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Collective Quadrupole Modes in Nuclei – Insights and Experiments

Explore low-energy quadrupole vibrations in nuclei, focusing on 94Mo, with theoretical calculations and experimental studies. Investigate fine structure of Giant Resonances in ISGQR. Damping mechanisms and dominant features analyzed.

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Collective Quadrupole Modes in Nuclei – Insights and Experiments

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  2. Collective Quadrupole Modes in Nuclei –New Insights into Old Problems* Peter von Neumann-Cosel Institut für Kernphysik, TU Darmstadt, Germany N. Botha2, O. Burda1,J. Carter3, R.W. Fearick2, S.V. Förtsch4, C. Fransen5, H. Fujita3, Y. Kalmykov, M. Kuhar, A. Lenhardt1, R. Neveling4, N. Pietralla6, V.Yu. Ponomarev1, A. Richter1, A. Shevchenko, E. Sideras-Haddad3, R. Smit4 and J. Wambach1 1 Technische Universität Darmstadt 2 University of Cape Town 3 University of the Witwatersrand 4 iThemba LABS, Somerset West 5 Universität zu Köln 6 State University of New York, Stony Brook * Supported by DFG under contracs SFB 634 and 445 SUA 113/6/0-1.

  3. First Part: Low-Energy Quadrupole Vibrations Quadrupole phonons as building blocks of low-energy structure High-resolution electron and proton scattering The case of 94Mo symmetric and mixed-symmetric phonons purity of two-phonon states

  4. Identification of Mixed-Symmetry States:Interacting Boson Model Pairing of nucleons to s- / d-bosons p boson: F0 = 1/2 F-Spin: u boson: F0 = -1/2 F = Fmax: symmetric states F < Fmax: mixed-symmetry states (ms) Q-phonon scheme:

  5. Identification of Mixed-Symmetry States:Q-Phonon Scheme F = Fmax (sym. states) F = Fmax – 1 (ms states) + + QsQms 0ms,…,4ms Strong E2 transitions for decay of symmetric Q-phonon + Qms 2ms Weak E2 transitions for decay of ms Q-phonon + + + QsQs 02,22,41 + 21 Qs Strong M1 transitions for decay of ms states to symmetric states + 01

  6. Why 94Mo? The low-energy spectrum of 94Mo is well studied and most one- and two-phonon states have been identified N.Pietrella et al, Phys. Rev. Lett. 83 (1999) 1303 N.Pietrella et al, Phys. Rev. Lett. 84 (2000) 3775 C.Fransen et al, Phys. Lett. B 508 (2001) 219 C.Fransen et al, Phys. Rev. C 67 (2003) 024307 Structure of basic phonons Purity of two-phonon states Study of 2+ states with (e,e´) and (p,p´) isoscalar / isovector decomposition sensitive to one-phonon components of the wave function

  7. Identification of Mixed-Symmetry States:Experiments High resolution required to resolve all 2+ states below 4 MeV Lateral dispersion matching techniques (e,e´): S-DALINAC, TU Darmstadt (p,p´): SSC, iThemba LABS Ee = 70 MeV Q = 93°- 165° DE =30 keV (FWHM) Ep = 200 MeV Q = 7°- 26° DE = 35 keV (FWHM)

  8. S-DALINAC and its Experimental Facilities

  9. LINTOTT- Spectrometer 1 m

  10. Focal Plane Detector System: Si-Microstrip Detectors 50 cm

  11. Separated-Sector Cyclotron Facility

  12. Measured Spectra + + + + + + - 21 23 31 22 24 25 2

  13. Theoretical Calculation of Cross Sections Wave functions: Quasi-Particle Phonon Model (QPM) pure one- and two-phonon states full (up to 3 phonons) Shell Model 88Sr core Surface Delta Interaction (SDI) Cross sections DWBA treatment Effective nucleon-target interaction (Paris, Love-Franey)

  14. QPM Predictions

  15. One-Phonon Symmetric State: Ex = 0.871 MeV

  16. One-Phonon Mixed-Symmetry State: Ex = 2.067 MeV

  17. Radial Transition Charge Densities

  18. pure two-phonon state two-step contributions? Two-Phonon Symmetric State: Ex = 1.864 MeV

  19. 5 – 10 % one-phonon admixture two-step contributions? Two-Phonon Mixed-Symmetry State: Ex = 2.87 MeV

  20. + 25 + 23 + 22 + 21 + 01 Admixture to two-phonon ms state confirmed Pure two-phonon symmetric state confirmed Coupled-Channel Analysis b = 1.23 b = 0.35 b = 0.0 b = 0.2

  21. Summary: First Part Study of one- and two-phonon 2+ states in 94Mo with high-resolution (e,e´) and (p,p´) experiments Combined analysis with QPM reveals symmetric and mixed-symmetric character of one-phonon states two-phonon symmetric state extremely pure about 25% admixtures in the two-phonon mixed-symmetric wave function (mostly 3-phonon) quantitatively consistent results after inclusion of two-step processes in (p,p´) Shell model description moderate to poor limited model space

  22. Second Part: Fine Structure of the ISGQR Introduction: Damping of giant resonances Evidence for fine structure of giant resonances Wavelet analysisand characteristic scales Dominant damping mechanisms Summary and outlook

  23. Damping of GR: Isoscalar Quadrupole Mode DL = 2 DT = 0 DS = 0 protons, neutrons DE = 200 keV Pitthan and Walcher (Darmstadt, 1972) Centroid energy: Ec ~ 63 A-1/3 MeV Width Damping mechanisms?

  24. Excitation and Decay of Giant Resonances DG G G Direct decay Pre-equilibrium and statistical decay Resonance decay width: G = DG G G + +

  25. Doorway-State Model

  26. Fine Structure of Giant Resonances ISGQR in 208Pb DE = 50 keV 1981 1991 DE = 50 keV High resolution is crucial Different probes but similar structures

  27. Fine Structure of Giant Resonances Global phenomenon? - Other nuclei - Other resonances Methods for characterization of fine structure? Goal: Dominant damping mechanisms

  28. New Experiments Place: iThemba LABS, South Africa RCNP, Osaka Reaction: ISGQR from (p,p´) GT from (3He,t) Beam energy: 200 MeV 140 MeV/u Scattering angles: 8° – 10° (DL = 2) 0° (DL = 0) DE = 35 - 50 keV DE = 50 keV Energy resolution: (FWHM)

  29. Fine Structure of the ISGQR (1981) (2004) Not a Lorentzian

  30. Fine Structure of the ISGQR in Other Nuclei Fine structure of the ISGQR is global

  31. Fine Structure in Deformed Nuclei?

  32. Fluctuation analysis using autocorrelation function Doorway-state analysis Fourier analysis Entropy index method Local scaling dimension Methods for Fine Structure Studies Wavelet transform from signal processing

  33. Wavelet Analysis Wavelet coefficients: Continuous: dE,Exare varied continuously and

  34. Application: ISGQR in 208Pb(p,p´)

  35. Three classes of scales Class I scales appear in all nuclei Class II scales change with mass number Class III scales gross structure (e.g. width) Summary of Scales

  36. 208Pb: Random Phase Approximation No scales from 1p-1h states

  37. 208Pb: Second RPA Coupling to 2p-2h generates fine structure and scales

  38. Microscopic Models: Case of 208Pb Wambach et al. (2000) Ponomarev (2003) Lacroix et al. (2001) Kamerdziev et al. (1997)

  39. Three classes of scales as in experiment Strong variations of class II and class III scales Experiment vs. Model Predictions

  40. Dissipation Mechanisms Two types of dissipation mechanisms: collective damping  low-lying surface vibrations  1p–1h  phonon non-collective damping  background states  coupling to 2p–2h states

  41. RMT: Gaussian distribution for 1p1h | V1p1h | 2p2h QPM: deviations at large and at small m.e. Large m.e. define the collective damping mechanism Small m.e. are responsible for the non-collective damping Distribution of the Coupling Matrix Elements (i) (ii) (i) 2p2h

  42. Collective vs. Non-collective Damping in 208Pb Collective part: all scales Non-collective part: no prominent scales

  43. Gaussian distribution for coupling matrix elements (RMT) Level spacing distribution according to GOE Average over statistical ensemble Similar results as for the non-collective damping mechanism Generic behavior of the non-collective damping? Stochastic Coupling Model

  44. Observed for the first time in a heavy nucleus Asymmetric fluctuations Selectivity: Jp = 1+ level density Fine Structure of the Spin-Flip GTR

  45. Orthogonal basis of wavelet functions Exact reconstruction of the spectrum is possible Relevance of scales Discrete Wavelet Analysis Wavelet coefficients: Discrete: dE=2 jandEx=kdE with j , k = 1, 2, 3, … • Resolution is limited to ranges of scales

  46. Decomposition

  47. Application: GTR from 90Zr(3He,t)90Nb Reconstruct the spectrum using important scales    

  48. Relevance of scale regions can be tested DWT and CWT results are consistent DWT: Reconstruction of the Spectrum sr(E)=A8+D8 + D6+ D4+ D3 sr(E) =A8+D7 + D5+ D2 +D1

  49. over a whole mass range in different types of resonances collective damping: low-lying surface vibrations non-collective damping: stochastic coupling Summary Fine structure is a general phenomenon of giant resonances Quantitative analysis with wavelets Origin of scales: Relevance of scales for discrete wave transform Model-independent level densities

  50. improvement of experimental resolution escape width Landau damping experiment and models Outlook Goal: next step in the hierarchy: 3p-3h Contribution of other damping mechanisms Quantitative analysis of scales:

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