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R Schwengner,
R Massarczyk,
G Rusev,
N Tsoneva,
D Bemmerer,
R Beyer,
R Hannaske,
A R Junghans,
J H Kelley, E Kwan,
H Lenske,
M Marta,
R Raut,
K D Schilling,
A Tonchev,
W Tornow,
A Wagner
[show abstract]
[hide abstract]
ABSTRACT: The dipole strength of the N = 50 nucleus 86 Kr was studied in photon-scattering experiments using bremsstrahlung produced with electron beams of energies of 7.9 and 11.2 MeV delivered by the linear accelerator ELBE as well as using quasimonoenergetic and linearly polarized γ rays of 10 energies within the range from 4.7 to 9.3 MeV delivered by the HIγ S facility. A high-pressure gas target was used. We identified 39 levels up to an excitation energy of 10.1 MeV. Simulations of γ -ray cascades were performed to estimate intensities of inelastic transitions and to correct the intensities of the ground-state transitions for their branching ratios. The photoabsorption cross section derived in this way up to the neutron-separation energy is combined with the photoabsorption cross section obtained from a (γ, n) experiment at HIγ S. The enhanced E1 strength found in the range from 6 to 10 MeV is compared with the ones in the N = 50 isotones 88 Sr, 90 Zr, and 92 Mo and with predictions of calculations within the quasiparticle-phonon model.
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G. Rusev,
N. Tsoneva,
F. Do¨nau,
S. Frauendorf,
R. Schwengner,
A. P. Tonchev,
† A. S. Adekola,
‡ S. L. Hammond,
J. H. Kelley, E. Kwan,
† H. Lenske,
W. Tornow,
and A. Wagner
Physical Review Letters 01/2013; · 7.37 Impact Factor
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G Rusev,
N Tsoneva,
F Dönau,
S Frauendorf,
R Schwengner,
A P Tonchev,
A S Adekola,
S L Hammond,
J H Kelley, E Kwan,
H Lenske,
W Tornow,
A Wagner
[show abstract]
[hide abstract]
ABSTRACT: The M1 excitations in the nuclide ^{90}Zr have been studied in a photon-scattering experiment with monoenergetic and linearly polarized beams from 7 to 11 MeV. More than 40 J^{π}=1^{+} states have been identified from observed ground-state transitions, revealing the fine structure of the giant M1 resonance with a centroid energy of 9 MeV and a sum strength of 4.17(56) μ_{N}^{2}. The result for the total M1 strength and its fragmentation are discussed in the framework of the three-phonon quasiparticle-phonon model.
Physical Review Letters 01/2013; 110(2):022503. · 7.37 Impact Factor
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J Beller,
J Wagner,
M Ahmed,
D Deleanu,
D M Filipescu,
T Glodariu,
J Isaak,
J H Kelley, E Kwan,
N Pietralla,
R Raut,
C Romig,
G Rusev,
M Scheck,
S C Stave,
A Tonchev,
W Tornow,
H R Weller,
N.-V Zamfir,
M Zweidinger
[show abstract]
[hide abstract]
ABSTRACT: The parity doublet of 1 + /1 − states of Ne-20 at 11.26 MeV excitation energy is one of the best known test cases to study the weak part of the nuclear Hamiltonian. The feasibility of parity violation experiments depend on the effective nuclear enhancement factor (RN /|E(1 +) − E(1 −)|) which amplifies the impact of the matrix element of the weak interaction on observables indicating parity mixing. An extreme large value of RN /|E(1 +) − E(1 −)| = (670 ± 7000) MeV −1 was reported for the doublet in 20 Ne. The large uncertainty depends amongst others on the large uncertainty of |E(1 +)−E(1 −)| = 7.7±5.5 keV of the parity doublet. Nuclear resonance fluorescence (NRF) experiments with linearly and circularly polarized photon beams were performed at the High Intensity Gamma-Ray Source at Duke University, Durham, NC, USA, to determine the energy difference of the parity doublet with higher precision. The different angular distributions for 0 + → 1 − → 0 + and 0 + → 1 + → 0 + NRF cascades in polarized γ-ray beams were used to determine the energy difference of the parity doublet to 2.9(13) keV.
Journal of Physics A General Physics 01/2012; 366.
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[show abstract]
[hide abstract]
ABSTRACT: Nuclear resonance fluorescence experiments were carried out at the High-Intensity γ -ray Source facility at Triangle Universities Nuclear Laboratory to characterize the low-energy dipole structure of 238 U using 100% linearly polarized photon beams from 2.0 to 6.2 MeV. 113 transitions corresponding to de-excitations to the ground state in 238 U were observed and the energy, spin, parity, integrated cross section, reduced width, and branching ratio were determined for each of these identified levels. The total E1 γ -ray interaction cross section was calculated and it was deduced that the observed concentration of low-lying E1 transitions were excited from the low-energy tail of the giant dipole resonance and were not a pygmy dipole resonance. Comparisons were made between quasiparticle random-phase approximation calculations and the experimentally observed strength. The observed and predicted M1 strength agreed well with each other. However, there was no similar agreement for the E1 strength.
Physical Review C 01/2012; 852090(27). · 3.31 Impact Factor
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R Massarczyk,
R Schwengner,
F Dönau,
E Litvinova,
G Rusev,
R Beyer,
R Hannaske,
A R Junghans,
M Kempe,
J H Kelley, [......],
C Nair,
R Raut,
K D Schilling,
G Schramm,
D Stach,
A P Tonchev,
W Tornow,
E Trompler,
A Wagner,
D Yakorev
[show abstract]
[hide abstract]
ABSTRACT: The electromagnetic dipole strength of the nucleus 136 Ba has been investigated. Two measurements were performed with electron energies of 7.0 and 11.4 MeV at the bremsstrahlung facility at the ELBE accelerator of the Helmholtz-Zentrum Dresden-Rossendorf. Photon scattering experiments on the same nucleus have been performed at the high-intensity gamma-ray source (HIγ S) facility of the Triangle Universities Nuclear Laboratory between 4.7 and 9.3 MeV. The GEANT4 code has been used to determine detector response and non-nuclear scattered events. Thus it is possible to account for the dipole strength in the quasicontinuum of unresolvable transitions. A statistical code was used to simulate inelastic transitions and to determine the branching ratios of transitions to the ground state. The resulting photoabsorption cross section is compared to quasiparticle random-phase approximation and relativistic quasiparticle time blocking approximation calculations.
Physical Review C 01/2012; 8660. · 3.31 Impact Factor
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G Rusev,
A S Adekola,
F Dönau,
S Frauendorf,
S L Hammond,
C Huibregtse,
J H Kelley, E Kwan,
R Schwengner,
A P Tonchev,
W Tornow,
A Wagner
[show abstract]
[hide abstract]
ABSTRACT: M1 excitations in the 90Zr nucleus have been studied in a photon-scattering experiment with monoenergetic and 100% linearly-polarized beams from 7 to 11 MeV. More than 40 Jπ = 1+ states were identified for the first time from the observed ground-state transitions revealing the fine structure of the giant M1 resonance with center of gravity of 9 MeV and sum strength of 3.8(3) μ2N. The results of the present work are compared with predictions from the shell model and the quasi-particle random-phase approximation.
Journal of Physics Conference Series 09/2011; 312(9):092053.
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[show abstract]
[hide abstract]
ABSTRACT: High resolution and sensitivity studies of the nuclear dipole response in 138Ba have been performed in (γ, γ') experiment. The electric dipole character of the "pygmy" mode was experimentally verified for excitations from 4.0 to 8.5 MeV. Experimental findings have been compared with the quasiparticle phonon model. A new understanding about the character and the evolution of the pygmy resonance emerges from comparison of the experimental results with the theoretical calculations.
Journal of Physics Conference Series 09/2011; 312(9):092058.
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R Raut,
A S Crowell,
B Fallin,
C R Howell,
C Huibregtse,
J H Kelley,
T Kawano, E Kwan,
G Rusev,
A P Tonchev,
W Tornow,
D J Vieira,
J B Wilhelmy
[show abstract]
[hide abstract]
ABSTRACT: Cross-section measurements for neutron-induced reactions on GaAs have been carried out at twelve different neutron energies from 7.5 to 15 MeV using the activation technique. The monoenergetic neutron beams were produced via the 2 H(d, n) 3 He reaction. GaAs samples were activated along with Au and Al monitor foils to determine the incident neutron flux. The activities induced by the reaction products were measured using high-resolution γ -ray spectroscopy. Cross sections for five reaction channels, viz., 69 Ga(n, 2n) 68 Ga, 69 Ga(n, p) 69 Zn m , 71 Ga(n, p) 71 Zn m , 75 As(n, 2n) 74 As, and 75 As(n, p) 75 Ge, are reported. The results are compared with the previous measurements and available data evaluations. Statistical-model calculations, based on the Hauser-Feshbach formalism, have been carried out using the TALYS and the COH3 codes and are compared with the experimental results.
Physical Review C 01/2011; 60. · 3.31 Impact Factor
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E Kwan,
G Rusev,
A S Adekola,
F Dönau,
S L Hammond,
C R Howell,
H J Karwowski,
J H Kelley,
R S Pedroni,
R Raut,
A P Tonchev,
W Tornow
[show abstract]
[hide abstract]
ABSTRACT: Gamma-ray transitions in 235 U were measured using the (γ, γ) reaction below 3 MeV. The nuclear-resonance-fluorescence experiment was carried out at the High-Intensity γ -ray Source facility using nearly monoenergetic and circularly polarized photon beams. More than 20 transitions corresponding to deexcitations to the ground state and low-lying levels in 235 U were observed. The integrated cross sections to the excited levels and intensities of branching transitions were deduced. The experimental results are compared with predictions from a quasiparticle random-phase approximation in a deformed basis.
Physical Review C 01/2011; · 3.31 Impact Factor
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[show abstract]
[hide abstract]
ABSTRACT: High-sensitivity studies of E1 and M1 transitions observed in the reaction 138Ba(gamma,gamma{'}) at energies below the one-neutron separation energy have been performed using the nearly monoenergetic and 100% linearly polarized photon beams of the HIgammaS facility. The electric dipole character of the so-called "pygmy" dipole resonance was experimentally verified for excitations from 4.0 to 8.6 MeV. The fine structure of the M1 "spin-flip" mode was observed for the first time in N=82 nuclei.
Physical Review Letters 02/2010; 104(7):072501. · 7.37 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 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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[show abstract]
[hide abstract]
ABSTRACT: We provide a detailed experimental validation of the concept of transmission-based isotope detection. The dominant background processes in this class of systems were measured by studying the detection of <sup>238</sup> U with a quasimonochromatic (ΔE/E∼3%) photon beam. A notch develops in the spectrum transmitted through our test objects due to the preferential attenuation of photons with an energy that resonantly excites a bound nuclear state in <sup>238</sup> U near 2 MeV. The notch was measured downstream of our test objects by means of resonant photon scattering from a secondary <sup>238</sup> U target. The dominant backgrounds measured in the notch detector due to radioactive decay and elastic scattering of the transmitted beam are presented. Processes that refill the notch with off-resonance photons will obscure the signal and result in a higher probability of false negatives. A measurement of the refill process produced a null result, and we report an upper limit on the magnitude of the notch fill factor.
Journal of Applied Physics 11/2009; · 2.17 Impact Factor
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A. Hutcheson,
C. Angell,
J. A. Becker,
A. S. Crowell,
D. Dashdorj,
B. Fallin,
N. Fotiades,
C. R. Howell,
H. J. Karwowski,
T. Kawano,
J. H. Kelley, E. Kwan,
R. A. Macri,
R. O. Nelson,
R. S. Pedroni,
A. P. Tonchev,
W. Tornow
[show abstract]
[hide abstract]
ABSTRACT: Precision measurements of 238U(n,n'γ) and 238U(n,2nγ) partial cross sections have been performed at Triangle Universities Nuclear Laboratory (TUNL) to improve crucial data needed for testing nuclear reaction models in the actinide mass region. A pulsed and monoenergetic neutron beam was used in combination with high-resolution γ-ray spectroscopy to obtain partial cross sections for incident neutron energies between 5 and 14 MeV. γ-ray yields were measured with high-purity germanium clover and planar detectors. Measured partial cross-section data are compared with previous results using white and monoenergetic neutron beams and calculations from the GNASH and TALYS Hauser-Feshbach statistical-model codes. Present experimental results are in fair to good agreement with most of the existing data for the 238U(n,n'γ) reaction. However, significant discrepancies are observed for the 238U(n,2nγ) reaction.
Physical Review C 07/2009; 80(1). · 3.31 Impact Factor
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A. P. Tonchev,
C. R. Howell,
A. Hutcheson, E. Kwan,
G. Rusev,
W. Tornow,
S. Hammond,
H. J. Karwowski,
C. Huibregtse,
J. H. Kelley,
D. J. Vieira,
J. B. Wilhelmy
[show abstract]
[hide abstract]
ABSTRACT: The photodisintegration cross section of radioactive {sup 241}Am has been obtained for the first time using monoenergetic {gamma}-ray beams from the HI{gamma}S facility. The induced activity of {sup 240}Am produced via the {sup 241}Am({gamma},n) reaction in the {gamma}-ray energy range from 9.5 to 16 MeV was measured by the activation technique utilizing high resolution HPGe detectors. The {sup 241}Am({gamma},n) cross section was determined both by measuring the absolute {gamma}-ray flux and by comparison to the {sup 197}Au({gamma},n) and {sup 58}Ni({gamma},n) cross section standards. The experimental data for the {sup 241}Am({gamma},n) reaction in the giant dipole resonance energy region is compared with statistical nuclear-model calculations.
AIP Conference Proceedings 03/2009; 1099(1).
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G. Rusev,
A. L. Hutcheson, E. Kwan,
A. P. Tonchev,
W. Tornow,
Durham NC Triangle Universities Nuclear Laboratory,
C. T. Angell,
S. L. Hammond,
H. J. Kawowski,
Chapel Hill NC University of North Carolina at Chapel Hill, [......],
M. Erhard,
E. Grosse,
S. Frauendorf,
A. R. Junghans,
J. Klug,
K. Kosev,
C. Nair,
N. Nikolov,
K.-D. Schilling,
R. Schwengner
[show abstract]
[hide abstract]
ABSTRACT: Dipole-strength distributions in the stable even-mass molybdenum isotopes up to the neutron-separation energies have been studied in photon-scattering experiments with bremsstrahlung at the superconducting electron accelerator ELBE at the Research Center Dresden-Rossendorf, Germany, and with mono-energetic photon beams at the High Intensity Gamma-ray Source facility at Triangle Universities Nuclear Laboratory. In order to determine the dipole-strength distribution, statistical methods were developed for the analysis of the measured spectra. The data obtained for the stable even-mass molybdenum isotopes from the present ({gamma},{gamma}') experiments are combined with ({gamma},n) cross sections from the literature resulting in a photoabsorption cross section covering the full range from about 4 to 15 MeV, which is of interest for nuclear structure as well as for nuclear astrophysics network calculations. Novel information about the low-energy tail of the Giant Dipole Resonance and the energy spreading of its strength is derived.
AIP Conference Proceedings 03/2009; 1099(1).
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[show abstract]
[hide abstract]
ABSTRACT: High-sensitivity studies of E1 and M1 excitations observed in the {sup 138}Ba({gamma},{gamma}') reaction at energies below the neutron emission threshold have been performed. The electric dipole character of the so-called 'pygmy' mode was experimentally verified for excitations from 4.0-8.6 MeV. The fine structure of the M1 'spin-flip' mode was observed for the first time in N = 82 nuclei.
AIP Conference Proceedings 01/2009; 1090(1).
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[show abstract]
[hide abstract]
ABSTRACT: Nuclear resonance fluorescence experiments were carried out at the High-Intensity γ-ray Source facility at Triangle Universities Nuclear Laboratory to characterize the low-energy dipole structure of 238U using 100% linearly polarized photon beams from 2.0 to 6.2 MeV. 113 transitions corresponding to de-excitations to the ground state in 238U were observed and the energy, spin, parity, integrated cross section, reduced width, and branching ratio were determined for each of these identified levels. The total E1 γ-ray interaction cross section was calculated and it was deduced that the observed concentration of low-lying E1 transitions were excited from the low-energy tail of the giant dipole resonance and were not a pygmy dipole resonance. Comparisons were made between quasiparticle random-phase approximation calculations and the experimentally observed strength. The observed and predicted M1 strength agreed well with each other. However, there was no similar agreement for the E1 strength.
Phys. Rev. C. 85(4).
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A. P. Tonchev,
S. L. Hammond,
C. R. Howell,
C. Huibregtse,
A. Hutcheson,
J. H. Kelley, E. Kwan,
R. Raut,
G. Rusev,
W. Tornow,
T. Kawano,
D. J. Vieira,
J. B. Wilhelmy
[show abstract]
[hide abstract]
ABSTRACT: The photodisintegration cross section of the radioactive nucleus 241Am has been obtained using activation techniques and monoenergetic γ-ray beams from the HIγS facility. The induced activity of 240Am produced via the 241Am(γ,n) reaction was measured in the energy interval from 9 to 16 MeV utilizing high-resolution γ-ray spectroscopy. The experimental data for the 241Am(γ,n) reaction in the giant dipole resonance energy region are compared with statistical nuclear-model calculations.
Phys. Rev. C. 82(5).
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[show abstract]
[hide abstract]
ABSTRACT: Properties of low-energy dipole states in 232Th have been investigated with the nuclear resonance fluorescence technique. The present work used monoenergetic γ-ray beams at energies of 2–4 MeV from the high-intensity γ-ray source at Triangle Universities Nuclear Laboratory. Over 40 transitions corresponding to deexcitation to the ground state and first excited state were observed for the first time. Excitation energies, integrated cross sections, decay widths, branching ratios, and transition strengths for those states in 232Th were determined and compared with quasiparticle random-phase-approximation calculations. A large number of E1 transitions were observed for the first time in actinide nuclei with summed strength of 3.28(69)×10-3 e2 fm2. The observed summed M1 strength of 4.26(63)μN2 is in good agreement with the other actinides and with the systematics of the scissors mode in deformed rare-earth nuclei.
Phys. Rev. C. 83(3).
