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

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

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

Available differential data for 235 U 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);

Evaluation methods Two Watt spectra

Evaluation methods Scale method

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.

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?

All experimental data together Ratio to “scale method” evaluation

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

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

Different microscopic experiments were applied for measurement of PFNS 1.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). 2.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).

ENDF/B-7 and PNPI experiment

Comparison of IRMM and PNPI experiments

Scattering on the FF detector materials

Model of PNPI experiment

Integration over angles

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!

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.

The motivation of the LD parameter reduction

Weisskopf’s assumption

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

Spectrum from (p,n) reaction

Spectrum from 181 Ta(p,n) reaction

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!!!!!

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)

Model for Prompt Fission Neutron Emission N. Kornilov et al, ISINN-12, Dubna, 2004 N. Kornilov et al, NPA 786, 2007, 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 1.Neutron emission from excited, moving FF (full acceleration) 2.Total excitation energy U= U h +U l = Er-TKE 3.U h and U l from equilibrium (correction is possible) This model = LANL model (Weisskopf assumption)

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

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

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

ν(TKE)), S=(N-Z)/A? Experimental slope: 235 U; 19 MeV/n, S= Pu; 16 MeV/n, S= U; 13 MeV/n, S= Cf; 13 MeV/n, S=0.222 Calculated result ~10 MeV/n

Slope estimation Average is ~6 MeV. The ν~2.5 in this eq. If ~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.

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

What we can describe and it means what we understand? Experimental dataYesNo Absolute value and energy dependence Yes, for any isotopes Macroscopic dataNo Microscopic PFNS (total)No Angular and LR effectsNo Dependence of ν(A) 235 U?No CMS neutron energy e(A) No CMS spectra for selected ANo Dependence ν(TKE) !!!!!No

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;

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.

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 e ff ects of reabsorption of the unbound neutrons by the imaginary potential and of the simultaneous separation of the fragments has to be included.

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

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); 235 U spectrum should be measured relative to 252 Cf; 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.

The end

Total acceleration

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

Sharing of energy between fragments

Conclusion for future experimental efforts New experimental efforts are necessary to answer the following very important questions: what is 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.