1 / 33

STUDIES OF FISSION DYNAMICS THROUGH THE SEARCH O F SCISSION NEUTRONS

Skt. Petersburg Nuclear Physics Institute of RAS. STUDIES OF FISSION DYNAMICS THROUGH THE SEARCH O F SCISSION NEUTRONS. CONTENT. INTRODUCTION SEARCH FOR “SCISSION RADIATIONS” AND INVESTIGATIONS OF THEIR CHARACTERISTICS

keiko-suarez
Download Presentation

STUDIES OF FISSION DYNAMICS THROUGH THE SEARCH O F SCISSION NEUTRONS

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Skt. Petersburg Nuclear Physics Institute of RAS STUDIES OF FISSION DYNAMICS THROUGH THE SEARCH OF SCISSION NEUTRONS "The scission process: The last stage of nuclear fission"

  2. CONTENT • INTRODUCTION • SEARCH FOR “SCISSION RADIATIONS” AND INVESTIGATIONS OF THEIR CHARACTERISTICS • - Scission neutrons emitted near the rupture point - Scission gamma-radiation emitted near the rupture point • ESTIMATES OF SCISSION NEUTRONS YIELDS FROM 233,235U(n,f) 239Pu(n,f) AND 252Cf(s,f) REACTIONS • - Main results of the neutron energy and angular distributions measurements for different fragment energies and masses in 233,235U(n,f) reactions - Main results of the (n-n)-coincidence measurements in 233,235U(n,f), 239Pu(n,f) and 252Cf(s,f) reactions • 4. TRI- AND ROT-EFFECTS OF THE LIGHT CHARGED and NEUTRAL PARTICLE EMISSION ASYMMETRIES • - general mechanisms of the T-odd asymmetry effects appearance in ternary and binary fission of polarized heavy nucleus - first results of the effects investigations for the fast neutrons and gamma-rays in 233,235U(n,f) fission "The scission process: The last stage of nuclear fission"

  3. Qualitative pictures of the low excitation energy fission Excitation energy Potential energy TOTAL ENERGY Kinetic energy R, fm Rupture point, 20 fm "The scission process: The last stage of nuclear fission"

  4. EXISTING SITUATION WITH SCISSION NEUTRON YIELDS IN FISSION • From general point of view one may expect relatively high probability of scission neutron emission near the time of fissioning nuclei rupture (no Coulomb barrier!) • In1962 R. Fuller presented the first estimate of neutron emission as a result of the fast nonadiabatic change of nuclear potential in the rupture process (about 0.4 neutron at t ~ 1,5 10-21 sec) • 3.. Prompt neutron emission in non-adiabatic passage from the barrier top to the scission • point (“pre-scission neutrons” ~10-21s. ) R. Fuller (1962). J. Boneh (1978). ( Well known • effect in heavy ion induced fission, but smaller ~ 1% if  > 10-20 s.) • Prompt neutron emission in rupture point (time scale – a few of ~10-22 s)J. Negele • (1982) • Instantaneous neutron emission as a result of the neck remnant “snatching” (Catapult • ” mechanism of neutron emission. (Time scale – about ~10-22s.).K.Dietrich • (1981), Madler (1985). • G.Val’ski (2002) under statistical consideration of light particle emission in ternary fission and using interpolation method had obtained estimate about 0.55(9)1/ffor the case of 235U(n,f) and 0.18(4)1/ffor 252Cf spontaneous fission. • Prompt neutron emission from highly excited fission fragments (up to 90% of total neutron yield. "The scission process: The last stage of nuclear fission"

  5. EXPERIMENTAL RESULTS OF SCISSION NEUTRON SEARCH "The scission process: The last stage of nuclear fission"

  6. BASIC REASONS AND PECULIARITIES OF OUR INVESTIGATIONS Because of strong scattering of available experimental data about scission neutrons existence and their properties we concentrated our attention only on the scission neutron yields and on general type of their energy spectra. As this takes place: 1.We tried to use all known methods of scission neutron observation on the existed background of fast neutrons evaporated from the excited fission fragments, namely: - correlation measurements of neutron energy and angular distributions for different masses and energies of fission fragments - angular correlations of (n-n)-coincidences for different neutron energy thresholds under conditions of integration over all the other features of neutron emission - new TRI and ROT-effects of T-odd emission asymmetry recently observed for the light charged particles in ternary fission 2. We plan to perform, where possible, such types of investigations for the fission reactions 252Cf(s.f.), 233,235U(n,f), and 239Pu(n,f) in the same experimental conditions. 3. Under the experimental data analysis and evaluation we used the following main suppositions: - taking into account very small probability of cascade emission we supposed Weiskopf energy spectra for scission neutrons isotropic emitted in CMS, - we have taken into account the big fragment angular momenta oriented relatively fission axis, - we have neglected possibility of neutron evaporation during fragments acceleration. "The scission process: The last stage of nuclear fission"

  7. 8 1 7 2 6 3 5 4 4 5 3 6 2 7 1 8 SCHEMATIC VIEW OF EXPERIMENTAL SET-UP Reaction Chamber: 235U target(Ø15mm) – 280 μg/сm2 UF4 onto 70 μg/сm2 Ti backing; start MWPC: (68 x 92 mm2) located within 7 mm range from the 235U target; stop MWPC: (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 distance: from 235U target – ~ 50 cm " "The scission process: The last stage of nuclear fission" 7

  8. ENERGY - ANGULAR DISTRIBUTIONS OF THE NEUTRONS IN 235U FISSION "The scission process: The last stage of nuclear fission"

  9. The neutron yields from different fragment masses for 235U(n,f) and total neutron yields as a function of pre-neutron fragment masses "The scission process: The last stage of nuclear fission"

  10. AVERAGE NEUTRON ENERGY AS A FUNCTION OF EMISSION ANGLE AND N(00)/N(900) AND N(1800)/N(900)RATIOFOR DIFFERENT NEUTRON ENERGIES IN 235U FISSION "The scission process: The last stage of nuclear fission"

  11. AVERAGE NEUTRON ENERGY AS A FUNCTION OF EMISSION ANGLE AND N(00)/N(900) AND N(1800)/N(900) RATIO FOR DIFFERENT NEUTRON ENERGIES IN 233U FISSION Energy "The scission process: The last stage of nuclear fission"

  12. ANISOTROPY OF NEUTRON EMISSION IN THE C.M.S. OF 235U(n,f) FISSION FRAGMENTS (I. Guseva, PNPI) The red lines are the Monte-Carlo calculations of neutron emission anisotropy. The blue lines are neutron spectra in the center-of-mass system. At the average <JLF>=7ћ and <JHF> = 8ћ average values of anisotropy were found to be: 6.3% - for light fragments and 9.5% - for heavy fission fragments. "The scission process: The last stage of nuclear fission"

  13. The total prompt neutron spectra for 233,235U(n,f): experiment – full circles, calculations – the lines (see legends). Angular distribution of prompt neutrons in the center-of-mass system of fragmentsare given approximately by: φ(E c.m.s. ,  c.m.s. ) = 1 + A2 Ec.m.s (3  cos2( c.m.s ) - 1) / 2 "The scission process: The last stage of nuclear fission"

  14. AVERAGE NEUTRON YIELDS FOR DIFFERENT ANGLES IN LABORATORY COORDINATE SYSTEM Comparison of the 235U and 233U fission data obtained with two different neutron detectors 233U 235U Model calculations were performed with taking into account the angular anisotropy of fast neutron emission from excited fission fragments (A2 = 0.06) "The scission process: The last stage of nuclear fission" 14

  15. AVERAGE NEUTRON ENERGIES FOR DIFFERENT ANGLES IN LABORATORY COORDINATE SYSTEMComparison of the 235U and 233U fission data obtained with two different neutron detectors Model calculations were performed with taking into account the angular anisotropy of fast neutron emission from excited fission fragments (A2 = 0.06) "The scission process: The last stage of nuclear fission"

  16. NEUTRON YIELDS AND AVERAGE ENERGIES FOR DIFFERENT ANGLES OF NEUTRON EMISSION IN 235U(n,f) REACTION (SCISSION NEUTRONS YIELDS IS NOT MORE THEN 5%) "The scission process: The last stage of nuclear fission"

  17. NEUTRON YIELDS AND AVERAGE ENERGIES FOR DIFFERENT ANGLES OF NEUTRON EMISSION IN 233U(n,f) REACTION (SCISSION NEUTRONS YIELDS IS NOT MORE THEN (4 -5%) "The scission process: The last stage of nuclear fission"

  18. SCHEMATIC VIEW OF EXPERIMENTAL SET-UP FOR (n-n)-COINCIDENCE INVESTIGATIONS "The scission process: The last stage of nuclear fission"

  19. GENERAL VIEW OF EXPERIMENTAL SPECTRUM OF (-), (-n), (n-), (n-n) COINCIDENCES AND ITS EXPANSION "The scission process: The last stage of nuclear fission"

  20. . SENSITIVITY OF (n-n)-COINCIDENCE METHOD TO THE SCISSION NEUTRON ADMIXTURE (a) AND TO THE ENERGY THRESHOLD OF NEUTRON REGISTRATIONS (b). Nsc/Ntot- 7% "The scission process: The last stage of nuclear fission"

  21. (n-n)-COINCIDENCES IN 252Cf(s,f), 233,235U(n,f) AND 239Pu(nf) REACTIONS Angular dependence of (n-n)-coincidences in 235U fission Angular dependence of (n-n)-coincidences in 252Cf fission Angular dependence of (n-n)- coincidences 233U fission Angular dependence of (n-n)-coincidences in 239Pu fission "The scission process: The last stage of nuclear fission"

  22. ESTIMATES FOR SCISSION NEUTRON YIELDS AND TEMPERATURES OF THE WEISKOPF ENERGY SPECTRA *(10 ± 2)% *(5 ± 2)% *(7 ± 2)% *(14 ± 2)% NOT MORE THEN (4-5)% *0.8 MeV *1.1 MeV *1 MeV *0.9 MeV • The data asterisked have been obtained from measurements (n-n)-coincidences • The yield estimates for 233,235U marked off by yellow color have been obtained from the energy and angular distributions of neutrons emitted from separated fragments "The scission process: The last stage of nuclear fission"

  23. TRI-effect of T-odd emission asymmetry for the third particle pTP σ0 W() = 1 + DTRI∙σn∙[pfxpTP] pLF pHF For the LCP: 235U: DTRI=+(1.7  0.2)10-3 233U: DTRI = - (3.9  0.1)10-3 From theoretical point of view such effect exists only for the particles appeared simultaneously! (A. Barabanov, 2001) "The scission process: The last stage of nuclear fission"

  24. θ 2 θ 1 Schematic diagram of ROT- effect appearance in ternary fission (Shift of LCP angular distributions) Observed shift of LCP angular distribution In 235U: 2 - 1 ~ 0.20 in 239Pu: 2 - 1 ~ 0.020 Anticlockwise rotation Clockwise rotation "The scission process: The last stage of nuclear fission"

  25. ROT: 0.215(5)o TRI: + 0.0017 Shift of the third particle angular distribution (ROT- effect) pTP σ0 JL 235U pLF pHF JH W() = 1+DROT∙σn∙[pfxpTP]∙(pf∙pTP) In the contrast to TRI-effect ROT-effect can exist for neutrons and -raysemitted from fragments as well (big oriented angular momenta!) "The scission process: The last stage of nuclear fission"

  26. PM MWPC start polarized neutron beam MWPC stop n LF HF σ+ MWPC stop σ– n Fissile target PM Experimental set-up TRI-effect of Т-odd asymmetry of scission neutron emission • The search for TRI and ROT- effects in scission neutron emission is a subject of much current interest. • Non zero value of these effects would indicate the scission neutrons existence in fission process. Existing information: 235U: <Dn> = - (9 ± 5)·10-4 233U: <Dn> = - (3 ± 7)·10-4 PNPI-2005 235U: │<Dn>│ < 410-5FRM-2-2010 Expected value of TRI-effect at 5% scission neutron admixture has to be about 10-4 The absence of TRI-effects for neutrons may be indicative of different mechanism for scission neutrons emission compared to the LCP in ternary fission "The scission process: The last stage of nuclear fission"

  27. Where “scission” -raysmay be emitted? ● During fissioning system descent from the barrier top to the rupture  Just in the rupture of strongly deformed fissioning system  In the moment of the excited light charged particle decay (for example 5He*  ~ 10-21 s.; 7He*  ~ 4∙10-21 s.; 8Li  ~ 2∙10-20 s.)  In the process of fission fragments acceleration (“bremstraglung”) But in all these cases the yield of such “scission” -rayscompared to the -rays from fission fragments is extremely low !!! (less then 10-2) "The scission process: The last stage of nuclear fission"

  28. J Jl Neutrons or γ-rays from fragments + φ Jh System rotation MECHANIZM OF FALSE “ROT-EFFECT” APPEARENCE FOR NEUTRONS AND -RAYS System rotation + - - φ 90+ 90- -90 -45 0 45 90 In the contrast to the ROT-effect of asymmetry for LCP emission in ternary fission, the “ROT-effect” for neutrons and -rays from fragments may arise as a result of existence of large oriented moments in fission fragments appeared in the rupture process! "The scission process: The last stage of nuclear fission"

  29. Results of the search investigations of ROT-effect for -rays Result of G. Danilyan et al. FRM-2 reactor Result of G. Petrov et al. WWR-M reactor Observed angular distribution shift for -rays: 0.10(3)0 compared to0.215(3)0for LCP "The scission process: The last stage of nuclear fission"

  30. Expected ROT-effects for the neutrons emitted in polarized 236U* fission in comparison with -rays one Expected form of “ROT-effect” for neutrons from fission fragments Expected form of ROT-effect for scission neutrons Observed form and value of “ROT-effect” for -rays ROT-effect form for “scission neutrons” has to be similar to the -rays one "The scission process: The last stage of nuclear fission"

  31. Perspectives of the further search investigations of ROT-effects for neutrons and -rays • Observed shift of -rays angular distributionis direct confirmation of fissioning system rotation around the direction of its polarization. From this point of view this effect together with ROT- and TRI-effects for the LCPs in ternary fission is useful for fission dynamics investigations. • Although shift of -ray angular distribution observed nowcan not testify ”scission” -raysexistence in fission, they can be emitted in principal from physical point of view. • However, it is doubtful if they may be selected in the further investigations at the background ofthe shift effect for -rays from fission fragments. • Unlike -rays scissionneutrons yield my be estimated through the ROT-effect "The scission process: The last stage of nuclear fission"

  32. THANK YOU FOR ATTENTION "The scission process: The last stage of nuclear fission"

  33. "The scission process: The last stage of nuclear fission"

More Related