N. Kornilov, CRP-PFNS, Vienna, 21-25 Oct. 2013

1. Open problems for neutron emission in fission N. Kornilov, CRP-PFNS, Vienna, 21-25 Oct. 2013

2. ENDF/B-7 Keff=1 ????????

3. EXPERIMENT 2009. I.Guseva, et al, ISINN-16, 370,2009, EXFOR41516; (PNPI) 2010.N.Kornilov, F.-J. Hambsch, et al, NSE, 165(1), 2010; NSE 169, 2011; (IRMM) EVALUATION Can predict macroscopic experiment (Mac-data) 2006. ENDF/B-7. “Theoretical” Madland-Nix (LANL) model with adjusted parameters. Cannot predict Mac-data 1999. N.V.Kornilov, A.B.Kagalenko, F.-J. Hambsch, Physics of Atomic Nuclei, 62(2), Semi-empirical “two Watt” model 2010. N.V.Kornilov, NS&E, 169, 2011,290, Semi-empirical “scale method” 2013. V.G. Pronyaev, GMA code for data evaluation. Covariance was included. What was happened recently? (10-15years)

4. Available differential data for 235U • Thermal point: • Starostov et al (1983), 3 spectra, • Lajtai et al (1985), • Yufeng et al (1989), • Kornilov et al (2010), 3 spectra, • Vorobyev et al (2009);

5. Evaluation methods • Two Watt spectra

6. Evaluation methods • Scale method

7. Evaluation methods • Evaluation with GMA approach, (2013) • GMA approach to the evaluation of the standards • GMA: generalized least-squares fit of experimental data developed by Wolfgang Poenitz for standard neutron reaction cross sections evaluation; • Non-model fit: no physical or mathematical model used in the fit; • Parameters of the fit are cross sections in the energy nodes (or groups) and normalization constants.

8. Are microscopic data (Mic-data) wrong? Criteria!? Only one conclusion we should made: are experimental data measured in different laboratory, different time, and so on…. in agreement or not!!!! So they are true OR not? What we have for PFNS at thermal point?

9. All experimental data together Ratio to “scale method” evaluation

10. All evaluations together Obvious conclusion: • ENDF/B-7 evaluation contradict to Mic data! • This conclusion dos not depend on method used for evaluation!

11. Mic-Mac problem Average cross sections: Open points - 252Cf ; Mannhart. Solid point - 235U; Scale method

12. Different microscopic experiments were applied for measurement of PFNS • Total Fission Fragment integrated experiment. In this type of experiment all fission fragment are integrated over TKE, masses, and emission angle relative to neutron detector. We should avoid any possible selection which may destroy PFNS shape (IRMM). • Differential Fission Fragment experiment. In this type of experiment PFNS are measuring relative to fixed direction of FF . The total PFNS may be calculated with integration of measured angular distributions over emission angle (PNPI).

13. ENDF/B-7 and PNPI experiment

14. ENDF/B-7 and PNPI experiment

15. Comparison of IRMM and PNPI experiments

16. Scattering on the FF detector materials

17. Model of PNPI experiment

18. Integration over angles

19. Differential experiment (conclusion) • So, Differential Experiments (DE) may contain systematical distortion effects! • Results of these experiments after proper corrections should be verified with total integral experiments and evaluations. • ONLY these experiments (DE) gives us important information about fission mechanism!

20. Madland-Nix model • Two FF or realistic distribution versus FF masses was included; • Triangle “temperature distribution” was assumed to simulate wide spread of excitation energy according to TKE distribution, and multiple neutron emission; • Optical model for absorption cross section; • Selection of the slope for Level Density parameter a=A/c; • Constant temperature assumption (Weisskopf type) for spectrum shape in CMS. Assumptions 3 and 4 are weekest points.

21. The motivation of the LD parameter reduction

22. Weisskopf’s assumption

23. Neutron spectra from (p,n) reactions • 94Zr(p,n); Ep=8, 11 MeV (Zhuravlev et all, IPPE) • 109Ag(p,n); Ep=7, 8, 9,10 MeV (Lovchikova et all, IPPE) • 113Cd(p,n); Ep=7, 8, 9,10 MeV (Lovchikova et all, IPPE) • 124Sn(p,n); Ep=10.2, 11.2 MeV (Zhuravlev et all, IPPE) • 165Ho(p,n),181Ta; Ep=11.2MeV (Zhuravlev et all, IPPE) • 181Ta(p,n); Ep=6, 7, 8, 9, 10 MeV (Lovchikova et all, IPPE) • 103Rh(p,n), 104-106,108,110Pd(p,n), 107,109Ag(p,n), Ep=18, 22, 25 MeV (Grimes et al, LLNL) • 51V(p,n), Ep=18, 22, 24, 26 MeV (Grimes et al, LLNL) • 159Tb(p,n), 169Tm(p,n), Ep=25 MeV (Grimes et al, LLNL) • 92-100Mo(p,n), Ep=25.6 MeV (Mordhorst et al, Un Hamburg)

24. Spectrum from (p,n) reaction

25. Spectrum from 181Ta(p,n) reaction

26. Madland-Nix model (conclusion) Madland-Nix model (LANL model) is semi-empirical model; Parameters of this model were selected to describe macroscopic results. The PFNS shape predicted with this model does not agree with microscopic experimental data. So, “Mic-Mac problem” was not solved till now!!!!!

27. Traditional assumption • Main assumptions for modeling of neutron emission in fission: 1. formation of compound and decay to Fission Fragment; 2. neutron emission from excited FF after total acceleration • Experimental data analysis: Neutron energy distributions measured in Laboratory System LS are transformed to CMS. These data are described by equation with fitted parameters λ,T. After this the data return back to LS with following conclusion about reliability of main assumption. It seems this procedure may provide misunderstanding. Model result should be compared with experiment in LS PFNS ν(TKE) ν(A) ν(μ,E) ν(μ)

28. Model for Prompt Fission Neutron EmissionN. Kornilov et al, ISINN-12, Dubna, 2004N. Kornilov et al, NPA 786, 2007, 55-72 Neutron spectra for selected fission parameters are available now Input data • Y(A,TKE) • Level density. (Level density model should be applied to extrapolate into FF mass range) • Absorption cross section (optical model) • Energy release and binding energies (G.Audi and A.H.Wapstra) Assumptions • Neutron emission from excited, moving FF (full acceleration) • Total excitation energy U= Uh +Ul = Er-TKE • Uh and Ul from equilibrium (correction is possible) This model = LANL model (Weisskopf assumption)

29. Experimental and calculated data (PFNS) 235U(th) 252Cf(sf)

30. Experimental and calculated data (ν(A)) 235U(th) 252Cf(sf)

31. Experimental and calculated data (ν(TKE)) 235U(th) 252Cf(sf)

32. ν(TKE)), S=(N-Z)/A? • Experimental slope: 235U; 19 MeV/n, S=0.217 239Pu; 16 MeV/n, S=0.213 233U; 13 MeV/n, S=0.210 252Cf; 13 MeV/n, S=0.222 • Calculated result ~10 MeV/n

33. Slope estimation Average <Bn> is ~6 MeV. The ν~2.5 in this eq. If <ε>~1.5 one may estimate the slope in eq.1, dU/dν~9 MeV. So, we can explain what does mean value estimated with detail calculation in the model.

34. Experimental and calculated data (ν(μ)) 235U(th) 252Cf(sf)

35. What we can describe and it means what we understand?

36. Conclusion 1 • Theoretical model can not describe simultaneously numerous experimental data. So, this model is wrong in main assumption; • ν(TKE) is the crucial point. May be if we will explain this huge slope (~19 MeV/n instead of ~9 MeV/n) we will understand the mechanism of neutrons emission in fission; • It seems that some of fissions happened due to simultaneous emission several particles (2 FFs and neutron(s)), providing continuous energy distribution;

37. Conclusion 2 • Until detailed understanding of mechanism of neutron emission in fission we have in hand only “semi-empirical models” for practical application; • Contradiction between microscopic and macroscopic data (Mic-Mac problem) is still exist. May be this connected with energy-angular selection. So, we should spend more effort to investigate the influence of complicate nature of the neutron emission on macro-results. • New experimental and theoretical efforts are extremely necessary to clarify the problem, to suggest new model, and to formulate new experiments for its investigation.

38. N.Corjan model. • IV. CONCLUSIONS • During the neck rupture neutrons are released (become unbound) due to the non-adiabaticity of this process. They leave the fissioning system during the next few 10E−21 sec after scisssion, i.e., during the acceleration of the fission fragments. Even if the neutrons are released predominantly in the inter fragment region, they do not move perpendicular to the fission axis but instead they are focused (by the fragments) along the fission axis. This feature is unexpected. The resulting angular distribution of these neutrons with respect to the fission axis resembles with the experimental data for all prompt neutrons. This re-opens the 50 years old debate on the origin of the fission neutrons. For a quantitative comparison the effects of reabsorption of the unbound neutrons by the imaginary potential and of the simultaneous separation of the fragments has to be included.

39. N.Corjan model. T=40E-22 s

40. Request for future total PFNS experiment • All fission fragments should be integrated over angle, masses, and TKE. The efficiency of FF counting should be ~1 (as close as possible); • Mass of fission chamber for fission counting should be reduced (as small as possible); • 235U spectrum should be measured relative to 252Cf; • Cf-source should be placed in the same chamber, and provide similar count rate; • Time resolution <2ns (FWHM), and flight path ~3m; • Shielding neutron detector to reduce counting of re-scattering neutrons (room neutrons); • The scattering on the FF counter material should be simulated taking into account angular-energy selection effect.

41. The end

42. Total acceleration

43. 2D distribution TKE * A FF masses TKE-100, MeV

44. Sharing of energy between fragments

45. Conclusion for future experimental efforts • New experimental efforts are necessary to answer the following very important questions: • whatis the nature of the “angular effect”, why the shape of the prompt fission neutron spectrum may change so drastically, • what is the physical reason responsible for the formation of a more energetic spectrum in the integral experiments in comparison with microscopic data, and • what is happening inside nuclear reactors.