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Evaluation of Neutron Emission Models in Prompt Fission Neutron Spectra of Various Nuclei

This study examines neutron emission from fission fragments (FF) using a model incorporating level density, absorption cross-section, and excitation energy. Key elements include the calculation of neutron spectra from (p,n) reactions across different nuclei, such as Zr, Ag, Cd, Sn, Ho, and Ta. Notably, the research challenges existing models' assumptions, particularly the Weisskopf assumption, highlighting discrepancies in neutron multiplicity predictions versus experimental data. The findings underscore the complexity of neutron emission mechanisms and call for refined models for accurate PFNS calculations.

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Evaluation of Neutron Emission Models in Prompt Fission Neutron Spectra of Various Nuclei

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  1. n n n CRP, 13-16 December 2011

  2. PFNS evaluation with “scale method” NS&E 169, 290-295 (2011)

  3. Model of PNPI experiment

  4. Results

  5. Model for Prompt Fission Neutron EmissionN. Kornilov et al, ISINN-12, Dubna, 2004N. Kornilov et al, NPA 786, 2007, 55-72 I added Neutron spectra calculation 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 This model = LANL model (Weisskopf assumption)

  6. 2D distribution TKE * A FF masses TKE-100, MeV

  7. Weisskopf assumption or LD?

  8. 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); Ep=11.2MeV (Zhuravlev et all, IPPE) • 181Ta(p,n); Ep=6, 7, 8, 9, 10 MeV (Lovchikova et all, IPPE)

  9. Ignatjuk’s Level Density is GOOD for (p,n) reaction!

  10. BUT Ignatjuk’s Level Density failed for PFNS235U, thermal <E>=1.85 MeV

  11. PFNS description contradict to experimental data from (p,n) reactions

  12. 235U neutron multiplicity versus FF mass

  13. Neutron energy in CMS

  14. Neutron spectrum in CMS

  15. Neutron multiplicity versus TKE

  16. 252Cf neutron multiplicity versus FF mass

  17. Self-checking of experimental data • Experimental data have been analysed with assumption: Neutron emission from excited, moving FF (full acceleration) We have: ν(A), <ε>(A), Y(A), TKE=>Ew(A) for LF and HF. So we can return back and calculate ν and <E> - average neutron energy in LS.

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

  19. ν(TKE) is a crucial point

  20. 235U “3 sources model” <Escn> = 2.38 MeV

  21. Intrinsic feature CMS => LS transformation

  22. MC simulation and experiment • There is NOT CMS=>LS transformation • E1/2 instead of E

  23. Conclusion • Theoretical model can not describe simultaneously numerous experimental data. So, this model is wrong(>90%) in main assumption; • Many neutrons are emitted along FF direction BUT not due to “evaporation from excited, moving fragments”; • 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; • After this we will create (may be) new model for PFNS calculation; • Until this we have in hand only “semi-empirical models” for practical application; • LS experimental spectra for different angles should be compared with model calculations directly.

  24. Intrinsic feature CMS => LS transformation E1/2 ε1/2 μ Ew1/2 μ – LS cosine ε>Ew(1-μ2)

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