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
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
235U: 10 - 15 % of total neutron yield
252Cf: 3 - 25 % of total neutron yield.
• 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)).
• For a start we selected 235U 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. 2
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-offlight criteria
time-of-flight distance from 235U
target – ~ 50 cm
8
1
7
2
6
3
5
4
4
5
3
6
2
7
1
8
3
Raw experimental data:
position spectrum of the fission fragments
7000
1
6000
8
2
3
5
4
6
7
Counts
5000
4000
3000
2000
1000
-1200
-800
-400
0
400
800
1200
T11 - T12, channel
Number of registered fission events as a function of MWPDs pulse timing delay
from both ends of Arc N1
4
Raw experimental data:
fission fragments time-of-flight
500
800
(b)
(a)
400
Counts
Counts
600
300
400
200
200
100
1600
1800
2000
2200
Fragment TOF channel
2400
-300
-200
-100
0
100
200
300
T11 - T22 , channel
(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
5
Raw experimental data:
neutron -  - quanta separation method
300
Partial Integral [arb. units]
Neutrons
250
200
150
100
 - quanta
50
100
200
300
400
500
Total Integral [arb. units]
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.
6
Raw experimental data:
total prompt neutron time-of flight spectrum
3500
10000


3000
2500
Counts
(a)
1000
2000
1500
Counts
1000
500
100
850
875
900
925
950
Neutron TOF Channel
(b)
10
Background
1
1000
1500
2000
2500
3000
3500
Neutron TOF Channel
initial prompt neutron TOF spectrum
corrected for the pulse-height dependence of timing jitter of the start MWPD
corrected for the dependence on the integral of neutron detector pulse
corrected for the fragment flight time from the target to start MWPD
7
Results (all registered events):
n(En ,) [neutron / fission / sr / MeV]
n(En , ) [neutron / fission / sr / MeV]
prompt neutron spectra in the laboratory system
0.21
 = 0
( light fragments )
0
0.18
0.15
0.12
0.09
0.06
0.03
1
2
3 4 5 6 7 8 9
Neutron energy, En [MeV]
10
0.12
 = 180
( heavy fragments )
0
0.10
0.08
0.06
0.04
0.02
1
2
3
4
5
6
7
8
9
10
Neutron energy, En [MeV]
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
8
Results (all registered events):
Ratio to Maxwellian T = 2.3
0.75*<Ec.m.>
ratio of the prompt neutron spectrum from fission fragments
in the center-of-mass system to the Maxwellian spectrum
2.0
1.8
1.6
This experiment
LANL model
Light fragments
This experiment
LANL model
Heavy fragments
1.4
1.2
1.0
0.8
0.01
0.1
1
10
Neutron energy, Ec.m. [MeV]
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.
9
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
neutron detector N1
neutron detector N2
average
0.6
Model calculation
0.4
neutron detector N1
neutron detector N2
average
1.2
n()exp / n()model
n() [neutron / fission / sr]
0.8
Skarsvag (1963)
0.2
anisotropy A2 = 0.04
1.1
1.0
0.9
anisotropy A2 = 0
0
18
36
54
72
90
108
 [degree]
126
144
162
180
0
18
36
54
72
90
108
 [degree]
126
144
162
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(Ec.m. ,  c.m. ) = 1 + A2  Ec.m.  (3  cos2( c.m. ) - 1) / 2
the parameter A2  0 defines a value of the angular anisotropy
10
180
Results (all registered events):
Average neutron energy, <En()> [MeV]
angular distribution of the average prompt neutron
emission energy in the lab. system
neutron detector N1
neutron detector N2
average
2.8
2.6
2.4
2.2
Model calculation
2.0
1.8
1.6
Skarsvag (1963)
1.4
0
18
36
54
72
90
108
 [degree]
126
144
162
180
11
Results (all registered events):
ratio of the prompt neutron yields at 00 and 900
(1800 and 900) as a function of energy in the lab. system
0
0
N(0 ) / N(90 )
This experiment
Model calculation
Ratio
100
10
0
0
N(180 ) / N(90 )
1
1
2
3
4
5
6
7
8
9
10
Neutron energy, En [Ì[MeV]
ýÂ]
12
Results (all registered events):
n(En), [ neutron / fission / MeV ]
total prompt neutron spectra in the laboratory system
reference spectrum
Model calculation
ENDF/B-VII
0.8
235
U
0.6
0.4
0.2
1
2
3
4
5
6
7
8
9
10
Neutron energy, En [MeV]
13
Results (coincident fission fragments):
average prompt neutron multiplicity vs fragment mass
Number of neutrons, (m)
4.0
3.5
3.0
Nishio (small-angle geometry)
Maslin (large liquid detector)
Maslin tot
Mueler (2E-2V - method)
Present work
Present work tot
2.5
2.0
1.5
1.0
0.5
0.0
80
100
120
140
160
Pre-neutron fragment mass, m [a.m.u.]
14
Results (coincident fission fragments):
Number of neutrons, tot(TKE)
average prompt neutron multiplicity vs TKE
Nishio
Maslin
Present data tot
Present data L
Present data H
4
3
2
1
120
140
160
180
200
Pre-neutron fragment TKE [MeV]
15
Conclusion
• The prompt neutron angle-energy distribution has been measured
for thermal - neutron induced fission of 235U.
• 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 ~ 300 and ~ 1500 a model calculation gives
overestimated values of fission neutron yield as compared with the
experiment. Introduction of anisotropy (A2 = 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 angleenergy 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
233U(n , f).
th
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Thank you very much for your attention
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