1. Measurements of Angular and Energy Distributions of Prompt Neutron Emission from Thermal Induced Fission Vorobyev A.S., Shcherbakov O.A., Gagarski A.M., Pleva Yu.S., Val’ski G.V., Petrov G.A., Petrova V.I., Zavarukhina T.A. Petersburg Nuclear Physics Institute. 188300, Gatchina, Leningrad district, Russia E-mail: alexander.vorobyev@pnpi.spb.ru

2. 2 Motivation The investigations of the fission neutron angular and energy distribution relative to the fragment direction depending on mass split and fragment kinetic energy gives possibility to estimate the yield of neutrons with the other formation nature than evaporation from fully accelerated fragments. Because of such (“scission”) neutrons are generate near the scission point and don’t undergo Coulomb forces the research of their behavior allows to obtain an unique information about the neutron emission mechanism and the fission process itself. Present estimations of “scission” neutron yield from experimental data exist only for 235 U: 10 - 15 % of total neutron yield 252 Cf: 3 - 25 % of total neutron yield. Scope of the experimental data available for end-to-end analysis is limited by 1 experiment for 235 U (Skarsvag et.al.(1963)) and 3 experiments for 252 Cf (Bowman et.al.(1962), Seregina et.al.(1985), Budtz-Jorgensen et.al.(1988)). For a start we selected 235 U as the object for investigation since from the experiments performed earlier and systematic of light charge particle yield in ternary fission it should be expected to obtain the highest relative yield of “scission” neutrons exactly for that nucleus.

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

4. 4 Raw experimental data: position spectrum of the fission fragments Number of registered fission events as a function of MWPDs pulse timing delay from both ends of Arc N1

5. 5 Raw experimental data: fission fragments time-of-flight (a) fission fragments time-of-flight spectrum detected by 2 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

6. 6 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.

7. 7 Raw experimental data: total prompt neutron time-of flight spectrum

8. 8 Results (all registered events): prompt neutron spectra in the laboratory system red points – measured neutron yield after corrections for neutron detector background, angular resolution of fragment detectors, neutron registration efficiency and not full separation of the light and heavy fragment groups blue points – calculated contribution from complementary fission fragment

9. 9 Results (all registered events): ratio of the prompt neutron spectrum from fission fragments in the center-of-mass system to the Maxwellian spectrum LANL model: neutrons are evaporated by fully accelerated fragments; average velocities and masses of light and heavy fragments are used in calculation; the cross section for the inverse process of compound-nucleus formation is constant. 2.3

10. 10 Results (all registered events): yield of prompt neutrons as a function of angle relative to the direction of light fission fragment in the lab. system angular distribution of prompt neutrons in the center-of-mass system of fragment should be given by (if the fragments have angular momenta normal to the fragment direction) φ(E c.m.,  c.m. ) = 1 + A 2  E c.m.  (3  cos 2 (  c.m. ) - 1) / 2 the parameter A 2  0 defines a value of the angular anisotropy

11. 11 Results (all registered events): angular distribution of the average prompt neutron emission energy in the lab. system

12. 12 Results (all registered events): ratio of the prompt neutron yields at 0 0 and 90 0 (180 0 and 90 0 ) as a function of energy in the lab. system [MeV]

13. 13 Results (all registered events): total prompt neutron spectra in the laboratory system

14. 14 Results (coincident fission fragments): average prompt neutron multiplicity vs fragment mass

15. 15 Results (coincident fission fragments): average prompt neutron multiplicity vs TKE

16. 16 Conclusion The prompt neutron angle-energy distribution has been measured for thermal - neutron induced fission of 235 U. Comparison of this distribution measured and calculated on the base of neutron evaporation from fully accelerated fragments enables to estimate the contribution of “scission” neutrons as about 5% of total neutron yield in an assumption of isotropic evaporation in the laboratory system. For angles ~ 30 0 and ~ 150 0 a model calculation gives overestimated values of fission neutron yield as compared with the experiment. Introduction of anisotropy (A 2 = 0.04) into the model calculation eliminates this discrepancy but leads to an increase of “scission” neutron yield to about 8% of total neutron yield. Now we are doing more careful analysis of the obtained angle- energy distribution which includes using the mass-energy distribution of fission fragments instead of average values. In future we are planning to carry out the same experiment for 233 U(n th, f).

17. 17 Thank you very much for your attention