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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.
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"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.
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"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.
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"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.
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"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.
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"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.
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"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.
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"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.
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"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.
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"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.
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"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 .
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"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.
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