The “Scission Neutron Emission” is last or first stage of nuclear fission?

1. The “Scission Neutron Emission” is last or first stage of nuclear fission? Nikolay Kornilov

2. Microscopic (differential) experiments (as a rule TOF) Macroscopic experiments Integral experiments Benchmark experiments Keff= 1 ± β Experiments

3. IRMM-2007

4. PFNS from microscopic experiments at thermal energy do not agree with integral data and can not reproduce Keff for benchmark experiments 1972, Islam and Knitter motivated new experiment "…though the results of the above measurements, characterized by the average fission neutron energy, <E>, agree reasonably with each other, a number of integral experiments shows higher values for <E> indicating a harder fission neutron spectrum than that given by differential measurements". 2003, Madland "…no calculated thermal spectrum has been found that simultaneously reproduces either of the two modern thermal differential measurement and the set of measured integral cross-sections to within an acceptable level” Problem during more than 50 years of the experimental efforts!

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

6. Available differential data 235U • Thermal point: Starostov et al (1983), 3 spectra, Lajtai et al (1985), Yufeng et al (1989), Kornilov et al (2008), 3 spectra; • 0.5 MeV: Trufanov (1994), Staples (1995), IRMM (2006-08) 8 spectra; • 1-5MeV: Boykov (1991, 2.9 MeV), Trufanov (1994, 5MeV), Staples (1995, 1.5, 2.5, 3.0MeV)

7. Sources of information

8. Angular distribution of SCN relative to FF E1-E2=0.5 - 6 MeV

9. Energy spectrum of SCN in LS

10. Non “direct” method for SCN estimation • Shape of the PFNS at thermal point requires the SCN incorporation (problem #1). χ2=3.5

11. 3 sources model

12. “3 sources model” spectrum and thermal data

13. Experimental results for 235U and 252Cf Vorob’ev (2007) et al estimated ~ 10% for 235U at thermal point

14. <Eth> = 2.031MeV ; <E05> = 2.045MeV IRMM(th) (2008) and Johansson (1977) 0.5MeV data What is happened at 0.5MeV? • The difference of the PFNS shape between thermal and 0.5 MeV input energy can not be predicted with existing theoretical model (problem #2).

15. What is happened at 0.5MeV (cont)?

16. What does mean angular effect? • Problem #3

17. What does mean left-right asymmetry?

18. Angular effect can be reproduced…. WHY share of SCN is changing……????

19. There is NO any idea to explain the difference between differential and integral data! (problem#4)

20. Integral data

21. Possible explanation ? • One may conclude that a factor exists which has a rather strong influence on the PFNS shape and asymmetry effects but was not fixed in experimental investigations at 0.5MeV input neutron energy • All experiments which results were used in the report were made with 7Li(p,n) reaction as a neutron source and pulsed mode. One may assume that this factor is the neutron polarization. • We should take into account the possible proton polarization also due to pulsed mode of the accelerators (chopper, bunching high voltages, analyzing and switching magnets). In the preparation stage of any PFNS experiment it was assumed that this factor is not important or by definition should be equal to zero. • If this explanation is true, the transmission mechanism of the information from the incident neutron to the secondary fission neutron should be found. The only possibility might be scission neutron emission, a fast process without formation of the compound nucleus. This may provide the link between the incident neutron and the secondary fission neutron. • So, for real clarification of this effect we need new experiments with polarized thermal neutron beam. When we will confirm and verify this effect new theoretical model should be developed. • The most difficult for understanding is the problem #4. There are not any realistic ideas for its solving. May be they will come after new experimental efforts mentioned above.

22. Neutron decay from excited states in the second minimum?

23. Conclusion • New experimental and theoretical efforts are necessary to answer the following very important questions: • whatis the mechanism of neutron emission in fission and fission process itself. ~40% of fission should be happened without COMPOUND nucleus formation; • why the shape of the prompt fission neutron spectrum may change so drastically. Thermal-0.5 MeV data, angular effect. SC Neutrons should be emitted at beginning stage of fission, in any case, the low energy component. We should construct the mechanism; • 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.

24. Team • F.-J. Hambsch, I. Fabry, S. Oberstedt EC-JRC-Institute for Reference Materials and Measurements, Retieseweg 111, B-2440 Geel, Belgium • T. Belgya, Z. Kis , L. Szentmiklosi Institute of Isotopes HAS, Dept. of Nuclear Research, Budapest, Hungary • S. Simakov Forschungszentrum Karlsruhe, Institut für Neutronenphysik und Reaktortechnik, D-76344 Eggenstein-Leopoldshafen, Germany