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Photon Strength Functions below GDR Maximum: Present Status and Outlook

Photon Strength Functions below GDR Maximum: Present Status and Outlook. An attempt to summarize experimental information about photon ( g -ray) strength functions at the low-energy tail of GDER with an emphasis on (possible) resonance structures. Milan Krtička. Photoabsorption x-section.

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Photon Strength Functions below GDR Maximum: Present Status and Outlook

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  1. Photon Strength Functions below GDR Maximum: Present Status and Outlook An attempt to summarize experimental information about photon (g-ray) strength functions at the low-energy tail of GDER with an emphasis on (possible) resonance structures Milan Krtička CGS13, Cologne, August 25, 2008

  2. Photoabsorption x-section γ abs=(γ, x) + (γ,γ’) Bn Eγ (γ,γ’) Porter-Thomas fluctuations (χ2ν=1 distribution) ≈10 eV ≈0.1 eV <γ abs> …an energy-smoothed photoabsorption x-section Eγ Fragmentation of the GDER Photonuclear x-section (γ, x) Eγ Bn ≈15 MeV CGS13, Cologne, August 25, 2008

  3. Detailed balance principle photoabsorption g-decay • PSF describes energy distribution of photon emission from “highly-excited” states excitation energy Photon Strength Functions CGS13, Cologne, August 25, 2008

  4. What quantities can PSFs depend on: type of transitions (E1, M1, E2, …) ü gamma-ray energy ü microscopic properties of the level (energy,Jp) ? ÞBrink hypothesis Restrictions: Energies below  10 MeV Low spins Medium-weight and heavy nuclei Near the valley of stability Photon Strength Functions CGS13, Cologne, August 25, 2008

  5. Brink hypothesis • The energy dependence of the photoeffect is independent of the detailed structure of the initial state Þ dependence only on g-ray but not on excitation energy (T), Jp,… Validity of the hypothesis? • at least approximately - from (n,g) reactions and hot nuclei • some signs for temperature dependence CGS13, Cologne, August 25, 2008

  6. Where could we learn about PSFs from? • photoexcitation techniques • (g,particle) • NRF experiments – (g,g’) • primaries from (n,g) reaction • two-step cascades spectra - (n,g) • spectrum fitting methods • single spectra • coincidence spectra • inelastic scattering of charged particles(e,e’), (p,p’), … • sequential extraction (Oslo - 3He-induced) • Coulomb dissociation • … Bn CGS13, Cologne, August 25, 2008

  7. photoabsorption - above particle threshold (g,particle) only GS PSF can be obtained does not allow to distinguish among different transition typesÞ dominance of E1 assumed Lorentzian shape of the resonance (classical ED, basic principles) photonuclear data on majority of nuclei (not all) confirms the shape above the neutron threshold how is it below Bn? Photonuclear experiments From B.L. Berman, S.C. Fultz, Rev. Mod. Phys. 47 (1975) 713 CGS13, Cologne, August 25, 2008

  8. Does an universal model of PSFs exist for all nuclei? Is the E1 PSF of Lorentzian shape? Is the decay dominated by E1 in the whole energy region? Are the PSFs “smooth” or are there any resonance-like structures? PSFs below Bn CGS13, Cologne, August 25, 2008

  9. Eg1 Eg2 Eg3 Eg4 Spectra from 4p ball CGS13, Cologne, August 25, 2008

  10. Does a universal model exist? CGS13, Cologne, August 25, 2008

  11. Does an universal model of PSFs exist for all nuclei? Is the E1 PSF of Lorentzian shape? Is the decay dominated by E1 in the whole energy region? Are the PSFs “smooth” or are there any resonance-like structures? PSFs below Bn CGS13, Cologne, August 25, 2008

  12. Does an universal model of PSFs exist for all nuclei?NO Is the E1 PSF of Lorentzian shape? Is the decay dominated by E1 in the whole energy region? Are the PSFs “smooth” or are there any resonance-like structures?There ARE structures PSFs below Bn CGS13, Cologne, August 25, 2008

  13. Does an universal model of PSFs exist for all nuclei?NO Is the E1 PSF of Lorentzian shape? Is the decay dominated by E1 in the whole energy region? Are the PSFs “smooth” or are there any resonance-like structures?There ARE structures PSFs below Bn CGS13, Cologne, August 25, 2008

  14. Does an universal model of PSFs exist for all nuclei?NO Is the E1 PSF of Lorentzian shape? Is the decay dominated by E1 in the whole energy region? Are the PSFs “smooth” or are there any resonance-like structures?There ARE structures PSFs below Bn CGS13, Cologne, August 25, 2008

  15. Does an universal model of PSFs exist for all nuclei?NO Is the E1 PSF of Lorentzian shape? NO Is the decay dominated by E1 in the whole energy region? Are the PSFs “smooth” or are there any resonance-like structures?There ARE structures PSFs below Bn CGS13, Cologne, August 25, 2008

  16. Does an universal model of PSFs exist for all nuclei?NO Is the E1 PSF of Lorentzian shape? NO Is the decay dominated by E1 in the whole energy region? Are the PSFs “smooth” or are there any resonance-like structures?There ARE structures Pigmy (small) resonances in nuclei Scissors mode in deformed nuclei Low-energy mode in medium-weight (Fe, Mo) nuclei Resonances near Bn: Gold region Spherical N=82 nuclei PSFs below Bn CGS13, Cologne, August 25, 2008

  17. Does an universal model of PSFs exist for all nuclei?NO Is the E1 PSF of Lorentzian shape? NO Is the decay dominated by E1 in the whole energy region? Are the PSFs “smooth” or are there any resonance-like structures?There ARE structures Pigmy (small) resonances in nuclei Scissors mode in deformed nuclei Low-energy mode in medium-weight (Fe, Mo) nuclei Resonances near Bn: Gold region Spherical N=82 nuclei PSFs below Bn CGS13, Cologne, August 25, 2008

  18. H. Maser et al., PRC 53 (1995) 2749 Scissors mode • Experimentally observed in many reactions: (e,e’), (g,g’), (n,g), (3He,a) • Majority of data from NRF: • Dynamitron, S-DALINAC, ELBE, HIgS • only GS excitations • in principle, allow to distinguish among different XL(Compton polarimeter, HIgS) • problems with exp. threshold Þ relation between exp. and actual strength? • M1 strength is stronger than E1 near 3 MeV CGS13, Cologne, August 25, 2008

  19. Even-even Odd or odd-odd 164Dy (NRF) Summing interval for B(M1): 2.5 - 4.0 MeV - deduced from data in original paper of J. Margraf et al., PRC 52, 2429 (1995) Phenomenological sum rule: Lo Iudice, Richter, Phys. Lett. B304 193 (1993) Scissors mode - strength 6 4 B(M1) (mN2) Summing interval for B(M1): 2.5 - 4.0 MeV Summing interval for B(M1): 2.7- 3.7 MeV 2 0 180 140 160 200 A Redrawn from J. Enders et al., PRC 59 R1851 (1999) CGS13, Cologne, August 25, 2008

  20. capture of thermal neutrons known initial Jp Eg1 HPGe detectors target beam of thermal neutrons Eg2 Eg1+ Eg2 Eg1 Eg2 TSC spectra đ CGS13, Cologne, August 25, 2008

  21. TSCs in the162Dy(n,γ)163Dy reaction SRs assumed to be built only on all levels below 2.5 MeV CGS13, Cologne, August 25, 2008

  22. TSCs in the162Dy(n,γ)163Dy reaction Scissors resonances assumedto be built on all163Dy levels CGS13, Cologne, August 25, 2008

  23. Even-even Odd or odd-odd Value from TSCs in 163Dy: total sum ΣB(M1) 163Dy (TSC) Value from TSCs in 163Dy; summing interval for B(M1): 2.5 - 4.0 MeV 164Dy (NRF) 163Dy (TSC) Summing interval for B(M1): 2.5 - 4.0 MeV - deduced from data in original paper of J. Margraf et al., PRC 52, 2429 (1995) Phenomenological sum rule: Lo Iudice, Richter, Phys. Lett. B304 193 (1993) Scissors mode - strength 6 4 B(M1) (mN2) Summing interval for B(M1): 2.5 - 4.0 MeV Summing interval for B(M1): 2.7- 3.7 MeV 2 0 180 140 160 200 A Redrawn from J. Enders et al., PRC 59 R1851 (1999) CGS13, Cologne, August 25, 2008

  24. SR ≈ 1.45 MeV SR ≈ 1.30 MeV Scissors from (3He,γ) and (3He,3He’γ) 167Er (3He,γ)168Er 167Er (3He,3He’γ)167Er f (E1)(Eγ) +f(M1)(Eγ)(MeV) Eγ (MeV) E. Melby et al., PRC 63 (2001) 044309 CGS13, Cologne, August 25, 2008

  25. SR ≈ 1.45 MeV SR ≈ 1.30 MeV Scissors from (3He,γ) and (3He,3He’γ) 167Er (3He,γ)168Er 167Er (3He,3He’γ)167Er f (E1)(Eγ) +f(M1)(Eγ)(MeV) Eγ (MeV) E. Melby et al., PRC 63 (2001) 044309 Contradictions with the NRF and TSC data: (1) the strength of the 3 MeV resonance is too small compared to the E1 strength at 3 MeV (2) the resonance is wider by a factor of two or more CGS13, Cologne, August 25, 2008

  26. It does not seem to be the case: – the position of the 3 MeV peak is remarkably stable with changing the initial excitation Scissors from (3He,γ) and (3He,3He’γ) 172Yb(3He,3He’γ)172Yb Data taken from A. Schiller et al., Physics of Atomic Nuclei 62 (2001) 1186 CGS13, Cologne, August 25, 2008

  27. Does an universal model of PSFs exist for all nuclei?NO Is the E1 PSF of Lorentzian shape? NO Is the decay dominated by E1 in the whole energy region? Are the PSFs “smooth” or are there any resonance-like structures?There ARE structures Pigmy (small) resonances in nuclei Scissors mode in deformed nuclei Low-energy mode in medium-weight (Fe, Mo) nuclei Resonances near Bn: Gold region Spherical N=82 nuclei PSFs below Bn CGS13, Cologne, August 25, 2008

  28. Does an universal model of PSFs exist for all nuclei?NO Is the E1 PSF of Lorentzian shape? NO Is the decay dominated by E1 in the whole energy region? NO - at least in deformed nuclei Are the PSFs “smooth” or are there any resonance-like structures?There ARE structures Pigmy (small) resonances in nuclei Scissors mode in deformed nuclei Exists, but problems (strength, width) Low-energy mode in medium-weight (Fe, Mo) nuclei Resonances near Bn: Gold region Spherical N=82 nuclei PSFs below Bn CGS13, Cologne, August 25, 2008

  29. Does an universal model of PSFs exist for all nuclei?NO Is the E1 PSF of Lorentzian shape? NO Is the decay dominated by E1 in the whole energy region? NO - at least in deformed nuclei Are the PSFs “smooth” or are there any resonance-like structures?There ARE structures Pigmy (small) resonances in nuclei Scissors mode in deformed nuclei Exists, but problems (strength, width) Low-energy mode in medium-weight (Fe, Mo,…) nuclei Resonances near Bn: Gold region Spherical N=82 nuclei PSFs below Bn CGS13, Cologne, August 25, 2008

  30. Data from Oslo  strong enhancement of PSF at low g-ray energies Fe, Mo, … – softpole CGS13, Cologne, August 25, 2008

  31. TSC data - 96Mo CGS13, Cologne, August 25, 2008

  32. TSC data - 96Mo CGS13, Cologne, August 25, 2008

  33. Pictures with comparison similar but correct statistical analysis excludes also this model at 99.8 % confidence levelKrticka et al., PRC 77 054319 (2008)  the enhancement is very weak if any analysis of data from DANCE confirm this TSC data - 96Mo CGS13, Cologne, August 25, 2008

  34. DANCE data (4p BaF2 detector) - 96Mo Exactly the same results were obtained from analysis of spectra with different multiplicities measured with the DANCE detectorSheets et al., submitted to PRC Strong enhancement No enhancement J p = 2+ J p = 2+ CGS13, Cologne, August 25, 2008

  35. Does an universal model of PSFs exist for all nuclei?NO Is the E1 PSF of Lorentzian shape? NO Is the decay dominated by E1 in the whole energy region? NO - at least in deformed nuclei Are the PSFs “smooth” or are there any resonance-like structures?There ARE structures Pigmy (small) resonances in nuclei Scissors mode in deformed nuclei Exists, but problems (strength, width) Low-energy mode in medium-weight (Fe, Mo,…) nuclei Resonances near Bn: Gold region Spherical N=82 nuclei PSFs below Bn CGS13, Cologne, August 25, 2008

  36. Does an universal model of PSFs exist for all nuclei?NO Is the E1 PSF of Lorentzian shape? NO Is the decay dominated by E1 in the whole energy region? NO - at least in deformed nuclei Are the PSFs “smooth” or are there any resonance-like structures?There ARE structures Pigmy (small) resonances in nuclei Scissors mode in deformed nuclei Exists, but problems (strength, width) Low-energy mode in medium-weight (Fe, Mo,…) nucleiWeak – if any Resonances near Bn: Gold region Spherical N=82 nuclei PSFs below Bn CGS13, Cologne, August 25, 2008

  37. Does an universal model of PSFs exist for all nuclei?NO Is the E1 PSF of Lorentzian shape? NO Is the decay dominated by E1 in the whole energy region? NO - at least in deformed nuclei Are the PSFs “smooth” or are there any resonance-like structures?There ARE structures Pigmy (small) resonances in nuclei Scissors mode in deformed nuclei Exists, but problems (strength, width) Low-energy mode in medium-weight (Fe, Mo) nucleiWeak – if any Resonances near Bn: Gold region Spherical N=82 nuclei PSFs below Bn CGS13, Cologne, August 25, 2008

  38. ? Pygmy resonance in 198Au data obtained before 1980 197Au(n,)198Au reaction Photonuclear data Isolated neutron resonances Fast n-capture, CTF Fast n-capture, BSFG Lorentzian GDR “Depressed” Lorentzian GDR Is there, indeed, an E1 pygmy resonance at 5.5 MeV? … or a mere deficiency and/or a redistribution of E1 photon strength? Redrawn from S. Joly, D. M. Drake and L. Nilsson, PRC 20 (1979) 2072 CGS13, Cologne, August 25, 2008

  39. No pygmy resonance postulated Pygmy resonance at 5.5 MeV Suppression of PSF below 5 MeV No difference in fits Pygmy resonance in 198Au revisited data from the Karlsruhe 4π BaF2 γ calorimeter … but postulating a pygmy resonance leads to too large total radiation width CGS13, Cologne, August 25, 2008

  40. Pygmy resonance in 198Au revisited CGS13, Cologne, August 25, 2008

  41. NRF in N = 82 nuclei A.Zilges et al., Phys. Lett. B542 (2002) 43 Profile of g beam CGS13, Cologne, August 25, 2008

  42. NRF in N = 82 nuclei A.Zilges et al., Phys. Lett. B542 (2002) 43 • Only G.S. strength • Relation actual vs. observed strength very complicated – decays to excited levels expected • Is the fragmentation of PSF valid here? CGS13, Cologne, August 25, 2008

  43. NRF experiment 1- many weak (unobservable) transitions – corrections are needed • relation actual GS vs. observed GS strength very complicated • some estimates can be made from measurement with “monoenergetic beam” (HIgS experiment)see talk of A.P.Tonchev G.S. (0+) CGS13, Cologne, August 25, 2008

  44. NRF in N = 82 nuclei Is there really a resonance? CGS13, Cologne, August 25, 2008

  45. Does an universal model of PSFs exist for all nuclei?NO Is the E1 PSF of Lorentzian shape? NO Is the decay dominated by E1 in the whole energy region? NO - at least in deformed nuclei Are the PSFs “smooth” or are there any resonance-like structures?There ARE structures Pigmy (small) resonances in nuclei Scissors mode in deformed nuclei Exists, but problems (strength, width) Low-energy mode in medium-weight (Fe, Mo,…) nucleiWeak – if any Resonances near Bn: Gold region Spherical N=82 nuclei PSFs below Bn CGS13, Cologne, August 25, 2008

  46. Does an universal model of PSFs exist for all nuclei?NO Is the E1 PSF of Lorentzian shape? NO Is the decay dominated by E1 in the whole energy region? NO - at least in deformed nuclei Are the PSFs “smooth” or are there any resonance-like structures?There ARE structures Pigmy (small) resonances in nuclei Scissors mode in deformed nuclei Exists, but problems (strength, width) Low-energy mode in medium-weight (Fe, Mo,…) nucleiWeak – if any Resonances near Bn: Gold region Change in a shape of a PSF Spherical N=82 nuclei PSFs below Bn CGS13, Cologne, August 25, 2008

  47. Does an universal model of PSFs exist for all nuclei?NO Is the E1 PSF of Lorentzian shape? NO Is the decay dominated by E1 in the whole energy region? NO - at least in deformed nuclei Are the PSFs “smooth” or are there any resonance-like structures?There ARE structures Pigmy (small) resonances in nuclei Scissors mode in deformed nuclei Exists, but problems (strength, width) Low-energy mode in medium-weight (Fe, Mo,…) nucleiWeak – if any Resonances near Bn: Gold region Change in a shape of a PSF Spherical N=82 nuclei Further study required Summary CGS13, Cologne, August 25, 2008

  48. Many thanks to F. Becvar(Charles University Prague) I. Tomandl, J. Honzatko(NPI Rez near Prague) F. Kappeler, F. Voss, K. Wisshak, M. Heil, R. Reifarth,…(FzK Karlsruhe) G.E. Mitchell(NCSU) U. Agvaanluvsan (LLNL, Stanford) M. Guttormsen, S. Siem, …(University of Oslo) … CGS13, Cologne, August 25, 2008

  49. CGS13, Cologne, August 25, 2008

  50. Simulations within statistical model I L L U S T R A T I O N Transitions to 21 + HIgS G.S. transitions in 138Ba By courtesy of A.P. Tonchev IGS/Iprim = 6(1) Eini = 8.4 MeV M1 E1 CGS13, Cologne, August 25, 2008

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