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N. Cooper,
F. Reichel,
V. Werner,
L. Bettermann,
B. Alikhani,
S. Aslanidou,
C. Bauer,
L. Coquard, M. Fritzsche,
Y. Fritzsche, [......],
D. Savran,
L. Schnorrenberger,
F. Siebenhühner,
V. V. Simon,
K. Sonnabend,
M. K. Smith,
C. Walz,
S. W. Yates,
O. Yevetska,
M. Zweidinger
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ABSTRACT: The dipole strength distribution of 76Se has been investigated via photon scattering in the energy region below 9 MeV utilizing bremsstrahlung produced at the S-DALINAC facility at the TU Darmstadt. About 0.20(1)% of the classical E1 sum rule is exhausted by observed J=1 states of justifiably assumed negative parity. An extrapolation of the GDR below 9 MeV suggests that considerable strength may remain unobserved due to background, finite detector resolution, and fragmentation. The observed strength thus represents a lower limit. Candidates for the 21,ms+ state and a fragment of the 1s+c mixed-symmetry states are presented.
Physical Review C 09/2012; 86(3). · 3.31 Impact Factor
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N Cooper,
F Reichel,
V Werner,
L Bettermann,
B Alikhani,
S Aslanidou,
C Bauer,
L Coquard, M Fritzsche,
Y Fritzsche, [......],
D Savran,
L Schnorrenberger,
F Siebenhühner,
V V Simon,
K Sonnabend,
M K Smith,
C Walz,
S W Yates,
O Yevetska,
M Zweidinger
Phys. Rev. C. 09/2012; 86(3):034313.
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D. Savran,
M. Elvers,
J. Endres, M. Fritzsche,
B. Löher,
N. Pietralla,
V. Yu. Ponomarev,
C. Romig,
L. Schnorrenberger,
K. Sonnabend,
A. Zilges
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ABSTRACT: The low-lying electric dipole (E1) strength in the semimagic nucleus 136Xe has been measured, which finalizes the systematic survey to investigate the so-called pygmy dipole resonance (PDR) in all stable even N=82 isotones with the method of nuclear resonance fluorescence using real photons in the entrance channel. In all cases, a fragmented resonance-like structure of E1 strength is observed in the energy region 5–8 MeV. An analysis of the fragmentation of the strength reveals that the degree of fragmentation decreases toward the proton-deficient isotones, while the total integrated strength increases, indicating a dependence of the total strength on the neutron-to-proton ratio. The experimental results are compared to microscopic calculations within the quasiparticle phonon model. The calculation includes complex configurations of up to three phonons and is able to reproduce also the fragmentation of the E1 strength, which allows us to draw conclusions on the damping of the PDR. Calculations and experimental data are in good agreement on the degree of fragmentation and also on the integrated strength if the sensitivity limit of the experiments is taken into account.
Phys. Rev. C. 06/2012; 84(2).
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J Isaak,
D Savran, M Fritzsche,
D Galaviz,
T Hartmann,
S Kamerdzhiev,
J H Kelley,
E Kwan,
N Pietralla,
C Romig,
G Rusev,
K Sonnabend,
A P Tonchev,
W Tornow,
A Zilges
[show abstract]
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ABSTRACT: The dipole-strength distribution in the semi-magic nucleus 44 Ca has been measured up to 10 MeV excitation energy in photon-scattering experiments using bremsstrahlung and monoenergetic and 100 % linearly polarized photon beams. The combination of both measurements allows a clear determination of spin and parity quantum numbers of the excited states as well as absolute cross sections and transition probabilities. The results show that the majority of the dipole strength in 44 Ca below 10 MeV is due to E1 transitions while M 1 strength plays only a minor role. The experimental results are compared to the strength in the neighboring doubly-magic nuclei 40,48 Ca and to microscopic calculations within the Extended Theory of Finite Fermi Systems in order to investigate the evolution of the low-lying E1 strength in this isotopic chain. Both, experiment and calculations, show a non-trivial dependence of the total E1 strength as a function of the neutron number.
Physical Review C 01/2010; 20. · 3.31 Impact Factor
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ABSTRACT: Spectra of the 235U(γ,γ′) reaction were measured at the S-DALINAC at bremsstrahlung end-point energies E0=3.5 and 4.4 MeV and scattering angles of 90° and 135° with respect to the beam axis. Discrete transitions are observed at excitation energies below 2.3 MeV only. The deduced cross sections are in rough agreement with the findings of Bertozzi et al. Phys. Rev. C 78 041601 (2008) except for the most prominent transition, where the present result is lower by a factor of about 2. Evidence for unresolved dipole strength is found in the spectra by means of a fluctuation analysis, which was carried out up to an excitation energy of 2.8 MeV. If this unresolved strength exhibits the same ratio of E1/M1 cross sections as observed in the even-mass neighbor 236U, then the energy centroid E̅ x=2.5(3) MeV and total strength ∑B(M1)↑=3.6(1.3)μN2 of the M1 part are in good agreement with the systematics of the scissors mode in even-even actinide nuclei.
Physical Review C. 01/2010; 81(4):044309 1-6.
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ABSTRACT: 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.
Phys. Rev. C. 10/2008; 78(4).
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ABSTRACT: The photoresponse of the semimagic N=82 nucleus (136)Xe was measured up to the neutron separation energy S(n) using the (gamma, gamma') reaction. A concentration of strong dipole excitations is observed well below S(n) 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.
Physical Review Letters 06/2008; 100(23):232501. · 7.37 Impact Factor
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K. Sonnabend,
D. Savran,
J. Beller,
M.A. Büssing,
A. Constantinescu,
M. Elvers,
J. Endres, M. Fritzsche,
J. Glorius,
J. Hasper,
J. Isaak,
B. Löher,
S. Müller,
N. Pietralla,
C. Romig,
A. Sauerwein,
L. Schnorrenberger,
C. Wälzlein,
A. Zilges,
M. Zweidinger
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ABSTRACT: The Darmstadt High-Intensity Photon Setup (DHIPS) for photon-scattering and photodissociation experiments at the superconducting electron linear accelerator S–DALINAC has been improved. We report on its present performance. The developments include the installation of a segmented radiator for an online evaluation of the beam energy and the construction of a second photon-scattering setup with a Clover detector to deduce parity assignments. The enhancement of the existing photon-scattering setup with an additional 100% HPGe detector and some minor changes are described as well as the installation of two setups for photon-induced activation. In addition, we present a comparison of experimental data to Monte Carlo simulations using the codes Geant 3.21 and GEANT4.
Nuclear Instruments and Methods in Physics Research Section A Accelerators Spectrometers Detectors and Associated Equipment 640(1):6-12. · 1.21 Impact Factor
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J. Isaak,
D. Savran, M. Fritzsche,
D. Galaviz,
T. Hartmann,
S. Kamerdzhiev,
J. H. Kelley,
E. Kwan,
N. Pietralla,
C. Romig,
G. Rusev,
K. Sonnabend,
A. P. Tonchev,
W. Tornow,
A. Zilges
[show abstract]
[hide abstract]
ABSTRACT: The dipole-strength distribution in the semimagic nucleus 44Ca has been measured up to 10 MeV excitation energy in photon-scattering experiments using bremsstrahlung and monoenergetic 100% linearly polarized photon beams. The combination of both measurements allows a clear determination of spin and parity quantum numbers of the excited states as well as absolute cross sections and transition probabilities. The results show that the majority of the dipole strength in 44Ca below 10 MeV is due to E1 transitions while M1 strength plays only a minor role. The experimental results are compared to the strength in the neighboring doubly magic nuclei 40,48Ca and to microscopic calculations within the extended theory of finite Fermi systems in order to investigate the evolution of the low-lying E1 strength in this isotopic chain. Both, experiment and calculations, show a nontrivial dependence of the total E1 strength as a function of the neutron number.
Phys. Rev. C. 83(3).
