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Physical Review C

(Nuclear Physics)

November 2009

Volume 80, Number 5 , partial issue

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RAPID COMMUNICATIONS
Rapid

Gamow shell-model calculations of drip-line oxygen isotopes
K. Tsukiyama, M. Hjorth-Jensen, and G. Hagen
Published 4 November 2009 (5 pages)
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We employ the Gamow shell model (GSM) to describe low-lying states of the oxygen isotopes 24O and 25O. The many-body Schrödinger equation is solved starting from a two-body Hamiltonian defined by a renormalized low-momentum nucleon-nucleon (NN) interaction and a spherical Berggren basis. The Berggren basis treats bound, resonant, and continuum states on an equal footing and is therefore an appropriate representation of loosely bound and unbound nuclear states near threshold. We show that the inclusion of continuum effects has a significant effect on the low-lying 1+ and 2+ excited states in 24O. On the other hand, we find that a correct description of binding energy systematics of the ground states is driven by the proper treatment and inclusion of many-body correlation effects. This is supported by the fact that we get 25O unstable with respect to 24O in both oscillator and Berggren representations starting from a 22O core. Furthermore, we show that the structure of these loosely bound or unbound isotopes is strongly influenced by the 1S0 component of the NN interaction. This has important consequences for our understanding of nuclear stability.
Rapid

Four-body continuum-discretized coupled-channels calculations
M. Rodríguez-Gallardo, J. M. Arias, J. Gómez-Camacho, A. M. Moro, I. J. Thompson, and J. A. Tostevin
Published 2 November 2009 (5 pages)
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The development of a continuum-bin scheme of discretization for three-body projectiles is necessary for studies of reactions of Borromean nuclei such as 6He within the continuum-discretized coupled-channels approach. Such a procedure, for constructing bin states on selected continuum energy intervals, is formulated and applied for the first time to reactions of a three-body projectile. The continuum representation uses the eigenchannel expansion of the three-body S matrix. The method is applied to the challenging case of the 6He+208Pb reaction at 22 MeV, where an accurate treatment of both the Coulomb and the nuclear interactions with the target is necessary.
Rapid

Scaling patterns of the suppression of pi0 yields in Au+Au collisions at sqrt(s[sub NN])=200 GeV: Links to the transport properties of the QGP
Roy A. Lacey, N. N. Ajitanand, J. M. Alexander, X. Gong, J. Jia, A. Taranenko, and Rui Wei
Published 3 November 2009 (4 pages)
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Suppression measurements for neutral pions (pi0) are used to investigate the predicted path length L and transverse momentum pT dependent jet quenching patterns of the hot QCD medium produced in Au+Au collisions at sqrt(s[sub NN])=200 GeV. The observed scaling patterns show the predicted trends for jet-medium interactions dominated by radiative energy loss. They also allow simple estimates of the transport coefficient q-hat and the ratio of viscosity to entropy density eta/s. These estimates indicate that the short mean free path lambda in the QCD medium leading to hydrodynamic-like flow with a small value of eta/s is also responsible for the strong suppression observed.

ARTICLES

Nuclear Structure

Excited state g factors in 125Te
S. K. Chamoli, A. E. Stuchbery, and M. C. East
Published 3 November 2009 (7 pages)
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The transient-field technique has been used to measure, with considerably improved precision, the g factors of the 3/2+ and 5/2+ states in 125Te at 444 and 463 keV, respectively, relative to the g factor of the first excited state in 126Te. Together with shell model and weak-coupling core-excitation model calculations, the g-factor measurements provide insight into the orbital occupation of the odd neutron for the low-excitation states in 125Te. A new 9/2+ level at 1029 keV, together with a firm 7/2+ spin assignment for the level at 1018 keV, identifies candidate states for the coupling of the s1/2 neutron to the 4+ core excitation.

Electric quadrupole moments of the 2<sub>1</sub><sup>+</sup> states in 100,102,104Cd
A. Ekström, J. Cederkäll, D. D. DiJulio, C. Fahlander, M. Hjorth-Jensen, A. Blazhev, B. Bruyneel, P. A. Butler, T. Davinson, J. Eberth, C. Fransen, K. Geibel, H. Hess, O. Ivanov, J. Iwanicki, O. Kester, J. Kownacki, U. Köster, B. A. Marsh, P. Reiter, M. Scheck, B. Siebeck, S. Siem, I. Stefanescu, H. K. Toft, G. M. Tveten, J. Van de Walle, D. Voulot, N. Warr, D. Weisshaar, F. Wenander, K. Wrzosek, and M. Zielińska
Published 3 November 2009 (11 pages)
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Using the REX-ISOLDE facility at CERN the Coulomb excitation cross sections for the 0gs<sup>+</sup>-->21<sup>+</sup> transition in the beta-unstable isotopes 100,102,104Cd have been measured for the first time. Two different targets were used, which allows for the first extraction of the static electric quadrupole moments Q(21<sup>+</sup>) in 102,104Cd. In addition to the B(E2) values in 102,104Cd, a first experimental limit for the B(E2) value in 100Cd is presented. The data was analyzed using the maximum likelihood method. The provided probability distributions impose a test for theoretical predictions of the static and dynamic moments. The data are interpreted within the shell-model using realistic matrix elements obtained from a G-matrix renormalized CD-Bonn interaction. In view of recent results for the light Sn isotopes the data are discussed in the context of a renormalization of the neutron effective charge. This study is the first to use the reorientation effect for post-accelerated short-lived radioactive isotopes to simultaneously determine the B(E2) and the Q(21<sup>+</sup>) values.

Reviewing the evidence for two-proton emission from the high-spin isomer in 94Ag
David G. Jenkins
Published 4 November 2009 (6 pages)
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The emission of two protons, a highly exotic decay process, has been reported from the high-spin 21+ isomer in 94Ag [I. Mukha et al., Nature (London) 439, 298 (2006)]. Recent experimental results from gamma-ray spectroscopy and Penning trap mass measurements, as well as theoretical predictions, are difficult to reconcile with this claim. These anomalies motivate a closer look at the supporting evidence. A detailed review of this evidence suggests that is it unsafe to conclude that the methods used in the two-proton identification experiment demonstrate an “unambiguous signature” for two-proton emission. Because the apparatus used could not distinguish protons from positrons and electrons, the identification stands or falls on coincidences with gamma rays in the daughter nucleus 92Rh. It is shown that spurious peaks from Compton-scattered gamma rays associated with the dominant background from 94Ag beta decay could have been misidentified as 92Rh gamma rays.

beta decay of the pif5/2 ground state of 77Cu studied with 225 MeV and 0.2 MeV purified radioactive beams
S. V. Ilyushkin, J. A. Winger, C. J. Gross, K. P. Rykaczewski, J. C. Batchelder, L. Cartegni, I. G. Darby, C. Goodin, R. Grzywacz, J. H. Hamilton, A. Korgul, W. Królas, S. N. Liddick, C. Mazzocchi, S. Padgett, A. Piechaczek, M. M. Rajabali, D. Shapira, and E. F. Zganjar
Published 6 November 2009 (10 pages)
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Isobarically purified beams of 77Cu with energies of 225 and 0.2 MeV were used at the Holifield Radioactive Ion Beam Facility of Oak Ridge National Laboratory to study beta decay into states in 77Zn. Data taken at 225 MeV allowed the determination of absolute branching ratios relative to the decay of 77Cu for this beta decay as well as its daughters. From these we obtained a refined beta-delayed neutron emission probability of 30.3(22)% and a probability that the decay proceeds through 77Zng of 49.1(26)%. A total of 64  gamma rays were placed in a level scheme for 77Zn containing 35 excited states including one state above the neutron separation energy, whereas two gamma rays were observed for the betan branch to states in 76Zn. The growth and decay curves of some prominent gamma rays indicate a single beta-decaying state with a half-life of 480(9)  ms. The decay pattern for 77Cu, with observed feeding of 8(3)% to 7/2+  77Zng and 6(3)% to 1/2-  77Znm, in contrast to the large feeding observed for decay of pip3/2 73Cug to 1/2-  73Zng, strongly suggests a pif5/2 ground state for the studied 77Cu activity.

Relativistic Nuclear Collisions

Photoproduction of rho0 mesons in ultraperipheral heavy ion collisions at energies available at the BNL Relativistic Heavy Ion Collider (RHIC) and CERN Large Hadron Collider (LHC)
V. P. Gonçalves and M. V. T. Machado
Published 2 November 2009 (6 pages)
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We investigate the photoproduction of rho mesons in ultraperipheral heavy ion collisions at the BNL Relativistic Heavy Ion Collider (RHIC) and CERN Large Hadron Collider (LHC) energies in the dipole approach and within two phenomenological models based on the color glass condensate (CGC) formalism. We estimate the integrated cross section and rapidity distribution for meson production and compare our predictions with the data from the STAR Collaboration. In particular, we demonstrate that the total cross section at RHIC is strongly dependent on the energy behavior of the dipole-target cross section at low energies, which is not well determined in the dipole approach. In contrast, the predictions at midrapidities at RHIC and in the full rapidity at LHC are under theoretical control and can be used to test QCD dynamics at high energies.

Light-cone wave function approach to open heavy flavor dynamics in QCD matter
Rishi Sharma, Ivan Vitev, and Ben-Wei Zhang
Published 4 November 2009 (17 pages)
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We calculate the lowest-order charm and beauty parton distribution functions in and fragmentation functions into D and B mesons using the operator definitions of factorized perturbative quantum chromodynamics (QCD). In the vacuum, we find the leading corrections that arise from the structure of the final-state hadrons. Quark-antiquark potentials extracted from the lattice are employed to demonstrate the existence of open heavy flavor bound-state solutions in the quark-gluon plasma in the vicinity of the critical temperature. We provide first results for the in-medium modification of the heavy-quark distribution and decay probabilities in a comoving plasma. In an improved perturbative QCD description of heavy-flavor dynamics in the thermal medium, we combine D- and B-meson formation and dissociation with parton-level charm and beauty quark quenching to obtain predictions for the heavy-meson and nonphotonic-electron suppression in Cu+Cu and Pb+Pb collisions at the Relativistic Heavy Ion Collider and the Large Hadron Collider, respectively.

Initial eccentricity in deformed 197Au+197Au and 238U+238U collisions at sqrt(s[sub NN])=200 GeV at the BNL Relativistic Heavy Ion Collider
Peter Filip, Richard Lednicky, Hiroshi Masui, and Nu Xu
Published 5 November 2009 (5 pages)
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Initial eccentricity and eccentricity fluctuations of the interaction volume created in relativistic collisions of deformed 197Au and 238U nuclei are studied using optical and Monte Carlo (MC) Glauber simulations. It is found that the nonsphericity noticeably influences the average eccentricity in central collisions, and eccentricity fluctuations are enhanced from deformation. Quantitative results are obtained for Au+Au and U+U collisions at energy sqrt(s[sub NN])=200 GeV.

Nuclear structure functions at small x
Jamal Jalilian-Marian
Published 6 November 2009 (6 pages)
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I study the nuclear structure function F2<sup>A</sup> and its logarithmic derivative in the high-energy limit (small-x region) using the color glass condensate formalism. In this limit the structure function F2 depends on the quark-antiquark dipole-target scattering cross section NF(xbj,rt,bt). The same dipole cross section appears in single-hadron and hadron-photon production cross sections in the forward rapidity region in deuteron (proton)-nucleus collisions at high energy, that is, at energies available at the BNL Relativistic Heavy Ion Collider (RHIC) and the CERN Large Hadron Collider (LHC). I use a parametrization of the dipole cross section, which has successfully been used to describe the deuteron-gold data at the RHIC, to compute the nuclear structure function F2<sup>A</sup> and its log Q2 derivative (which is related to gluon distribution function in the double log limit). I provide a quantitative estimate of the nuclear shadowing of F2<sup>A</sup> and the gluon distribution function in the kinematic region relevant to a future electron-ion collider.

Hadronic Physics and QCD

Photoproduction of pseudoscalar mesons off nuclei at forward angles
S. Gevorkyan, A. Gasparian, L. Gan, I. Larin, and M. Khandaker
Published 6 November 2009 (8 pages)
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With the advent of new photon-tagging facilities and novel experimental technologies, it has become possible to perform photoproduction cross-sectional measurements of pseudoscalar mesons on nuclei with percent-level accuracy. The extraction of the radiative decay widths from these measurements at forward angles is done by the Primakoff method, which requires theoretical treatment of all processes participating in these reactions at the same percent level. In this work, we review the theoretical approach to meson photoproduction amplitudes in the electromagnetic and strong fields of nuclei at forward angles. The most updated description of these processes are presented based on the Glauber theory of multiple scattering. In particular, the effects of final-state interactions, corrections for light nuclei, and photon shadowing in nuclei are discussed.

Photoproduction of eta and eta' mesons off protons
V. Crede et al. (CBELSA/TAPS Collaboration)
Published 6 November 2009 (12 pages)
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Total and differential cross sections for eta and eta'photoproduction off the proton have been determined with the CBELSA/TAPS detector for photon energies between 0.85 and 2.55 GeV. The eta mesons are detected in their two neutral decay modes, eta-->gammagamma and eta-->3pi0-->6gamma, and for the first time, cover the full angular range in cos  thetac.m. of the eta meson. These new eta photoproduction data are consistent with the earlier CB-ELSA results. The eta' mesons are observed in their neutral decay to pi0pi0eta-->6gamma and also extend the coverage in angular range.

BRIEF REPORTS

Analytic expressions for alpha particle preformation in heavy nuclei
H. F. Zhang, G. Royer, Y. J. Wang, J. M. Dong, W. Zuo, and J. Q. Li
Published 2 November 2009 (4 pages)
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Experimental alpha decay energies and half-lives are investigated systematically to extract alpha particle preformation in heavy nuclei. Formulas for the preformation factors are proposed that can be used to guide microscopic studies on preformation factors and perform accurate calculations of the alpha decay half-lives. There is little evidence for the existence of an island of long stability of superheavy nuclei.

Giant monopole resonance in Pb isotopes
E. Khan
Published 5 November 2009 (3 pages)
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The extraction of the nuclear incompressibility from isoscalar giant monopole resonance (GMR) measurements is analyzed. Pairing may play a role in the shift of the GMR energy between the doubly closed shell 208Pb nucleus and other Pb isotopes. Pairing effects are predicted microscopically using the constrained Hartree-Fock-Bogoliubov method. Accurate measurements of the GMR in open-shell Pb isotopes are necessary.

Production of new superheavy Z=108–114 nuclei with 238U, 244Pu, and 248,250Cm targets
Zhao-Qing Feng, Gen-Ming Jin, and Jun-Qing Li
Published 4 November 2009 (4 pages)
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Within the framework of the dinuclear system (DNS) model, production cross sections of new superheavy nuclei with charged numbers Z=108–114 are analyzed systematically. Possible combinations based on the actinide nuclides 238U, 244Pu, and 248,250Cm with the optimal excitation energies and evaporation channels are pointed out to synthesize new isotopes that lie between the nuclides produced in the cold fusion reactions and the 48Ca-induced fusion reactions experimentally, which are feasible to be constructed experimentally. It is found that the production cross sections of superheavy nuclei decrease drastically with the charged numbers of compound nuclei. Larger mass asymmetries of the entrance channels enhance the cross sections in 2n–5n channels.