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.

• "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.

• "X-ray spectroscopy - a new method to investigate field emission in superconducting accelerating cavities"
  
  W. Bayer, R. Eichhorn, M. Gopych, H.-D. Gräf, J. Hasper, A. Richter,and A. Zilges.
  
  Nucl. Inst. and Meth. Phys. Res. 575 (2007) 321.

  We report on a study of field emission accompanied by emission of visible light and by emission of characteristic X-rays in a 5 and a 20 cell superconducting niobium 3 GHz accelerating cavity of the S-DALINAC injector. The nearly absorption free measurement of characteristic niobium X-rays inside the beam pipe allowed for the first time an estimate of the field emission current during the normal operation of superconducting accelerating cavities within their cryostat. The field emission current for different accelerating field strengths is well described by the law of Fowler and Nordheim. Hence the field enhancement factors for the observed field emission sites could be determined. For electron tracking calculations based on the Leap-Frog method the field strength distribution of the 5 cell cavity was determined. Due to these calculations it can be assumed that light emitting spots are electron emitters too and that the characteristic niobium X-rays are emitted from the same areas. We additionally report on the cleaning method and heat treatment of the superconducting niobium accelerating cavities at the S-DALINAC with particular application to the 5 cell cavity. After that, up to the maximum applied field strength of Eacc=6.5 mV/m field emission as well as the emission of visible light and characteristic X-rays were not observed anymore.

• "Nature of the pygmy dipole resonance in 140Ce studied in (alpha, alpha' gamma) experiments"
  
  D. Savran, M. Babilon, A.M. van den Berg, M.N. Harakeh, J. Hasper, A. Matic, H.J. Wörtche, and A. Zilges.
  
  Phys. Rev. Lett. 97 (2006) 172502.

  A concentration of electric-dipole excitations below the particle threshold, which is frequently denoted as the pygmy dipole resonance, has been studied in the semimagic nucleus 140Ce in (alpha, alpha' gamma) experiments at E=136 MeV. The technique of alpha-gamma coincidence experiments allows the separation of E1 excitations from states of other multipolarities in the same energy region and provides an excellent energy resolution to allow a detailed analysis for each state. The experimental results show that the PDR splits into two parts with different nuclear structure: one part which is excited in (alpha, alpha' gamma) as well as (gamma, gamma') experiments and one part which is excited only in (gamma, gamma').

• "The photoresponse of stable N=82 nuclei below 10 MeV"
  
  S. Volz, N. Tsoneva, M. Babilon, M. Elvers, J. Hasper, R.-D. Herzberg, H. Lenske, K. Lindenberg, D. Savran, and A. Zilges.
  
  Nucl. Phys. A 779 (2006) 1.

  High resolution photon scattering experiments were performed at the 10 MeV bremsstrahlung facility of the electron accelerator S-DALINAC at TU Darmstadt to investigate the semi-magic N=82 nuclei 138Ba, 140Ce, 142Nd and 144Sm. Energies and absolute strengths of dipole transitions below 10 MeV were determined in a model independent way. A concentration of electric dipole excitations exhausting up to 1% of the isovector E1 energy weighted sum rule is observed in all four nuclei around 6.5 MeV. Calculations in the framework of the Quasiparticle-Phonon Model (QPM) reproduce the gross experimental features.

• REVIEW: "Direct determination of photodisintegration cross sections and the p-process"
  
  H. Utsunomiya, P. Mohr, A. Zilges, and M. Rayet.
  
  Nucl. Phys. A 777 (2006) 459.

  Photon-induced reactions play a key role in the nucleosynthesis of heavy neutron-deficient nuclei, the so-called p-nuclei. In this paper we review the present status of experiments on photon-induced reactions at energies of astrophysical importance and their relevance to p-process modeling.

• REVIEW: "Low-lying dipole modes in vibrational nuclei studied by photon scattering"
  
  U. Kneissl, N. Pietralla, and A. Zilges.
  
  J. Phys. G: Nucl. Part. Phys. 32 (2006) R217-R252.

  After a short introduction to the nuclear resonance fluorescence technique (NRF) and a brief description of typical photon scattering facilities, the present topical review focuses on three phenomena observed in recent NRF experiments on nuclei near the shell closures. These topics of current interest in low-energy nuclear structure physics are: the systematics of 1- quadrupole-octupole-coupled two-phonon states (2+ times 3-), the study of two-phonon mixed-symmetry states and the exploration of the E1 pygmy resonance near the particle emission threshold.