The two-step g cascade method as a tool for studying g -ray strength functions

1. The two-step g cascade method as a tool for studying g-ray strength functions Milan Krtička

2. Outline • The method of two-step γ-cascades following thermal neutron capture (setup at Rez near Prague) • Data processing - DICEBOX code • Examples

3. The method of two-step γ-cascades following TNC Geometry: Three-parametric, list-mode Data acquisition: - Energy Eg1 - Energy Eg2 - Detection-time difference

5. Spectrum of energy sums Bn-Ef Energy sum Eγ1+ Eγ2 TSC spectra Accumulation of the TSC spectrum from, say, detector #1: The contents of the bin, belonging to the energy E1, is incremented by q =ij, where ijis given by the position and the size of the corresponding window in the 2D space “detection time”דenergy sum E1+E2”.  background-free spectrum i=-1 i=0 i=+1 j=-1 Detection-time difference Time response function j=0 j=+1 List-mode data

6. Spectrum of energy sums Bn-Ef TSC spectrum - taken from only one of the detectors Energy sum Eγ1+ Eγ2 X 5 TSC spectra i=-1 i=0 i=+1 j=-1 Detection-time difference Time response function j=0 j=+1 List-mode data

7. Example of sum-energy spectra (57Fe)

8. Example of a TSC spectrum Dynamic range 1:1000

9. Example of a TSC spectrum

10. Normalization of experimental spectra 5 % • Knowing intensity of one g-ray cascade TSC intensities to all final levels can be normalized • Corrections to angular correlation and vetoing must be done 20 %

11. How to process data from this experiment? • Result of interplay of level density and g-ray SF • Comparison with predictions from decay governed by different level density formulas and g-ray strength functions • Code DICEBOX is used for making these simulations • Simulates gamma decay of a compound nucleus within extreme statistical model

12. Main assumptions: For nuclear levels below certain “critical energy” spin, parity and decay properties are known from experiments Energies, spins and parities of the remaining levels are assumed to be a random discretization of an a priori known level-density formula A partial radiation width if(XL), characterizing a decay of a level i to a level f, is a random realization of a chi-square-distributed quantity the expectation value of which is equal to f(XL)(Eγ) Eγ2L+1/(Ei), where f (XL) and ρ are also a priori known Selection rules governing the  decay are fully observed Any pair of partial radiation widths if(XL) is statistically uncorrelated Simulation of  cascades - DICEBOX algorithm

13. Simulation of the decay: “nuclear realization” (106 levels Ţ 1012Glgf) Ţ “precursors” are introduced fluctuations originating from nuclear realizations cannot be suppressed Modellingwithin ESM Deterministic character of random number generators is exploited Outcomes from modelling are compared with experimental data

14. Main feature of DICEBOX • There exists infinite number of artificial nuclei (nuclear realizations), obtained with the same set of level density and g-ray SFs models that differ in exact number of levels and intensities of transitions between each pair of them  leads to different predictions from different nuclear realizations • DICEBOX allows us to treat predictions from different nuclear realizations • The size of fluctuations from different nuclear realizations depend on the (observable) quantity - in our case intensity of TSC cascades - and nucleus • Due to fluctuations only “integral” quantities can be compared • Simulation of detector response must be applied

15. Results of GEANT3 simulations - 95Mo(n,g)96Mo

16. DEg = 2 MeV Integrated TSC spectra Example of a TSC spectrum Wide-bin TSC spectra

17. Examples of spectra (96Mo) Integrated TSC Simulation Experiment

18. distribution of values from 1400 nuclear realizations Some features of TSC spectra (1) Problems with presentation of results 100 nuclear realizations 2235 keV

19. Some features of TSC spectra (2)

20. And some results

21. Řež experimental data πf = + DICEBOX Simulations πf = - TSCs in the 162Dy(n,γ)163Dy reaction 1/2 + M1 E1 Entire absence of SRs is assumed

22. TSCs in the 162Dy(n,γ)163Dy reaction A “pygmy E1 resonance” with energy of 3 MeV assumed to be built on all levels

23. TSCs in the 162Dy(n,γ)163Dy reaction SRs assumed to be built only on all levels below 2.5 MeV

24. TSCs in the 162Dy(n,γ)163Dy reaction Scissors resonances assumedto be built on all163Dy levels

25. 163Dy:models for photon strength function used g-ray strength functions plotted refer to the  transitions to the ground state of 163Dy The role of E1 transitions to or from the ground state is reduced f(E,T=0) (MeV-3) EGLO SR SF KMF+BA

26. TSCs in the167Er(n,γ)168Er reaction Entire absence of scissors resonances is assumed

27. TSCs in the167Er(n,γ)168Er reaction Scissors resonances assumedto be built on all168Er levels

28. Enhanced PSF at low energies - 96Mo

29. Enhanced PSF at low energies - 96Mo

30. Enhanced PSF at low energies - 96Mo

31. Enhanced PSF at low energies - 96Mo 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

32. Pygmy resonance in 198Au revisited

33. Pygmy resonance in 198Au revisited No pygmy resonance postulated

34. Pygmy resonance in 198Au revisited Pygmy resonance at 5.9 MeV

35. Pygmy resonance in 198Au revisited Abrupt suppression of PSF below 5 MeV The best fit obtained – it does not seem that there is a pygmy resonance in 198Au

36. PSF used Exactly the same fit describes perfectly also data obtained with DANCE 4pdetector

37. Eg1 Eg2 Eg3 Eg4 Spectra from 4p ball (DANCE, n_TOF, …)

38. 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 4 BaF2 calorimeter … but postulating a pygmy resonance leads to too large total radiation width

40. Measurement of TSC cascades provides valuable information on g-ray strength functions DICEBOX simulations can be used for obtaining information of g-ray strength functions from many experiments Special thanks to: F. Becvar Charles University, Prague, Czech Republic I. Tomandl, J. Honzatko Nuclear Physics Institute, Rez, Czech Republic G. MitchellNCSU Conclusions

41. Thank you for your attention