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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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"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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"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.
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"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').
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"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.
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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.
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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.
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