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Nuclear Physics Tuesday HK 19 Nuclear Structure/Spectroscopy III Time: Tuesday 15:30–18:30 Room: D Group Report HK 19.1 Tue 15:30 D Fine Structure of the Isoscalar Giant Quadrupole Resonance High-Resolution Inelastic Proton Scattering Experiments ∗ — •A. Shevchenko 1 , J. Carter 2 , R.W. Fearick 3 , S.V. Förtsch 4 , Y. Fujita 5 , D. Lacroix 6 , J.J. Lawrie 4 , Y. Kalmykov 1 , P. von Neumann-Cosel 1 , V.Yu. Ponomarev 1 , A. Richter 1 , F.D. Smit 4 , and J. Wambach 1 — 1 Institut für Kernphysik, Technische Universität Darmstadt, Germany — 2 School of Physics, University of the Witwatersrand, Johannesburg, South Africa — 3 Physics Department, University of Cape Town, South Africa — 4 iThemba LABS, Somerset West, South Africa — 5 Physics Department, University of Osaka, Japan — 6 LPC Caen, France Fine structure of giant resonances carries important information on the coherent motion of nucleons in collective modes of excitations and on the role of different damping mechanisms of giant resonances. A systematic study of the fine structure of giant resonances for the example of the IS- GQR was carried out using high-resolution inelastic proton scattering at iThemba LABS on a variety of closed-shell and deformed nuclei. The fine structure is established as a global phenomenon. Novel wavelet analysis methods have been developed for the extraction of characteristic energy scales of the fluctuations in the fine structure. New experimental results will be presented together with an interpretation of their physical nature based on various microscopic approaches including the coupling to complex configurations. ∗ Supported by DFG, contracts SFB 634 and 445 SUA-113/6/0-1, and by the South African FRD. Group Report HK 19.2 Tue 16:00 D Dipole strength distributions in neutron-rich nuclei — •Nadezhda Tsoneva and Horst Lenske — Institut für Theoretische Physik, Universität Giessen, 35392 Giessen, Germany Dipole excitations below the neutron threshold in neutron-rich nuclei are studied microscopically by a method combining self-consistent mean-field results from Hartree-Fock-Bogoliubov (HFB) theory and the Quasiparticle-Phonon Model (QPM). Including up to three-phonon components in the model configuration space, a realistic description of nuclear spectra and their fragmentation pattern up to the particle threshold is obtained. Of special interest are the low-lying two-phonon 1 − states and the Pygmy Dipole Resonance (PDR). The evolution of the PDR strength in relation to the neutron skin thickness is investigated over the Z=50 isotopic and N=82 isotonic chains. For Sn isotopes a close correlation of the PDR excitation energy and transition strength with the neutron excess was found. The PDR strength increases with the neutron number while the centroid energy decreases. By comparison to the radii of HFB ground state densities a correlation with the size of the neutron skin could be identified. Hence, the PDR strength establishes a clear signature for the neutron skin size. The good agreement of our results with experiments confirms the success of such a microscopically inspired description. For the N=81 isotope 137 Ba with one neutron-hole in the closed N=82 shell the distribution of the low-lying dipole strength is studied microscopically within the framework of the QPM including quasiparticle+two-phonon coupling. The results are compared with recent experimental data from Darmstadt and Stuttgart. Work supported by DFG. HK 19.3 Tue 16:30 D New Insights into the Nuclear Structure of 42,44,46 Ca from g Factor and B(E2) measurements of 2 + 1 States + — •S. Schielke 1 , K.-H. Speidel 1 , O. Kenn 1 , J. Leske 1 , D. Hohn 1 , H. Hodde 1 , J. Gerber 2 , P. Maier-Komor 3 , O. Zell 4 , Y.Y. Sharon 5 , and L. Zamick 5 — 1 Helmholtz-Institut für Strahlen- und Kernphysik, Univ. Bonn — 2 Institut de Recherches Subatomiques, Strasbourg, France — 3 Physik-Dept. TU München — 4 Institut für Kernphysik, Univ. Köln — 5 Department of Physics and Astronomy, Rutgers Univ., USA The low-level structure of calcium isotopes is generally characterized by the valence neutrons in the fp shell and a 40 Ca core. The gradual filling of the f7/2 shell with neutrons for 42,44,46 Ca, between doubly-magic 40 Ca and 48 Ca, causes strong variations in the excitation energies and B(E2)’s of their 2 + 1 states. If fp shell neutrons are the dominant components in the nuclear wave functions one expects for these states negative g factor values (g ≃ −0.4). From recent g factor measurements, employing projectile Coulomb excitation and the transient field technique, however, very surprising results were obtained. For 42,44 Ca(2 + 1 ) positive values of g factors were observed which could only be explained by shell model calculations including substantial cross-shell configurations from 40 Ca core excitation [1]. In contrast, a recent first measurement on 46 Ca(2 + 1 ), using an enriched 46 Ca beam, however, yielded a negative g factor, in complete disagreement with the positive value prediction. Evidently, in this case the valence neutrons of the fp shell play the dominant role. + supported by the DFG under Sp 190/9-2 [1] S. Schielke et al., Phys. Lett. B 571(2002)29 HK 19.4 Tue 16:45 D Fragmentation of the E1 two-phonon strengths in odd-mass nuclei near the N=82 shell closure — •M. Scheck 1 , H. von Garrel 1 , P. von Brentano 2 , C. Fransen 2 , G. Friessner 2 , N. Hollmann 2 , J. Jolie 2 , U. Kneissl 1 , C. Kohstall 1 , A. Linnemann 2 , D. Mücher 2 , N. Pietralla 2 , H.H. Pitz 1 , F. Stedile 1 , C. Stoyanov 3 , N. Tsoneva 3,4 , S. Walter 1 , and V. Werner 2 — 1 Inst. für Strahlenphysik, Universität Stuttgart — 2 Inst. für Kernphysik, Universität zu Köln — 3 INRNE, Sofia — 4 Inst. für Theor. Physik, Universität Giessen Quadrupole-octupole coupled two-phonon E1 excitations (2 + ⊗3 − ) represent the dominant low-lying E1 excitations in spherical even-even nuclei near shell closures [1]. In the present studies the fragmentation of the dipole strength in odd-mass nuclei near the N=82 shell closure was investigated. The recent photon scattering experiments on 135,137 Ba, 139 La, and 141 Pr extend systematically our previous measurements on 143 Nd [2]. All experiments were performed at the photon scattering facility of the 4.3 MV Stuttgart Dynamitron using bremsstrahlung beams of 4 MeV end point energy. The fragmentation and partially observed reduction of the dipole strengths as compared to those in the neighboring even-even N=82 isotones [3] arediscussed and compared with our similar results for nuclei near the Z=50 shell closure (see [4]). [1] W. Andrejtscheff et al., Phys. Lett. B 506, 239 (2001). [2] A. Zilges et al., Phys. Rev. Lett. 70, 2880 (1993). [3] R.-D. Herzberg et al., Nucl. Phys. A 592, 211 (1995). [4] J. Bryssinck et al., Phys. Rev. C 65, 024313 (2002). Supported by DFG under contracts Kn 154/31 and Pi 393/1-2 HK 19.5 Tue 17:00 D E1 strength distribution in N=82 nuclei ∗ — •S. Volz, M. Babilon, W. Bayer, D. Galaviz, T. Hartmann, A. Kretschmer, K. Lindenberg, S. Müller, D. Savran, K. Sonnabend, K. Vogt, and A. Zilges — Institut für Kernphysik, Technische Universität Darmstadt, Germany Recently much effort in theory and experiment has been spent to get a deeper insight into low lying electric dipole excitations between about 5 and 10 MeV. This strength is usually denoted as Pygmy Dipole Resonance. A well suited experimental technique to determine the electric dipole strength distribution is Nuclear Resonance Fluorescence (NRF) [1]. With this method it was possible to determine the absolute transition strengths of excitations in the N=82 isotones 138 Ba, 140 Ce, and 144 Sm [2]. In a recent experiment at the injector of the S-DALINAC at TU Darmstadt the nucleus 142 Nd has been measured and it is now possible to discuss a systematic trend of the mode. In addition calculations in the Quasi Particle Phonon Model will be presented [3,4]. ∗ supported by the DFG (SFB 634 and Zi510/2-2) [1] U. Kneissl et al., Prog. Part. Nucl. Phys. 37, 349 (1996) [2] A. Zilges et al., Phys. Lett. B 542, 43 (2002) [3] V.Yu. Ponomarev, private communication [4] N. Tsoneva, private communication HK 19.6 Tue 17:15 D Spectral Statistics of the Fine Structure of the Electric Pygmy Dipole Resonance in N = 82 Nuclei ∗ — •A. Heine 1 , J. Enders 1 , T. Guhr 2 , P. von Neumann-Cosel 1 , V.Yu. Ponomarev 1 , A. Richter 1 , S. Volz 1 , and J. Wambach 1 — 1 Institut für Kernphysik, Technische Universität Darmstadt, 64289 Darmstadt, Germany — 2 Matematisk Fysik, LTH, Lunds Universitet, SE-22100 Lund, Sweden We present the first statistical analysis of the fine structure of the electric pygmy dipole resonance. To this extent we analyzed a data set composed of 184 E1 transitions (levels and strengths) measured in the semimagic N = 82 isotones 138 Ba, 140 Ce, 142 Nd, and 144 Sm [1]. Mi-
Nuclear Physics Tuesday croscopic calculations performed within the quasiparticle-phonon model (QPM) serve to understand the experimental findings. By taking the effects of the incompleteness of the experimental data into account, qualitative agreement between experiment and QPM can obtained for the level as well as the intensity statistics. [1] A. Zilges et al., Phys. Lett. B 542 (2002) 43. ∗ Supported by DFG under contract SFB 634. HK 19.7 Tue 17:30 D Dipole- and quadrupole strength in 45 Sc ∗ — •D. Savran 1 , M. Babilon 1,2 , W. Bayer 1 , D. Galaviz 1 , T. Hartmann 1 , A. Kretschmer 1 , K. Lindenberg 1 , S. Müller 1 , K. Sonnabend 1 , K. Vogt 1 , S. Volz 1 , and A. Zilges 1 — 1 Institut für Kernphysik, Technische Universität Darmstadt, Schlossgartenstrasse 9,D-64289 Darmstadt, Germany — 2 A. W. Wright Structure Laboratory, Yale University, New Haven, CT, USA Strong variations of the dipole excitation strengths have been observed in even-even nuclei of the A=30-50 mass region. We performed a Nuclear Resonance Fluorescence Experiment using bremsstrahlung at the Darmstadt S-DALINAC accelerator to measure dipole- and quadrupole strengths in the Z=21 nucleus 45 Sc up to 7 MeV. The energy and strength of 34 states have been observed for the first time. The results will be compared to neighbouring nuclei. * supported by the DFG (SFB 634 and Zi 510/2-2) HK 19.8 Tue 17:45 D Low lying dipole strength in nuclei around the Z = 20-shell closure — •T. Hartmann 1 , M. Babilon 1,2 , W. Bayer 1 , D. Galaviz 1 , S. Kamerdhziev 3 , A. Kretschmer 1 , K. Lindenberg 1 , E. Litvinova 3 , S. Müller 1 , D. Savran 1 , K. Sonnabend 1 , K. Vogt 1 , S. Volz 1 , and A. Zilges 1 — 1 Institut für Kernphysik, Technische Universität Darmstadt, Germany — 2 Physics Department - Wright Nuclear Structure Laboratory, Yale University, New Haven, CT, USA — 3 Institute of Physics and Power Engineering, Obninsk, Russia Photon-scattering experiments have been performed on the semi-magic nucleus 44 Ca to investigate the low-lying electric dipole strength below 10 MeV. Earlier experiments on the two doubly-magic nuclei 40,48 Ca had shown a significant difference in their E1-strength which exhaust 0.02% and 0.33% of the isovector EWSR, respectively [1]. The E1-strength in 44 Ca is more fragmented. About 0.39% of the EWSR is exhausted below 10 MeV. Theoretical predictions within the Extended Theory of Finite Fermi Systems (ETFFS) describe the general trend and the fragmentation of the strength in the three Calcium isotopes quite well, the absolute strength is reproduced within a factor of 2. ∗ supported by the DFG (SFB 634 and Zi510/2-2) [1] T. Hartmann et al., Phys. Rev. C65 (2002) 034301. HK 20 Theory III HK 19.9 Tue 18:00 D Photon scattering from 92 Mo, 98 Mo and 100 Mo up to the neutron-separation energy — •G. Rusev 1 , R. Schwengner 1 , A. Wagner 1 , K.D. Schilling 1 , M. Erhard 1 , F. Dönau 1 , A.R. Junghans 1 , K. Kosev 1 , and E. Grosse 1,2 — 1 Institut für Kern- und Hadronenphysik, Forschungszentrum Rossendorf, Dresden, Germany — 2 Technische Universität Dresden, Dresden, Germany We present the results of photon-scattering experiments on the nuclei 92 Mo, 98 Mo and 100 Mo up to the neutron-separation energy. The experiments were carried out at the new bremsstrahlung facility of the Rossendorf ELBE accelerator at electron energies of 9 and 12.6 MeV. Samples of about 3 g of highly enriched 92 Mo, 98 Mo and 100 Mo, respectively, were used in the nuclear-resonance-fluorescence experiments. Scattered photons were measured with four escape-suppressed HPGe detectors placed two by two at 90 ◦ and 127 ◦ , respectively, relative to the incident photon beam. The dependence of the observed dipole-strength distributions on the neutron number of the Mo isotopes is discussed. The experimental results are compared with predictions of RPA calculations. ⋆ Supported by the DFG under contracts Do 466/1-1 and Do 466/1-2. HK 19.10 Tue 18:15 D Low-Spin γ-Ray Spectroscopy of the Doubly-Magic Nucleus 56 Ni with the MINIBALL-Spectrometer — C. Fransen 1 , N. Pietralla 1,2 , •D. Mücher 1 , J. Eberth 1 , N. Warr 1 , and D. Weisshaar 1 for the MINIBALL collaboration — 1 Institut für Kernphysik, Universität zu Köln — 2Dept. of Physics, SUNY, Stony Brook, USA The doubly closed-shell N=Z nucleus 56 28Ni is of particular interest for understanding nuclear structure in the fp-shell. In recent years large-scale shell model calculations for this region became possible. The microscopic description of the excitation of 56Ni remained one of the biggest challenges for theory. While deformed high-spin structures of 56Ni are well investigated [1], information on low-spin states was sparse. We investigated 56Ni with the reaction 54Fe( 3He,nγ) 56Ni at the Cologne FN-Tandem accelerator with the highly efficient Miniball-spectrometer. γγ-coincidence data were measured for excited states up to an excitation energy of about 9 MeV. The known γ-decay scheme for low-spin states has been more than doubled and several branching ratios were determined. The set-up and data will be presented. The spectrometer performance will be discussed. The data will be compared to the newest large-scale Monte-Carlo shell model calculations by the Tokyo group using the GXPF1 interaction [2]. Supported by the Emmy Noether-Program of the DFG under Pi393/1-2 and by BMBF under 06OK958 [1] D. Rudolph et al.,Phys.Rev.Lett. 82, 3763 (1999) [2] M. Honma, T. Otsuka et al., priv. comm., submitted for publication Time: Tuesday 15:30–18:30 Room: E Group Report HK 20.1 Tue 15:30 E Rho-Omega Mixing in Matter — •Felix Riek and Jörn Knoll — Gesellschaft für Schwerionenforschung, Planckstr. 1, 64291 Darmstadt The in-medium properties of the light vector mesons in nuclear matter are studied within a self-consistent coupled Dyson scheme. For the nuclear matter part the nucleon, the ∆-isobar and the pion degrees of freedom are explicitly included. Besides the normal couplings short range repulsive correlations of Migdal type are added in order to arrive at a realistic description of the in-medium spectral properties of the pion. All spectral functions are calculated self-consistently as a function of energy and spatial momentum in the density and temperature range till twice ρ0 and T ≤ 120 MeV. The properties of the vector mesons are modified through their interaction with the virtual pion cloud by means of the ππρand πρω-couplings. With increasing density in particular the space-like parts of the pion spectral function provide a strong mixing between the two vector mesons. This leads to a strong increase the damping with of the ω-meson with increasing density and temperature. Consequences for the in-medium part of di-lepton yields from the electromagnetic decay of both vector mesons are discussed. Group Report HK 20.2 Tue 16:00 E Linking the Quark Meson Model with QCD at High Temperature — •Jens Braun 1 , Kai Schwenzer 2 , and Hans-Jürgen Pirner 1 — 1 Institut für Theoretische Physik, Universität Heidelberg, Philosophenweg 19, 69120 Heidelberg. — 2 Department of Physics, North Carolina State University, 2700 Stinson Drive, Raleigh, NC 27695, USA. The equation of state of QCD in extreme conditions of high temperature and high baryon density is at the focus of interest in heavy ion collisions and astrophysics. We model the transition of a system of quarks and gluons at high energies to a system of quarks and mesons at low energies in a consistent renormalization group approach. Flow equations interpolate between the physics of the high temperature degrees of freedom and the low temperature dynamics at a scale of 1 GeV. We discuss the dependence of the equation of state on baryon density and compare our results with recent lattice gauge simulations. HK 20.3 Tue 16:30 E Renormalization Group Flow Equations for the Linear sigma- Model at Finite Volume and Finite Quark Mass — •Bertram Klein, Jens Braun, Lars Jendges, and Hans-Jürgen Pirner — Institut für Theoretische Physik, Universität Heidelberg, Philosophenweg 19, 69120 Heidelberg.
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