Monday, March 11, 2002 - DPG-Tagungen

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Nuclear Physics Tuesday HK 17.3 Tue 17:45 B Study of the oblate deformed g9/2 band in the N=Z+1 nucleus 69 Se — •I. Stefanescu 1 , J. Eberth 1 , G. Gersch 1 , T. Steinhardt 1 , O. Thelen 1 , N. Warr 1 , D. Weisshaar 1 , G. de Angelis 2 , T. Martinez 2 , D. Curien 3 , K.P. Lieb 4 , A. Jungclaus 4 , R. Schwengner 5 ,andE. Stefanova 5 for the Euroball collaboration — 1 Institut für Kernphysik, Zülpicherstr. 77, D-50937 Köln, Germany — 2 Laboratori Nazionali INFN, I-35020 Legnaro, Italy — 3 CRN Strasbourg, F-67037 Strasbourg, France — 4 II. Phys. Institut, Universität Göttingen, D-37073 Göttingen, Germany — 5 Inst. für Kern- und Hadronenphysik , Forschungszentrum Rossendorf, Germany The γ-ray transitions in the 69 Se nucleus have been investigated in the 40 Ca( 32 S,2pn) 69 Se reaction at 105 MeV using the Euroball array coupled with ancillary detectors. The target consisted of a 800 µg/cm 2 99.965 % enriched self-suporting 40 Ca foil. The level scheme of 69 Se has been established by means of particlegated γ-γ and γ-γ-γ coincidences up to 13.7 MeV and J π =53/2 + for the prolate-deformed band which crosses the oblate band built on the g9/2 orbital. Some new lines have also been added to the structure resulting from the coupling of one octupole phonon to the 9/2 + state in the prolate band. The spins of the levels were deduced, whenever possible, from the analysis of the directional correlation ratios from oriented states (DCO) using the γ-γ and the neutron-γ-γ events. Funded by German BMBF under Contract No. 06OK958. HK 17.4 Tue 18:00 B Nature of the Scissors Mode near Shell Closure ⋆ — •P. von Neumann-Cosel 1 , E. Guliyev 2 , F. Hofmann 1 , A.A. Kuliev 3 ,and A. Richter 1 — 1 Institut für Kernphysik, Technische Universität Darmstadt — 2 Department of Engineering Physics, Ankara University, Turkey — 3 Department of Physics, Sarkara University, Turkey While the global features of the scissors mode in heavy nuclei are quite well understood, peculiarities remain to be solved in nuclei near shell closure. There, the simple geometrical picture of a scissors-like motion of deformed proton and neutron bodies breaks down. Two examples are discussed. QRPA calculations of the low-energy dipole strength in 122−130 Te are reported which provide new insight into the complex distributions which cannot be understood in spherical models. The extracted deformation dependence of the scissors mode strength is discussed with respect to various models. Another interesting case are the 194,196 Pt isotopes in the region of γ-softness near the N = 126 shell gap. Photon scattering experiments surprisingly suggest an upward shift of the scissors mode centroid in 194 Pt by about 500 keV with respect to the sys- HK18 Nuclear and Particle Astrophysics II tematics in rare-earth nuclei. First microscopic calculations of the M1 and E1 strengths in 194,196 Pt are presented using the above model. ⋆ Supported by the DFG under contract FOR 272/2-1. HK 17.5 Tue 18:15 B Phase transitions and two-neutron separation energies in algebraic models — •Ruben Fossion 1 , Kris Heyde 1 , and Jose-Enrique Garcia-Ramos 2 — 1 Department of Subatomic Physics, University of Gent, Proeftuinstraat,86 B-9000 Gent(Belgium) — 2 Departamento de Fiscia Aplicada. EPSLa Rabida, Universidad de Huelvam 21819 Palos de la Frontera (Spain) In the last few years, interest for the study of phase transitions and phase coexistence in atomic nuclei has been revived, in particular making use of algebraic methods such as the Interacting Boson Model (IBM). In the present study we consider the three transitional regions in which one observes rapid structural changes i.e. (a) the Nd-Sm-Gd region, (b) the Ru-Pd region and, (c) the Os-Pt region. Although these regions have been studied extensively emphasizing excited state properties, not so much attention has been given, up to now, concerning possible phase transitions in the nuclear ground-state properties. In this work, a new type of plot is presented that allows the study of phase transitions in finite systems, such as atomic nuclei. This approach allows to establish a connection between excited-state properties and binding energies. It can be shown that the binding energy and in particular the 2-neutron separation energy presents a very sensitive observable that allows to discriminate between apparently equivalent Hamiltonians. We also present results on the systematics of 2-neutron separation energies in long chains of isotopes. HK 17.6 Tue 18:30 B Cluster interpretation of the properties of the alternating parity bands in actinides — •T.M. Shneidman 1,2 , G.G. Adamian 1,2,3 , N.V. Antonenko 1,2 , R.V. Jolos 1,2 , and W. Scheid 1 — 1 Institut für Theoretische Physik der Justus-Liebig-Universität, D-35392 Giessen, Germany — 2 Joint Institute for Nuclear Research, 141980 Dubna, Russia — 3 Institute of Nucear Physics, Tashkent 702132, Uzbekistan A cluster model interpretation is suggested for the properties of the alternating parity bands in Ra, Th and U isotopes, which includes a description of the parity splitting [1] and the Eλ (λ=1,2,3) transition probabilities. The mass asymmetry and relative distance coordinates are the most important variables of the model. The characteristics of the Hamiltonian used are determined in investigations of heavy ion reactions at low energies. [1] T.M.Shneidman et al., Phys.Lett. B(<strong>2002</strong>), in press. Time: Tuesday 16:45–18:45 Room: C Group Report HK 18.1 Tue 16:45 C Nuclear Input Data for the r-Process: Cosmochronometer for Old Halo Stars — •B. Pfeiffer 1 , K.-L. Kratz 1 , H. Schatz 2 ,and J.J. Cowan 3 — 1 Inst. f. Kernchemie, Univ. Mainz — 2 NSCL, Michigan State Univ., East Lansing, USA — 3 Dep. of Phys. and Astr., Univ. Oklahoma, Norman, USA The description of the rapid neutron-capture process (r-process) requires environments with a high neutron density, where neutron captures are faster than β-decays, even for unstable nuclei up to 15 - 30 units from stability near the neutron drip-line. These exotic nuclei with a large neutron excess are of considerable interest not only for astrophysics but for nuclear structure studies. They experience dramatic changes giving rise to new shell structures characterized by vanishing gaps [1]. The updated nuclear physics input is applied for classical r-process models. The calculated abundances are compared to new observations of extremely metal-poor, old stars in the Galactic halo. The observed abundances for heavier neutron-capture elements (including the third rprocess peak elements) are consistent wth a scaled solar system distribution. Our theoretical production value for Th/U ratio combined with the new observations make more reliable chronometric age estimates possible [2]. These ages are lower limits for the age of the Universe. [1] B. Pfeiffer et al., Nucl. Phys. A693, 282 (2001) [2] R. Cayrel et al., Nature 409, 691 (2001); J.J. Cowan et al., submitted to Ap. J. HK 18.2 Tue 17:15 C Measurement of the 197 Au(γ,n) 196 Au cross section close above the reaction threshold ∗ — •P. Mohr, K. Vogt, M. Babilon, W. Bayer, T. Hartmann, C. Hutter, K. Lindenberg, K. Sonnabend, S . Volz, and A. Zilges — Institut für Kernphysik, Technische Universität Darmstadt, D-64289 Darmstadt, Germany Recently, the interest in (γ,n) reactions has been revived because of their astrophysical relevance for the nucleosynthesis of neutron-deficient p nuclei [1]. It has been shown that the astrophysical (γ,n) reaction rate depends on the (γ,n) cross section only in narrow energy window – a Gamow-like window – above the (γ,n) threshold [1,2]. We have measured (γ,n) reaction rates for several nuclei in a quasi-thermal photon bath at typical temperatures of a supernova explosion [1,3]. The cross section of the reaction 197 Au(γ,n) 196 Au has been measured at the S-DALINAC close above the reaction threshold at Ethr =8.071 MeV using the method of photoactivation. From a combination of our result and data from literature [4] we derive the 197 Au(γ,n) 196 Au cross section from threshold to the giant dipole resonance with very small uncertainties. This cross section can be used as standard in future experiments. [1] P. Mohr et al., Phys. Lett. B 488, 127 (2000). [2] P. Mohr et al., Nucl.Phys.A688, 82c (2001). [3] K. Vogt et al., Phys.Rev.C63, 055802 (2001). [4] B. L. Berman et al., Phys.Rev.C36, 1286 (1987); A. Veyssiere et al., Nucl.Phys.A159, 561 (1970); G. M. Gurevich et al., Nucl.Phys. A351, 257 (1981); H. Utsunomiya et al., to be published.
Nuclear Physics Tuesday ∗ supported by DFG (contracts Zi 510/2-1 and FOR 272/2-1). HK 18.3 Tue 17:30 C Resonance strengths for the reaction 28 Si(α, γ) 32 S at low energies — •M. Babilon, T. Hartmann, C. Hutter, P. Mohr, K. Sonnabend, K. Vogt, S.Volz,andA. Zilges — Institut für Kernphysik, Technische Universität Darmstadt, Schlossgartenstrasse 9, D-64289 Darmstadt, Germany Silicon burning is supposed to be the dominant mechanism for element synthesis in the mass range A = 28-65. Capture reactions like (α, γ), (p,γ), (n,γ) and the inverse photodisintegration processes are involved [1]. During a calibration of a Compton polarimeter [2] γ-decays of eight resonances in the 28 Si(α, γ) 32 Sreaction have been studied. The results for resonance strengths and decay branches from previous measurements [3-5] were verified, and in several cases the uncertainties in resonance strengths could be significantly reduced. ∗ supported by the DFG (contracts Zi510/2-1 and FOR272/2-1) [1] C. E. Rolfs, W. S. Rodney, ”Cauldrons in the Cosmos” (1988) [2] C. Hutter et al., this conference [3]D.W.O.Rogersel al., Nucl.Phys.A281, 345 (1977) [4]J.W.Toevsel al., Nucl.Phys.A172, 589 (1971) [5]P.J.M.Smuldersel al., Physica30, <strong>11</strong>97 (1964) HK 18.4 Tue 17:45 C Determination of the (n,γ) reaction rate of unstable 185 W in the astrophysical s-process via its inverse reaction — •K. Sonnabend, P. Mohr, K. Vogt, M. Babilon, W. Bayer, D. Galaviz, T. Hartmann, C. Hutter, S.Volz,andA. Zilges — Institut für Kernphysik, Technische Universität Darmstadt, D-64289 Darmstadt, Germany The β-unstable isotope 185 W is a branching point in the astrophysical s-process. Therefore the ratio of the (n,γ) reaction rate and β-decay rate of 185 W is of common interest. We have measured the inverse reaction 186 W(γ,n) 185 W due to the experimental difficulties in measuring the (n,γ) reaction rate of an unstable isotope directly. Our preliminary result is in agreement with theoretical predictions for the (γ,n) cross section [1] and available data [2-4]. Therefore, the predicted (n,γ) cross section under s-process conditions of about 700 millibarn and the derived neutron density [5] are confirmed. ∗ supported by the DFG (contracts Zi510/2-1 and FOR272/2-1) [1] T. Rauscher, F.-K. Thielemann, At. Data Nucl. Data Tables 75, 1 (2000) [2] B.L. Berman et al., Phys. Rev. 185, 1576 (1969) [3] A.M. Goryachev, G.N. Zalesnyĭ, IZV. An. KazSSR 6, 8 (1978) [4] G.M. Gurevich et al., Nucl. Phys. A351, 257 (1981) [5] F. Käppeler et al., ApJ. 366, 605 (1991) HK 18.5 Tue 18:00 C Electron Screening in Metals — •Francesco Raiola, Claus Rolfs, andFrank Strieder for the LUNA collaboration — Institut für Experimentalphysik III, Ruhr-Universität Bochum Recently, the electron screening effect on the d(d,p)t reaction has been studied in the metals Al, Zr, and Ta, where deuterated metals were produced via implantation of low-energy deuterons. The resulting S(E) data show the well known exponential enhancement, however the extracted electron screening potential values Ue are one order of magnitude larger than the values found in the corresponding gas-target experiment as well as that predicted from the adiabatic limit. In a new measurement at the 100 kV accelerator at the Ruhr-Universität Bochum the observation of the large electron screening effect in the d(d,p)t reaction using a deuterated Ta target has been confirmed using somewhat different experimental approaches. Studies using other deuterated metals are on the way and new results will be presented in this talk. HK 18.6 Tue 18:15 C ERNA: a status report — •Daniel Schürmann for the ERNA collaboration — Institut für Experimentalphysik III, Ruhr-Universität Bochum, Universitätsstr. 150, 44780 Bochum The European Recoil Separator for Nuclear Astrophysics (ERNA) is currently in an advanced stage of construction and testing at the Dynamitron Tandem Laboratory of the Ruhr-Universität Bochum. It is mainly devoted to a new measurement of the astrophysically important 12 C(α, γ) 16 O reaction cross section. Exploiting the separation of beam and recoil ions one measures the total cross section. Furthermore it is possible to clean up the γ-ray spectra by coincidence techniques and obtain information about the γ-ray angular distribution. Key parameters characterising the separator are the suppression factor of the carbon beam and the recoil acceptance. Measurements of these quantities and comparison with optical calculations are presented. HK 18.7 Tue 18:30 C The Trojan-Horse Method in Nuclear Astrophysics — •Stefan Typel 1 und Hermann Wolter 2 — 1 NSCL/Michigan State University, USA — 2 University of Munich, Germany The Trojan-Horse method (THM) is an indirect way to determine the energy dependence of nuclear cross sections at the very low energies of astrophysical interest. In this method the Coulomb barrier in the astrophysical reaction is effectively reduced by adding a spectator to one of the participating nuclei and by studying the breakup of the final three-body system in the particular region of the phase space where the momentum transfer to the spectator is small. In the plane-wave impulse approximation the cross section is a product of three factors: (1) a kinematical factor, (2) a momentum distribution, (3) an off-shell two-body cross section. The relation of the latter to the on-shell cross section was previously assumed in a heuristic approximation. We present inprovements in the theoretical description of the process starting from a distorted wave Born approximation. It yields a simple connection between the on-shell and off-shell cross sections. Applications of the THM in recent experiments, e.g. 7 Li(p,α)α and 6 Li(d,α)α, are discussed. HK19 Electromagnetic and Hadronic Probes II Time: Tuesday 16:45–18:45 Room: D Group Report HK 19.1 Tue 16:45 D Meson Spectroscopy with BABAR∗ — •Klaus Götzen for the BABAR collaboration — Institut für Experimentalphysik I, Ruhr-Universität Bochum The high luminosity of PEP-II in combination with the vertexing possibilities of the BABAR-Detector offers unique opportunities on light meson spectroscopy. The basic interest in this domain is the search for exotic states. In order to find those, the spectrum of conventional mesons must be precisely known. Earlier analyses were not able to resolve all ambiguities arising because of overlapping states. Some ambiguities can be resolved using a clean initial state which restricts the final state to specific quantum numbers, such as the weak decays of D ± s -mesons into three pseudoscalars, allowing only a few resonances to occur. Discussed in particular are the decays D ± s → K0 S K0 S π± and D ± s → π + π − π ± , and the data selection and Dalitz plot analyses are presented. These decays are of particular interest for the discussion of the existence and the spin of isoscalar resonances above 1.5 GeV/c 2 .Inthismassre- gion the glueball ground state is conjectured to lie and might mix strongly with other isoscalar J PC =0 ++ -states. ∗ supported by the BMB+F Group Report HK 19.2 Tue 17:15 D Measurement of time–dependent CP–Asymmetries in B 0 – mesondecays to CP–eigenstates and studies of charmonium decays of B–mesons — •Jens Brose for the BABAR collaboration — Institut für Kern- und Teilchenphysik, TU Dresden, 01062 Dresden The BABAR detector, at the PEP-II asymmetric B-meson factory at SLAC collected a sample of 52 events/fb whilst operating at energies near the Υ(4S) resonance in 2000/2001. A study of time-dependent CP–asymmetries in events where one neutral B meson is fully reconstructed in a charmonium final state (J/ΨK 0 S, J/ΨK 0 L , Ψ(2S)K0 S , χc1K 0 S and J/ΨK∗0 (K ∗0 → K 0 S π0 )) is presented. Since the final state J/ΨK ∗0 is an admixture of CP-even and CPodd states, an analysis of the different decay amplitudes is necessary.
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Page 65 and 66: Nuclear Physics Friday HK49 Plenary
Page 67 and 68: M. Ding3 , M.O. Distler3 , P. Drago
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Page 75 and 76: Friese, J. . HK 14.1, HK 14.3, HK 4
Page 77 and 78: Rusev, G. ..................HK 10.1

Monday, March 11, 2002 - DPG-Tagungen

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