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

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

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

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

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

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

7. CGS13, Cologne, August 25, 2008 Photonuclear experiments photoabsorption - above particle threshold ( ,particle) only GS PSF can be obtained does not allow to distinguish among different transition types  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 B n ? From B.L. Berman, S.C. Fultz, Rev. Mod. Phys. 47 (1975) 713

8. CGS13, Cologne, August 25, 2008 PSFs below B n 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?

9. CGS13, Cologne, August 25, 2008 Spectra from 4  ball E1E1 E2E2 E3E3 E4E4

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

11. CGS13, Cologne, August 25, 2008 PSFs below B n 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?

12. CGS13, Cologne, August 25, 2008 PSFs below B n 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

13. CGS13, Cologne, August 25, 2008 PSFs below B n 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

14. CGS13, Cologne, August 25, 2008 PSFs below B n 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

15. CGS13, Cologne, August 25, 2008 PSFs below B n 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

16. CGS13, Cologne, August 25, 2008 PSFs below B n 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 B n : Gold region Spherical N=82 nuclei

17. CGS13, Cologne, August 25, 2008 PSFs below B n 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 B n : Gold region Spherical N=82 nuclei

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

19. CGS13, Cologne, August 25, 2008 Scissors mode - strength 0 2  B(M1)  (  N 2 ) 200 180 160140 A 4 Redrawn from J. Enders et al., PRC 59 R1851 (1999) Summing interval for  B(M1)  : 2.5 - 4.0 MeV Summing interval for  B(M1)  : 2.7- 3.7 MeV 164 Dy (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) Odd or odd-oddEven-even 6 Phenomenological sum rule: Lo Iudice, Richter, Phys. Lett. B304 193 (1993)

20. CGS13, Cologne, August 25, 2008 TSC spectra E  1 + E   HPGe detectors target beam of thermal neutrons E1E1 E2E2 capture of thermal neutrons known initial J   E1E1 E2E2

21. CGS13, Cologne, August 25, 2008 TSCs in the 162 Dy(n,γ) 163 Dy reaction SRs assumed to be built only on all levels below 2.5 MeV

22. CGS13, Cologne, August 25, 2008 TSCs in the 162 Dy(n,γ) 163 Dy reaction Scissors resonances assumed to be built on all 163 Dy levels

23. CGS13, Cologne, August 25, 2008 Scissors mode - strength 0 2  B(M1)  (  N 2 ) 200 180 160140 A 4 Redrawn from J. Enders et al., PRC 59 R1851 (1999) Summing interval for  B(M1)  : 2.5 - 4.0 MeV Summing interval for  B(M1)  : 2.7- 3.7 MeV 164 Dy (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) Odd or odd-oddEven-even 163 Dy (TSC) Value from TSCs in 163 Dy: total sum Σ B(M1)  Value from TSCs in 163 Dy; summing interval for  B(M1)  : 2.5 - 4.0 MeV 163 Dy (TSC) 6 Phenomenological sum rule: Lo Iudice, Richter, Phys. Lett. B304 193 (1993)

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

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

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

27. CGS13, Cologne, August 25, 2008 PSFs below B n 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 B n : Gold region Spherical N=82 nuclei

28. CGS13, Cologne, August 25, 2008 PSFs below B n 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 B n : Gold region Spherical N=82 nuclei

29. CGS13, Cologne, August 25, 2008 PSFs below B n 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 B n : Gold region Spherical N=82 nuclei

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

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

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

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

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

35. CGS13, Cologne, August 25, 2008 PSFs below B n 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 B n : Gold region Spherical N=82 nuclei

36. CGS13, Cologne, August 25, 2008 PSFs below B n 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 Weak – if any –Resonances near B n : Gold region Spherical N=82 nuclei

37. CGS13, Cologne, August 25, 2008 PSFs below B n 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 Weak – if any –Resonances near B n : Gold region Spherical N=82 nuclei

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

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

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

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

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

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

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

45. CGS13, Cologne, August 25, 2008 PSFs below B n 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 Weak – if any –Resonances near B n : Gold region Spherical N=82 nuclei

46. CGS13, Cologne, August 25, 2008 PSFs below B n 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 Weak – if any –Resonances near B n : Gold region Change in a shape of a PSF Spherical N=82 nuclei

47. CGS13, Cologne, August 25, 2008 Summary 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 Weak – if any –Resonances near B n : Gold region Change in a shape of a PSF Spherical N=82 nuclei Further study required

48. CGS13, Cologne, August 25, 2008 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) …

49. CGS13, Cologne, August 25, 2008

50. HI  S By courtesy of A.P. Tonchev E1 M1 G.S. transitions in 138 Ba Transitions to 2 1 + Simulations within statistical model I L L U S T R A T I O N I GS /I prim = 6(1) E ini = 8.4 MeV

51. CGS13, Cologne, August 25, 2008 P R E L I M I N A R Y 95 Mo – comparison with simulations (2 + )

52. CGS13, Cologne, August 25, 2008 P R E L I M I N A R Y 95 Mo – comparison with simulations (2 + ) BA

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

54. CGS13, Cologne, August 25, 2008 E γ (MeV) f (E1) (E γ ) +f (M1) (E γ ) (MeV) Data taken from E. Melby et al., PRC 63 (2001) 044309 NRF Scissors from (3He,  γ) and (3He,3He’γ)

55. CGS13, Cologne, August 25, 2008

56. Primaries from (n,  ) reaction Two possibilities: 1.measurement from a large number of isolated resonances (absolute value of PSF can be deduced) 2.average resonance capture (ARC) - simultaneous population of a large number of resonances by the capture of n’s with energy spread over a relatively wide interval (only relative PSFs can be deduced)  f(E1)/f(M1)  5-7 for A > 100

57. CGS13, Cologne, August 25, 2008 Inelastic proton scattering Inelastic scattering experiment with protons with E p  200 MeV at forward angles is sensitive to spin-flip M1 transitions experiments performed on rare-earth (deformed) nuclei

58. CGS13, Cologne, August 25, 2008 How is it with M1 strength @ 7 MeV? Problem with M1 strength  data from (n,  ) and (p,p’) for deformed rare-earth nuclei are not in a good agreement a) shape b)assuming the ratio E1/M1 from (n,  ) data and Lorentz shape of E1 PSF  the strength from (p,p’) is  3x lower than those deduced from (n,  ) f(E1)/f(M1)  5-7 for A > 100

59. CGS13, Cologne, August 25, 2008 Scissors mode - strength Counts per 1.2 keV Gamma-ray energy (keV) A. Nord et al., PRC 67, 034307 (2003) Spectrum of γ -rays scattered off 163 Dy Even if all 163 Dy transitions observed are M1 we get only  B(M1)  = 1.53  N 2  94 lines from 163 Dy  41 lines from 164 Dy  background lines  -ray energies 2.3-3.3 MeV:  line spacing of 7 keV Even if all 163 Dy transitions observed are M1 we get only  B(M1)  = 1.53  N 2 … while TSC 163 Dy data yield total  B(M1)  = 6.2  N 2 Probably a large number of unresolved lines here... Limits of NRF?

60. CGS13, Cologne, August 25, 2008 KMF+BA SR SF EGLO f(E ,T=0) (MeV -3 ) SP wrong PSF model in deformed nucleus  2 ( 152 Eu g.s. ) = 0.29

61. CGS13, Cologne, August 25, 2008 Pygmy resonance in 198 Au revisited data from the Karlsruhe 4 π BaF 2 γ calorimeter Enhancement of level density at E exc ≈ 1.0 MeV by a factor of 8 … a not satisfactory explanation of the observed effects

62. CGS13, Cologne, August 25, 2008 Pygmy resonance in 198 Au revisited TSC data from Řež No pygmy resonance postulated

63. CGS13, Cologne, August 25, 2008 Pygmy resonance in 198 Au revisited TSC data from Řež Pygmy resonance at 5.9 MeV

64. CGS13, Cologne, August 25, 2008 Pygmy resonance in 198 Au revisited TSC data from Řež Abrupt suppression of PSF below 5 MeV The best fit obtained – it does not seem that there is a pygmy resonance in 198 Au

65. CGS13, Cologne, August 25, 2008

66. Photonuclear experiments Classical ED: response of a point-like charge with damping to external electric dipole field is given by Lorentzian shape majority of photonuclear data confirms above neutron threshold but there exist nuclei violating this shape - puzzle  how is it below n threshold

67. CGS13, Cologne, August 25, 2008 Decay of excited states decay of low-lying levels (low density) is experimentally known (structure effects - calculations) decay of highly excited levels? is it possible to predict it? Solution in medium-weight and hevavy nuclei: –use of statistical approach –description via photon strength functions

68. CGS13, Cologne, August 25, 2008 D. Savran et al., PRL 95 (2005) 132501

69. CGS13, Cologne, August 25, 2008

70. Scissors mode - strength 0 2  B(M1)  (  N 2 ) 200 180 160140 A 4 Summing interval for  B(M1)  : 2.5 - 4.0 MeV Summing interval for  B(M1)  : 2.7- 3.7 MeV 164 Dy (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) 163 Dy (TSC) Value from TSCs in 163 Dy: total sum Σ B(M1)  Value from TSCs in 163 Dy; summing interval for  B(M1)  : 2.5 - 4.0 MeV 163 Dy (TSC) 6 Phenomenological sum rule: Lo Iudice, Richter, Phys. Lett. B304 193 (1993) Theory of E. Lipparini and S. Stringari, Phys. Rep. 175 (1989) 103 Redrawn from J. Enders et al., PRC 59 R1851 (1999)