26Si reaction to be consistent with either a 2+ or 3+ assignment. We have calculated reaction rates under these assumptions and used them in a nova nucleosynthesis model to examine the effects of the remaining uncertainties in the 25Al(p,gamma)26Si rate on 26Al production in novae.

CNO and pep neutrino spectroscopy in Borexino: Measurement of the deep-underground production of cosmogenic 11C in an organic liquid scintillator
H. Back et al. (Borexino Collaboration)
Published 11 October 2006 (6 pages)
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Borexino is an experiment for low-energy neutrino spectroscopy at the Gran Sasso underground laboratories. It is designed to measure the monoenergetic 7Be solar neutrino flux in real time, via neutrino-electron elastic scattering in an ultrapure organic liquid scintillator. Borexino has the potential to also detect neutrinos from the pep fusion process and the CNO cycle. For this measurement to be possible, radioactive contamination in the detector must be kept extremely low. Once sufficiently clean conditions are met, the main background source is 11C, produced in reactions induced by the residual cosmic muon flux on 12C. In the process, a free neutron is almost always produced. 11C can be tagged on an event-by-event basis by looking at the threefold coincidence with the parent muon track and the subsequent neutron capture on protons. This coincidence method has been implemented on the Borexino Counting Test Facility data. We report on the first event-by-event identification of in situ muon-induced 11C in a large underground scintillator detector. We measure a 11C production rate of 0.130 ± 0.026(stat) ± 0.014(syst) day-1 ton-1, in agreement with predictions from both experimental studies performed with a muon beam on a scintillator target and ab initio estimations based on the 11C producing nuclear reactions.

Low density instability in relativistic mean field models
A. Sulaksono and T. Mart
Published 16 October 2006 (12 pages)
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The effects of symmetry energy softening of relativistic mean field (RMF) models on the properties of matter with neutrino trapping are investigated. It is found that the effects are less significant than those in the case without neutrino trapping. The weak dependence of the equation of state on the symmetry energy is shown as the main reason for this finding. Using different RMF models the dynamical instabilities of uniform matters, with and without neutrino trapping, have been also studied. The interplay between the dominant contribution of the variation of matter composition and the role of effective masses of mesons and nucleons leads to higher critical densities for matter with neutrino trapping. Furthermore, the predicted critical density is insensitive to both the number of trapped neutrinos as well as the RMF model used in the investigation. It is also found that additional nonlinear terms in the Horowitz-Piekarewicz and Furnstahl-Serot-Tang models prevent another kind of instability, which occurs at relatively high densities, because the effective sigma meson mass in their models increases as a function of matter density.

Neutron star matter in the quark-meson coupling model in strong magnetic fields
P. Yue and H. Shen
Published 19 October 2006 (6 pages)
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The effects of strong magnetic fields on neutron star matter are investigated in the quark-meson coupling (QMC) model. The QMC model describes a nuclear many-body system as nonoverlapping MIT bags in which quarks interact through self-consistent exchange of scalar and vector mesons in the mean-field approximation. The results of the QMC model are compared with those obtained in a relativistic mean-field (RMF) model. It is found that quantitative differences exist between the QMC and RMF models, whereas qualitative trends of the magnetic-field effects on the equation of state and composition of neutron star matter are very similar.

High-density symmetry energy and direct Urca process
Andrew W. Steiner
Published 30 October 2006 (5 pages)
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The symmetry energy of nucleonic matter is usually assumed to be quadratic in the isospin density. While this may be justified at subsaturation densities, there is no need to enforce this restriction at supersaturation densities. The presence of a quartic term can strongly modify the critical density for the direct Urca process which leads to faster cooling of neutron stars. Neutron star cooling predictions which lie below the observational data can, for some equations of state, be repaired with a quartic term which effectively turns off the direct Urca process.

Diagrammatic calculation of thermodynamical quantities in nuclear matter
V. Somà and P. Bozek
Published 30 October 2006 (7 pages)
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In medium T-matrix calculations for symmetric nuclear matter at zero and finite temperatures are presented. The internal energy is calculated from the Galitskii-Koltun's sum rule and from the summation of the diagrams for the interaction energy. The pressure at finite temperature is obtained from the generating functional form of the thermodynamic potential. The entropy at high temperature is estimated and compared to expressions corresponding to a quasiparticle gas.

beta decay of proton-rich nucleus 23Al and astrophysical consequences
V. E. Iacob, Y. Zhai, T. Al-Abdullah, C. Fu, J. C. Hardy, N. Nica, H. I. Park, G. Tabacaru, L. Trache, and R. E. Tribble
Published 31 October 2006 (6 pages)
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We present the first study of the beta decay of 23Al undertaken with pure samples. The study was motivated by nuclear astrophysics questions. Pure samples of 23Al were obtained from the momentum achromat recoil separator (MARS) of Texas A&M University, collected on a fast tape-transport system, and moved to a shielded location where beta and beta-gamma coincidence measurements were made. We deduced beta branching ratios and log ft values for transitions to states in 23Mg, and from them determined unambiguously the spin and parity of the 23Al ground state to be Jpi=5/2+. We discuss how this excludes the large increases in the radiative proton capture cross section for the reaction 22Mg(p,gamma)23Al at astrophysical energies, which were implied by claims that the spin and parity is Jpi=1/2+. The log ft for the Fermi transition to its isobaric analog state (IAS) in 23Mg is also determined for the first time. This IAS and a state 16 keV below it are observed, well separated in the same experiment for the first time. We can now solve a number of inconsistencies in the literature, exclude strong isospin mixing claimed before, and obtain a new determination of the resonance strength. Both states are resonances in the 22Na(p,gamma)23Mg reaction at energies important in novae. The reactions 22Mg(p,gamma)23Al and 22Na(p,gamma)23Mg have both been suggested as possible candidates for diverting some of the flux in oxygen-neon novae explosions from the A=22 into the A=23 mass chain.

BRIEF REPORTS

Model independent determination of the spin of the 180Ta naturally occurring isomer
M. L. Bissell, K. Baczynska, J. Billowes, P. Campbell, B. Cheal, T. Eronen, D. H. Forest, M. D. Gardner, I. D. Moore, B. Tordoff, G. Tungate, and J. Äystö
Published 6 October 2006 (4 pages)
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The hyperfine structures of the 33715.27 cm-1 and 33706.47 cm-1 transitions from the ground state of singly ionized Ta have been measured by collinear laser spectroscopy. The structures were found to contain a large second order contribution. From fitting the observed hyperfine components for both 181Ta and the 180Ta naturally occurring isomer it was possible to determine the first and second order hyperfine structure coefficients. As no model independent determination of the nuclear spin of the 180Ta isomer has been performed, fitting was attempted for a range of spins. A clear chi-squared minimum is obtained for a nuclear spin of 9, in agreement with model dependent measurements.

One-neutron knockout in the vicinity of the N=32 sub-shell closure: 9Be(57Cr,56Cr+gamma)X
A. Gade, R. V. F. Janssens, D. Bazin, B. A. Brown, C. M. Campbell, M. P. Carpenter, J. M. Cook, A. N. Deacon, D.-C. Dinca, S. J. Freeman, T. Glasmacher, M. Horoi, B. P. Kay, P. F. Mantica, W. F. Mueller, J. R. Terry, J. A. Tostevin, and S. Zhu
Published 11 October 2006 (4 pages)
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The one-neutron knockout reaction 9Be(57Cr,56Cr+gamma)X has been measured in inverse kinematics with an intermediate-energy beam. Cross sections to individual states in 56Cr were partially untangled through the detection of the characteristic gamma-ray transitions in coincidence with the reaction residues. The experimental inclusive longitudinal momentum distribution and the yields to individual states are compared to calculations that combine spectroscopic factors from the full fp shell model and nucleon-removal cross sections computed in a few-body eikonal approach.

Search for beta-delayed fission of 228Ac
Xu Yanbing, Zhang Shengdong, Ding Huajie, Yuan Shuanggui, Yang Weifan, Niu Yanning, Lu Xiting, Li Yingjun, and Xiao Yonghou
Published 20 October 2006 (3 pages)
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Radium was radiochemically separated from natural thorium. Thin 228Ra-->beta-228Ac sources were prepared and exposed to mica fission track detectors, and measured by an HPGe gamma-ray detector. The beta-delayed fission events of 228Ac were observed and its beta-delayed fission probability was found to be (5±2)×10-12.

Nuclear matter saturation point and symmetry energy with modern nucleon-nucleon potentials
Z. H. Li, U. Lombardo, H.-J. Schulze, W. Zuo, L. W. Chen, and H. R. Ma
Published 20 October 2006 (4 pages)
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We determine the saturation properties of nuclear matter within the Brueckner-Hartree-Fock approach based on a large set of modern nucleon-nucleon potentials and confirm the validity of the Coester band. The improvement of the saturation point when including nuclear three-body forces is pointed out and comparison with the Dirac-Brueckner-Hartree-Fock results is made.

Determination of the 22Nenucl/4Herad ratio in natural uranium-rich fluorite by mass spectrometry
Jesús Solé and Teresa Pi
Published 16 October 2006 (4 pages)
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A determination by noble gas mass spectrometry of 22Ne production through the combined reactions 19F(alpha,n)22Na(beta+)22Ne and 19F(alpha,p)22Ne on natural calcium fluoride is made for the first time. Six samples of U-rich fluorite from a fluorspar deposit in Mexico were used to determine the 22Nenucl/4Herad ratio generated by the spontaneous decay of U during the last 32 Ma. The obtained ratio (1.33 ± 0.11) ×10-5 (95% confidence), is compared to other experimental data on natural uranium oxides and theoretical values.

Maximum freeze-out baryon density in nuclear collisions
J. Randrup and J. Cleymans
Published 6 October 2006 (3 pages)
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Using simple parametrizations of the thermodynamic freeze-out parameters extracted from the data over a wide beam-energy range, we reexpress the hadronic freeze-out line in terms of the underlying dynamic quantities, the net baryon density rhoB and the energy density epsilon, which are subject to local conservation laws. This analysis reveals that rhoB exhibits a maximum as the collision energy is decreased. This maximum freeze-out density has µ=400–500 MeV, which is above the critical value, and it is reached for a fixed-target bombarding energy of 20–30 GeV/nucleon.