Investigation of the Prompt Neutron Emission in Low Energy Fission of 235 U

1. Investigation of the Prompt Neutron Emission in Low Energy Fission of 235U Vorobyev A.S., Shcherbakov O.A., Petrov G.A. Petersburg Nuclear Physics Institute. 188300, Gatchina, Leningrad district, Russia E-mail: alexander.vorobyev@pnpi.spb.ru

2. Motivation • Investigation of the prompt neutron emission mechanism. • Scope of the experimental data available for end-to-end analysis is limited by 1 experiment for 235U (Skarsvag et.al.(1963)) and 3 experiments for 252Cf (Bowman et.al.(1962), Seregina et.al.(1985), Budtz-Jorgensen et.al.(1988)). • To measure spectra of prompt fission neutrons from the 235U(nth,f) reaction at several angles relative to the light fragment direction to eliminate the absent of these data in literature. • From previous works: the contribution of scission neutrons to the total yield of PFN ranges from 1% to 20%, so even existence of scission neutrons can hardly be considered as proven. • The 235U is the most promising isotope for investigation as it was observed to provide the highest relative yield of supposed scission neutrons in previous experiments.

3. Methodology General features of the investigation of neutron emission mechanism: • It is well established that the main source of prompt neutrons is accelerated fission fragments ~90%; • The yield of neutrons with other emission mechanism is usually determined as difference between experimentally observable variables in the laboratory system and those calculated using known neutron spectra in the center-of-mass system; • As the prompt neutron energy spectra in the center-of-mass system of light and heavy fragments the Maxwellian or Weisskopf spectra are usually used. The parameters of these spectra are adjusted so as to show the best correlation with available experimental data. • The model of two fragments with average mass and kinetic energy is used frequently for calculation of neutron spectra in the laboratory system instead of real mass and kinetic energy distributions. Such substitution has a minor influence (3%) on the total neutron energy distribution. 3

4. Methodology D.G. Madland, IAEA Report INDC(NDS) – 251, Vienna, 1991, p. 201

5. Methodology Main features of calculation • The neutron spectra in the c.m.s of the fission fragments, nc.m.(Ec.m), are obtained using experimental data , nlab (En ,  lab), for 00, 180 and 360 angles relative to the fission direction free of any parameters derived from other experiments; no dependence on spectrum shape; • Using equations (two fragments approximation): nlab (En ,  lab) = (En/ Ec.m.) 1/2 φ(Ec.m. , c.m.) nc.m.(Ec.m) , Ec.m. = En + Ef - 2  cos( lab )  (EnEf) 1/2, Ef = TKE [ 1/mf – 1/A ], (Ec.m.) 1/2 cos(cm ) = (En) 1/2 cos( lab ) – (Ef ) 1/2; • Since the fragments have the angular momenta normal to the fragment direction the angular distribution of prompt neutrons in the center-of-mass system of fission fragment may be given by: φ(Ec.m.,  c.m. ) = 1 + A2Ec.m. (3 cos2( c.m. ) - 1) / 2 , where A2 = (1 - φ(1,900) / φ(1,00) )  0. 5

6. 8 1 7 2 6 3 5 4 4 5 3 6 2 7 1 8 Schematic view of the experimental set-up Reaction Chamber: 235U target(Ø15mm) – 280 μg/сm2 UF4 onto 70 μg/сm2 Ti backing; start MWPD(68 x 92 mm2) located within 7 mm range from the 235U target; stop MWPD(72 x 38 mm2) located at a distance of 140 mm from the chamber axis. Neutron detectors: stilbene crystals (50 x 50 mm2 and 40 x 60 mm2 mounted on the Hamamatsu - R6091) neutron registration threshold –150  200 keV; double-discrimination method – pulse shape and time-of-flight criteria time-of-flight distancefrom 235U target – ~ 50 cm

7. Raw experimental data: counts rate of fission fragments from different fragment detectors Number of registered fission events as a function of MWPDs pulse timing delay from both ends of Arc N1

8. Raw experimental data: fission fragments time-of-flight (a) fission fragments time-of-flight spectrum detected by second MWPD of Arc N1 (wasn’t shaded by start MWPD) (b) number of fragments as a function of TOF difference for fragments registered by two opposite detectors of Arc N1 and N2

9. Raw experimental data: neutron -  - quanta separation method Both integrals were measured for pulse of neutron detector in a time window of 300 nsec, while the partial integral window – with a delay ~30 nsec.

10. Raw experimental data:total prompt neutron time-of flight spectrum

11. Measurement of the Total Prompt Neutron Spectrum of 235U(nth, f) Relative to 252Cf(sf)(neutron detector efficiency determination) 252Cf target placed into the experimental set-up in place of 235U

12. Analysisfirst approximation where  is the angle between the neutron direction and the direction of motion of the light fragments, of T is the flight time in nanoseconds, c is the velocity of light, l is the flight path between source and the detector; is linear interpolation of the 252Cf prompt neutron spectrum evaluation (C.W.REICH, W MANNHART, T ENGLAD – ENDF-B/VII). 12

13. Analysisdegree of reliability – experimental errors determination • For 11 fixed angles between the neutron and light fragment direction (from 00 to 1800 in 180 interval) the prompt neutron spectra were obtained independently for two neutron detectors as weighted averages of 4 measurement cycles; • Each measurement cycle was analyzed separately • The errors of 235U spectra for fixed angle neutron emission relative to the light fragment direction are the RMS deviation from weighted means. These errors include the possible instability of electronic (uncertainties of neutron threshold determination …), the statistical and energy determination uncertainties as well as for total prompt neutron spectrum uncertainties due to the fact that in the measurements we have the experimental histogram distributions instead of continuous distributions. • Result is a weighted average of two neutron spectra obtained by individual detectors. • The errors of 252Cf spectra measurements were obtained as in the case with 235U . 13

14. Total prompt neutron spectrafirst approximation • There is a good agreement between spectra obtained by two individual neutron detectors

15. Total prompt neutron spectra first approximation • There is a good agreement between spectra obtained by two individual neutron detectors

16. Analysissecond approximation – experimental resolution was taken into account 16

17. Analysissecond approximation: neutron energy resolution Resolution function is supposed to be Gaussian in every energy point.

18. Analysissecond approximation: neutron energy resolution correction Ratio of corrected to non-corrected neutron energy spectra. 252Cf 235U

19. Analysissecond approximation: angular resolution function calculated using the real dimensions of fragment and neutron detectors

20. Analysissecond approximation: angular resolution correction Ratio of corrected to non-corrected neutron energy spectra. 252Cf 235U

21. Analysissecond approximation: total resolution corrections

22. Analysisdegree of reliability – experimental and systematic errors

23. Results: total prompt neutron spectrum of 235U(ntn,f) comparison to literature data Present experiment : <  > = 2.44 ± 0.05 ENDF/B-VII: <  > = 2.421 The obtained PFNS agrees with literature experimental data in full energy range 23

24. Results: total prompt neutron spectrum of 235U(ntn,f) comparison to evaluation Method 1 – summation over angles; Method 2 – calculated in a framework of neutron emission from accelerated fragments using the c.m.s. spectra for light and heavy fragments obtained from experimental spectra measured at small angles relative to fragment direction in the lab. system.

25. Results: total prompt neutron spectrum of 235U(ntn,f) Conclusion: • Our PFNS agrees with differential experimental data in full energy range. • Results of two different measurements are in a good agreement. • The average of two measurements is in a good agreement ENDF/B-VII.

26. Angle and Energy distributions of the Prompt Fission Neutrons of 235U(n, f)

27. Analysis of the data Applied corrections for **: • time uncertainties in TOF measurements; • neutron detector background (a double-discrimination method, true coincidence subtracted and the linear approximation of the remain part of background); • fission fragment detector efficiency; • complementary fission fragment contribution; • angular and neutron energy resolution; • neutron detector efficiency determined as the ratio of the measured total neutron spectrum of 252Cf to the reference standard spectrum; • normalization to the average fission neutron multiplicity of 235U recommended by ENDF/B-VII; ** - Measurements of angular and energy distributions of prompt neutrons from thermal neutron-induced fission A.S. Vorobyev, O.A. Shcherbakov, Yu.S. Pleva, A.M. Gagarski, G.V. Val’ski, G.A. Petrov, V.I. Petrova, T.A. Zavarukhina, NIM A598 (2009) 795 27

28. Analysis of the data Calculation procedure • At the first step, the neutron energy spectra in c.m.s are calculated on the assumption that neutrons registered at fixed angles relative to light fragment direction were emitted solely by the light and heavy fragments, respectively; • At the second step, the neutron contribution to the complementary fragment is subtracted and the energy spectra for these angles in the laboratory system are obtained; • Further, using these energy spectra in the laboratory system, the neutron energy spectra for light and heavy fragments are obtained in the center-of-mass system; • Finally, the spectra obtained in the center-of-mass system are used for calculation of neutron angular and energy distributions in the laboratory system. 28

29. Results U-235:yield of prompt neutrons as a function of angle relative to the direction of light fission fragment in the lab. system

30. Results U-235:angular distribution of the average prompt neutron emission energy in the lab. system

31. Results U-235:total prompt neutron spectra in the laboratory system

32. Conclusion • The angular and energy distributions of the prompt fission neutrons of 235U(nth,f) have been measured in neutron energy range 0.2 – 12 MeV. • Total prompt fission neutron spectrum of 235U is in a good agreement with ENDF/B-VII. • Comparison of experimentally obtained angular and energy distributions of prompt neutron for 235U and calculated ones on the base of neutron evaporation from accelerated fragments enables: to estimate the contribution of “scission” neutrons as not to exceed 5% of total neutron yieldin an assumption of isotropic evaporation in the laboratory system; • to conclude that the angular anisotropy of the neutron emission in the fragment center-of–mass system, which is alike to 1 + 0.06Ec.m.cos2( c.m.),should be included into any calculation of prompt neutron properties in the nuclear fission • Now we are doing the same analysis of the measured angle-energy distributions of 233U(nth,f). • In future we are planning to carry out the same experiment for 239Pu(nth,f).

33. Thank you very much for your attention