This is a selection of our latest publications in refereed journals. Online access to the articles may be restricted by the publisher. If you are interested to receive reprints, please contact Andreas Zilges.

A complete list of refereed publications can be downloaded here as a PDF-File.

• "High precision ⁸⁹Y(alpha,alpha)⁸⁹Y scattering at low energies"
  
  G.G. Kiss, P. Mohr, Zs. Fülöp, D. Galaviz, Gy. Gyürky, Z. Elekes, E. Somorjai, A. Kretschmer, K. Sonnabend, A. Zilges, and M. Avrigeanu.
  
  Physical Review C 80 (2009) 045807.

  Elastic scattering cross sections of the ⁸⁹(alpha,alpha)⁸⁹Y reaction have been measured at energies E(c.m.)=15.51 and 18.63 MeV. The high-precision data for the semimagic N=50 nucleus ⁸⁹Y are used to derive a local potential and to evaluate the predictions of global and regional alpha-nucleus potentials. The variation of the elastic alpha-scattering cross sections along the N=50 isotonic chain is investigated by a study of the ratios of angular distributions for ⁸⁹Y(alpha,alpha)⁸⁹Y and ⁹²Mo(alpha,alpha)⁹²Mo at E(c.m.)=15.51 and 18.63 MeV. This ratio is a very sensitive probe at energies close to the Coulomb barrier, where scattering data alone is usually not enough to characterize the different potentials. Furthermore, alpha-cluster states in ⁹³Nb=⁸⁹Y - alpha are investigated.

• "Competition between excited core states and 1hw single-particle excitations at comparable energies in ²⁰⁷Pb from photon scattering "
  
  N. Pietralla, T.C. Li, M. Fritzsche, M.W. Ahmed, T. Ahn, A. Costin, J. Enders, J. Li, S. Müller, P. von Neumann-Cosel, I.V. Pinayev, V.Yu. Ponomarev, D. Savran, A.P. Tonchev, W. Tornow, H.R. Weller, V. Werner, Y.K. Wu and A. Zilges.
  
  Phys. Lett. B 681 (2009) 134.

  The Pb(gamma,gamma') photon scattering reaction has been studied with the nearly monochromatic, linearly polarized photon beams at the High Intensity gamma-ray Source (HIGS) at the DFELL. Azimuthal scattering intensity asymmetries measured with respect to the polarization plane of the beam have been used for the first time to assign both the spin and parity quantum numbers of dipole excited states of ^206,207,208Pb at excitation energies in the vicinity of 5.5 MeV. Evidence for dominant particle-core coupling is deduced from these results along with information on excitation energies and electromagnetic transition matrix elements. Implications of the existence of weakly coupled states built on highly excited core states in competition with 1hw single particle (hole) excitations at comparable energies are discussed.

• "Splitting of the pygmy dipole resonance in ¹³⁸Ba and ¹⁴⁰Ce observed in the (alpha,alpha'gamma) reaction"
  
  J. Endres, D. Savran, A.M. van den Berg, P. Dendooven, M. Fritzsche, M.N. Harakeh, J. Hasper, H.J. Wörtche, and A. Zilges.
  
  Phys. Rev. C 80 (2009) 034302.

  The N=82 nuclei ¹⁴⁰Ce and ¹³⁸Ba have been investigated by means of the (alpha,alpha'gamma) coincidence method to study the pygmy dipole resonance (PDR). The experiments have been performed at the AGOR cyclotron at KVI, Groningen, at a primary beam energy of E(alpha)=136 MeV. The Big-Bite Spectrometer and seven large-volume high-purity germanium detectors were used in coincidence to perform a simultaneous spectroscopy of the scattered alpha particles and the gamma decay. The comparison with results of nuclear resonance fluorescence experiments reveals a splitting of the PDR into two components. Up to about 6 MeV the same states that could be observed in (gamma,gamma') are also excited in alpha-scattering experiments, whereas the higher-lying states are missing in the (alpha,alpha'gamma) reaction. This indicates a structural splitting of the PDR into two modes with different underlying structure.

• "Investigation of photoneutron reactions on ¹⁹²Os and ^191,193Ir at energies of relevance for the astrophysical p process"
  
  J. Hasper, D. Galaviz, S. Müller, A. Sauerwein, D. Savran, L. Schnorrenberger, K. Sonnabend, and A. Zilges.
  
  Phys. Rev. C 79 (2009) 055807.

  We have investigated the photoneutron reactions on the isotopes ¹⁹²Os and ^191,193Ir for astrophysically relevant photon energies just above the neutron separation energy. The experiments were performed using the photoactivation technique at the superconducting Darmstadt linear electron accelerator (S-DALINAC). The measurements extend the existing experimental database on photoneutron reactions in this mass region and serve as an important test for the state-of-the-art statistical model calculations commonly used for the prediction of stellar reaction rates.

• "The (d,²He) reaction on ⁹⁶Mo and the double-beta decay matrix elements for ⁹⁶Zr"
  
  H. Dohmann, C. Bäumer, D. Frekers, E.-W. Grewe, M.N. Harakeh, S. Hollstein, H. Johansson, L. Popescu, S. Rakers, D. Savran, H. Simon, J. H. Thies, A.M. van den Berg, H. J. Wörtche, and A. Zilges.
  
  Phys. Rev. C 78 (2008) 041602(R).

  The 96Mo(d,2He)96Nb charge-exchange reaction was investigated at an incident energy of Ed=183.5 MeV. An excitation-energy resolution of 110 keV was achieved. The experiment was performed at KVI, Groningen, using the magnetic spectrometer BBS at three angular positions: 0°,2.5°, and 6°. We found that below 6 MeV almost the entire Gamow-Teller (GT+) strength is concentrated in a single state at 0.69 MeV excitation energy. As 96Mo is the daughter of the beta-beta decay nucleus 96Zr, the present result provides information about the nuclear matrix elements active in the 2neutrino-beta-beta decay of 96Zr.

• "Parity assignments in ¹⁴⁰Ce up to 7 MeV using Compton polarimetry"
  
  M.A. Büssing, M. Elvers, J. Endres, J. Hasper, A. Zilges, M. Fritzsche, K. Lindenberg, S. Müller, D. Savran, and K. Sonnabend,
  
  Phys. Rev. C 78 (2008) 044309.

  Parity quantum numbers of J=1 states up to 7 MeV in the region of the Pygmy Dipole Resonance in ¹⁴⁰Ce were determined model independently by combining the methods of Nuclear Resonance Fluorescence and Compton polarimetry. For the first time the well-established method of Compton polarimetry was applied at such high energies. The experiment was performed using a fourfold segmented HPGe clover detector for the detection of the scattered photons. For all investigated dipole transitions asymmetries are found which correspond to negative parity of the excited states.

• "The (d,²He) reaction on ⁷⁶Se and the double-beta-decay matrix elements for A=76"
  
  E.-W. Grewe, C. Bäumer, H. Dohmann, D. Frekers, M.N. Harakeh, S. Hollstein, H. Johansson, L. Popescu, S. Rakers, D. Savran, H. Simon, J. H. Thies, A.M. van den Berg, H. J. Wörtche, and A. Zilges.
  
  Phys. Rev. C 78 (2008) 044301.

  The (d,²He) charge-exchange reaction on ⁷⁶Se was studied at an incident energy of 183 MeV. The outgoing two protons in the ¹S₀ state, referred to as ²He), were both momentum analyzed and detected by the same spectrometer and detector. The experiment was performed at KVI, Groningen, using the magnetic spectrometer BBS at three angular positions.

• "Fine Structure of the Pygmy Dipole Resonance in ¹³⁶Xe"
  
  D. Savran, M. Fritzsche, J. Hasper, K. Lindenberg, V.Yu. Ponomarev, K. Sonnabend, and A. Zilges.
  
  Phys. Rev. Lett. 100 (2008) 232501.

  The photoresponse of the semimagic N=82 nucleus ¹³⁶Xe was measured up to the neutron separation energy Sn using the (gamma,gamma') reaction. A concentration of strong dipole excitations is observed well below Sn showing a fragmented resonancelike structure. Microscopic calculations in the quasiparticle phonon model including complex configurations of up to three phonons agree well with the experimental data in the total integrated strength, in the shape and the fragmentation of the resonance, which allows us to draw conclusions on the damping mechanism of the pygmy dipole resonance.

• "Studies on the double-beta decay nucleus ⁶⁴Zn using the (d,²He) reaction"
  
  E.-W. Grewe, C. Bäumer, H. Dohmann, D. Frekers, M.N. Harakeh, S. Hollstein, H. Johansson, K. Langanke, G. Martínez-Pinedo, F. Nowacki, I. Petermann, L. Popescu, S. Rakers, D. Savran, K. Sieja, H. Simon, J. H. Thies, A.M. van den Berg, H. J. Wörtche, and A. Zilges.
  
  Physical Review C 77(2008)064303.

  The (d,²He) charge-exchange reaction on the double-bet a decay nucleus ⁶⁴Zn has been studied at an incident energy of 183 MeV. The two protons in the ¹S₀ state (indicated as ²He) were both momentum analyzed and detected simultaneously by the BBS magnetic spectrometer and its position-sensitive detector. ²He spectra with a resolution of about 115 keV (FWHM) have been obtained allowing identification of many levels in the residual nucleus ⁶⁴Cu with high precision. ⁶⁴Zn is one of the rare cases undergoing a beta-beta decay in beta+ direction. In the experiment presented here, Gamow-Teller (GT+) transition strengths have been extracted. Together with the GT- transition strengths from ⁶⁴Ni(³He,t) data to the same intermediate nucleus ⁶⁴Cu, the nuclear matrix elements of the beta-beta decay of ⁶⁴Zn have been evaluated. Finally, the GT± distributions are compared with shell-model calculations and a critical assessment is given of the various residual interactions presently employed for the pf shell.

• "The photoresponse of ⁴⁵Sc up to 7 MeV"
  
  D. Savran, W. Bayer, D. Galaviz, S. Müller, K. Sonnabend, and A. Zilges.
  
  European Physical Journal 36 (2008) 17.

  High-resolution photon scattering experiments have been performed on the nucleus ⁴⁵Sc at the Darmstadt superconducting electron accelerator S-DALINAC using bremsstrahlung beams with end point energies of 5.0 and 7.0MeV. Energies, absolute cross-sections and decay widths of 50 states, most of them previously unknown, have been determined. The results are compared to (gamma,gamma') experiments on the neighbouring closed proton shell isotope ⁴⁴Ca .

• "Investigation of photoneutron reactions close to and above the neutron emission threshold in the rare earth region"
  
  J. Hasper, S. Müller, D. Savran, L. Schnorrenberger, K. Sonnabend, and A. Zilges.
  
  Phys. Rev. C 77 (2008) 015803.

  We have investigated the photoneutron cross section of the isotopes ^148,150Nd, ¹⁵⁴Sm, and ^154,160Gd close to the neutron emission threshold in photoactivation experiments at the superconducting Darmstadt electron linear accelerator S-DALINAC. Naturally composed targets were activated with a high-intensity bremsstrahlung beam at various energies and the reaction yields have been determined by measuring the activity of the produced radioactive isotopes with HPGe detectors. The results are compared to two different statistical model calculations.





Ältere Publikationen