STUDIES OF FISSION DYNAMICS THROUGH THE SEARCH OF SCISSION NEUTRONS

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Skt. Petersburg Nuclear Physics
Institute of RAS
STUDIES OF FISSION DYNAMICS THROUGH THE SEARCH OF
SCISSION NEUTRONS
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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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Qualitative pictures of the low excitation energy
fission
Excitation energy
Potential energy
TOTAL ENERGY
Kinetic energy
R, fm
Rupture point, 20 fm
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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 ? gt 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.Valski (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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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)
235U nth ? ? 10 3.2 Kapoor (1963)
235U nth (n-n) correlations 20 ? Franklyn (1978)
235U nth (n-f) correlations (10 ? 2) ? Samant (1995)
235U nth Compilations 14 0.98 and 2.74 Kornilov (2001)
235U nth Total spectra 15 ? Kornilov (2002)
252Cf sp. fission (n-n) correlations 10 ? Pringle (1975)
252Cf sp. fission (n-f) correlations 20 gt 1.5 Piksaikin (1977)
252Cf sp. fission ? ? 10 2.6 Bowman (1962)
252Cf sp. fission ? ? (13.2 ? 3.1) ? 1 Riechs (1981)
252Cf sp. fission ? ? ?10 (1.5 ?0.3) Seregina (1985)
252Cf sp. fission ? ? 1.1 0.39 B.-Jorgenson (1988)
252Cf sp. fission ? ? No scis. neutrons! ? Marten (1989)
252Cf sp. fission ? ? lt 1 ? Blinov (1989)
252Cf sp. fission Compilations 10 0.9 and 3.1 Kornilov (2001)
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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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SCHEMATIC VIEW OF EXPERIMENTAL SET-UP
Reaction Chamber 235U target (Ø15mm) 280
µg/?m2 UF4 onto 70 µg/?m2 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-fl
ight distance from 235U target 50 cm
"
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ENERGY - ANGULAR DISTRIBUTIONS OF THE NEUTRONS IN
235U FISSION
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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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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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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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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 ltJLFgt7? and ltJHFgt 8? 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 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 f(E c.m.s. , ?
c.m.s. )  1  A2 ? Ec.m.s ? (3 ? cos2(?
c.m.s ) - 1) / 2
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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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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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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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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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SCHEMATIC VIEW OF EXPERIMENTAL SET-UP FOR
(n-n)-COINCIDENCE INVESTIGATIONS

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GENERAL VIEW OF EXPERIMENTAL SPECTRUM OF (?-?),
(?-n), (n-?), (n-n) COINCIDENCES AND ITS EXPANSION
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. 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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(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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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
(5 2)
(10 2)
(14 2)
(7 2)
NOT MORE THEN (4-5)
1 MeV
1.1 MeV
0.8 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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TRI-effect of T-odd emission asymmetry for the
third particle
pTP
s0
W(?) 1 DTRIsnpfxpTP
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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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
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Shift of the third particle angular distribution
(ROT- effect)
pTP
s0
JL
235U
pLF
pHF
JH
W(?) 1DROTsnpfxpTP(pfpTP)
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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TRI-effect of ?-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 ltDngt - (9
5)10-4 233U ltDngt - (3 7)10-4
PNPI-2005 235U ltDngt lt 4 ?10-5 FRM-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
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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 ? 10-21 s. 7He
? 410-21 s. 8Li ? 210-20 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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MECHANIZM OF FALSE ROT-EFFECT APPEARENCE FOR
NEUTRONS AND ?-RAYS
System rotation
? -?
- f
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!
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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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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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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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THANK YOU FOR ATTENTION
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