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

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

3. Qualitative pictures of the low excitation energy fission
TOTAL ENERGY
Potential energy
Kinetic energy
R, fm
Rupture point, 20 fm
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"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, 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  > 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/f for the case of 235U(n,f) and 0.18(4)1/f for 252Cf spontaneous fission.
Prompt neutron emission from highly excited fission fragments (up to 90% of total neutron yield.
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"The scission process: The last stage of nuclear fission"

5. EXPERIMENTAL RESULTS OF SCISSION NEUTRON SEARCH
Nuclear reaction
Type of experiment
% of scission
neutrons
Typical average SN energy, Mev
Reference
235U + nth
(n-f) correlations
15%
1.58
Skarsvag (1963)
 “ 
10%
3.2
Kapoor (1963)
(n-n) correlations
20% 
Franklyn (1978)
(10  2)%
Samant (1995)
Compilations
14%
0.98 and 2.74
Kornilov (2001)
Total spectra
Kornilov (2002)
252Cf sp. fission
Pringle (1975)
> 1.5
Piksaikin (1977)
2.6
Bowman (1962)
(13.2  3.1)%
 1
Riechs (1981)
10%
(1.5 0.3)
Seregina (1985)
1.1%
0.39
B.-Jorgenson (1988)
No scis. neutrons!
Marten (1989)
< 1%
Blinov (1989)
0.9 and 3.1
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"The scission process: The last stage of nuclear fission"

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

7. 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 1 2 3 4 5 6 7 8
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"The scission process: The last stage of nuclear fission" 7

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

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

10. "The scission process: The last stage of nuclear fission"
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 235U FISSION
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"The scission process: The last stage of nuclear fission"

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

12. ANISOTROPY OF NEUTRON EMISSION IN THE C. M. S
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.
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"The scission process: The last stage of nuclear fission"

13. "The scission process: The last stage of nuclear fission"
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 fragments are given approximately by:
φ(E c.m.s. ,  c.m.s. ) = 1 + A2  Ec.m.s  (3  cos2( c.m.s ) - 1) / 2
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"The scission process: The last stage of nuclear fission"

14. "The scission process: The last stage of nuclear fission"
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)
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"The scission process: The last stage of nuclear fission" 14

15. "The scission process: The last stage of nuclear fission"
AVERAGE NEUTRON ENERGIES FOR DIFFERENT ANGLES IN LABORATORY COORDINATE SYSTEM Comparison 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)
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"The scission process: The last stage of nuclear fission"

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

17. "The scission process: The last stage of nuclear fission"
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%)
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"The scission process: The last stage of nuclear fission"

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

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

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

22. *239-Pu Parameters 252-Cf 233-U 235-U Scission neutron yields
ESTIMATES FOR SCISSION NEUTRON YIELDS AND TEMPERATURES OF THE WEISKOPF ENERGY SPECTRA
Parameters
252-Cf
233-U
235-U
*239-Pu
Scission neutron yields
Temperature of spectrum
*(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
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"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)
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"The scission process: The last stage of nuclear fission"

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

25. Shift of the third particle angular distribution (ROT- effect)
pTP σ0
ROT: (5)o
TRI: 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 -rays emitted from fragments as well (big oriented angular momenta!)
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"The scission process: The last stage of nuclear fission"

26. 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.
Experimental set-up σ–
polarized neutron beam
MWPC
stop
Fissile target PM n LF HF σ+
start
Existing information:
235U: <Dn> = - (9 ± 5)·10-4
233U: <Dn> = - (3 ± 7)· PNPI-2005
235U: │<Dn>│ < 4  FRM
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
Last experimental results had been obtained at the PNPI of RAS reactor with the main parameters of neutron beam: n-Flux ~107 n/сm2s, polarization ~80%
After all necessary corrections upper limits (at 95% c. l.) look as following:
233U: │<Dn>│ < 1,8 10-3, U: │<Dn>│ < 1,9 10-3
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"The scission process: The last stage of nuclear fission"

27. Where “scission” -rays may 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*  ~ 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” -rays compared to the -rays from fission fragments is extremely low !!!
(less then 10-2)
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"The scission process: The last stage of nuclear fission"

28. MECHANIZM OF FALSE “ROT-EFFECT” APPEARENCE FOR NEUTRONS AND -RAYS
System rotation
System rotation
Neutrons or γ-rays from fragments
+ φ Jh J Jl + -
- φ
90+
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!
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"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 to 0.215(3)0 for LCP
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"The scission process: The last stage of nuclear fission"

30. Expected ROT-effects for the neutrons emitted in polarized 236U
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
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"The scission process: The last stage of nuclear fission"

31. "The scission process: The last stage of nuclear fission"
Perspectives of the further search investigations of ROT-effects for neutrons and -rays
Observed shift of -rays angular distribution is 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 now can not testify ”scission” -rays existence 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 of the shift effect for -rays from fission fragments.
Unlike -rays scission neutrons yield my be estimated through the ROT-effect
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"The scission process: The last stage of nuclear fission"

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

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