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Title: Study of Collective Dipole Excitations below the Giant Dipole Resonance at HI{gamma}S

Abstract

The High-Intensity Gamma-ray Source utilizing intra-cavity back-scattering of free electron laser photons from relativistic electrons allows one to produce a unique beam of high-flux gamma rays with 100% polarization and selectable energy and energy resolution which is ideal for low-energy {gamma}-ray scattering experiments. Nuclear resonance fluorescence experiments have been performed on N=82 nuclei. High sensitivity studies of E1 and M1 excitations at energies close to the neutron emission threshold have been performed. The method allows the determination of excitation energies, spin, parities, and decay branching ratios of the pygmy dipole mode of excitation. The observations are compared with calculations using statistical and quasi-particle random-phase approximations.

Authors:
; ;  [1];  [2]; ; ;  [3];  [2]; ;  [4];  [2];  [5];  [6];  [2]
  1. Duke University, Department of Physics, Box 90308, Durham, NC 27708-0308 (United States)
  2. (United States)
  3. University of North Carolina, Department of Physics and Astronomy, Chapel Hill, NC 27599-3255 (United States)
  4. North Carolina State University, Department of Physics, Box 8202, Raleigh, NC 27695-8202 (United States)
  5. Institute of Theoretical Physics, University of Giessen, Heinrich-Buff-Ring 16, D-35392, Giessen (Germany)
  6. Duke University, Durham, NC 27708-0319 (United States)
Publication Date:
OSTI Identifier:
21056759
Resource Type:
Journal Article
Resource Relation:
Journal Name: AIP Conference Proceedings; Journal Volume: 891; Journal Issue: 1; Conference: 6. Symposium on nuclear physics, Tours (France), 5-8 Sep 2006; Other Information: DOI: 10.1063/1.2713535; (c) 2007 American Institute of Physics; Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
73 NUCLEAR PHYSICS AND RADIATION PHYSICS; BACKSCATTERING; BRANCHING RATIO; ELECTRONS; ENERGY RESOLUTION; EXCITATION; GAMMA RADIATION; GAMMA SOURCES; GIANT RESONANCE; NEUTRON EMISSION; PARITY; PHOTON-NUCLEON INTERACTIONS; PHOTONUCLEAR REACTIONS; POLARIZATION; QUASI PARTICLES; RANDOM PHASE APPROXIMATION; RESONANCE FLUORESCENCE; SENSITIVITY ANALYSIS; SPIN; STATISTICAL MODELS

Citation Formats

Tonchev, A. P., Howell, C. R., Tornow, W., TUNL, Department of Physics, Box 90308, Durham, NC 27708-0308, Angell, C., Boswell, M., Karwowski, H. J., TUNL, Department of Physics and Astronomy, Chapel Hill, NC 27599-3255, Chyzh, A., Kelley, J. H., TUNL, Department of Physics, Box 8202, Raleigh, NC 27695-8202, Tsoneva, N., Wu, Y. K., and Duke Free Electron Laser Laboratory, Durham, NC 27708-0319. Study of Collective Dipole Excitations below the Giant Dipole Resonance at HI{gamma}S. United States: N. p., 2007. Web. doi:10.1063/1.2713535.
Tonchev, A. P., Howell, C. R., Tornow, W., TUNL, Department of Physics, Box 90308, Durham, NC 27708-0308, Angell, C., Boswell, M., Karwowski, H. J., TUNL, Department of Physics and Astronomy, Chapel Hill, NC 27599-3255, Chyzh, A., Kelley, J. H., TUNL, Department of Physics, Box 8202, Raleigh, NC 27695-8202, Tsoneva, N., Wu, Y. K., & Duke Free Electron Laser Laboratory, Durham, NC 27708-0319. Study of Collective Dipole Excitations below the Giant Dipole Resonance at HI{gamma}S. United States. doi:10.1063/1.2713535.
Tonchev, A. P., Howell, C. R., Tornow, W., TUNL, Department of Physics, Box 90308, Durham, NC 27708-0308, Angell, C., Boswell, M., Karwowski, H. J., TUNL, Department of Physics and Astronomy, Chapel Hill, NC 27599-3255, Chyzh, A., Kelley, J. H., TUNL, Department of Physics, Box 8202, Raleigh, NC 27695-8202, Tsoneva, N., Wu, Y. K., and Duke Free Electron Laser Laboratory, Durham, NC 27708-0319. Mon . "Study of Collective Dipole Excitations below the Giant Dipole Resonance at HI{gamma}S". United States. doi:10.1063/1.2713535.
@article{osti_21056759,
title = {Study of Collective Dipole Excitations below the Giant Dipole Resonance at HI{gamma}S},
author = {Tonchev, A. P. and Howell, C. R. and Tornow, W. and TUNL, Department of Physics, Box 90308, Durham, NC 27708-0308 and Angell, C. and Boswell, M. and Karwowski, H. J. and TUNL, Department of Physics and Astronomy, Chapel Hill, NC 27599-3255 and Chyzh, A. and Kelley, J. H. and TUNL, Department of Physics, Box 8202, Raleigh, NC 27695-8202 and Tsoneva, N. and Wu, Y. K. and Duke Free Electron Laser Laboratory, Durham, NC 27708-0319},
abstractNote = {The High-Intensity Gamma-ray Source utilizing intra-cavity back-scattering of free electron laser photons from relativistic electrons allows one to produce a unique beam of high-flux gamma rays with 100% polarization and selectable energy and energy resolution which is ideal for low-energy {gamma}-ray scattering experiments. Nuclear resonance fluorescence experiments have been performed on N=82 nuclei. High sensitivity studies of E1 and M1 excitations at energies close to the neutron emission threshold have been performed. The method allows the determination of excitation energies, spin, parities, and decay branching ratios of the pygmy dipole mode of excitation. The observations are compared with calculations using statistical and quasi-particle random-phase approximations.},
doi = {10.1063/1.2713535},
journal = {AIP Conference Proceedings},
number = 1,
volume = 891,
place = {United States},
year = {Mon Feb 26 00:00:00 EST 2007},
month = {Mon Feb 26 00:00:00 EST 2007}
}
  • The study of the gamma decay of the giant dipole resonance (GDR) has allowed to investigate nuclear structure properties at excitation energies up to the Fermi energy. It continues to be a useful tool to explore the basic nuclear properties at finite temperature and angular momentum, such as nuclear shapes and thermal effects. In particular, the dependence of the GDR width as a function of temperature and angular momentum provides information on the evolution of the nuclear shapes and of the damping mechanisms of this collective state. This is an interesting topic under discussion which is presently addressed with newmore » and more exclusive measurements. In reviewing the activity in this field selected exclusive data concerning the extreme region of temperature and angular momentum will be presented. In particular data for the two mass regions A=120-130 and A=180-200 will be discussed, with results based on measurements made at the INFN laboratories LNL using the HECTOR and GARFIELD arrays.« less
  • The system of equations of motion for the components of the one-particle density matrix is linearized, for a finite system of fermions interacting through a two-body residual'' force. Exchange terms are included. Equations corresponding to those of the extended shell models' method are obtained for calculating collective states of spherical nuclei of definite angular momentum, parity, spin, and isobaric spin, in both the j-j and L-S coupling schemes. Particular application and solution of such equations is obtaincd for the case of the giant dipole state of O/sup 16/ by the use of finite range potentials with two different mixtures ofmore » exchange forces and radial shapes. The matrix elements are antisymmetrized and the backward going graphs'' are included. The left- over terms bilinear in the density operator are then minimized'' by extracting their lowest order linear contribution. The resulting correction terms are estimated for the giant dipole state in O/sup 16/ and found to be small, giving support to the Handom Phase Approximation. (auth)« less
  • 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 fullmore » 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.« less