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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: [email protected]
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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.
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1
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 onthe 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
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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
-1200 -800 -400 0 400 800 1200
1000
2000
3000
4000
5000
6000
7000
8
765432
1
Co
un
ts
T11
- T12
, channel
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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
1600 1800 2000 2200 2400
100
200
300
400
500
(a)
Co
un
ts
T11
- T22
, channel
Co
un
ts
Fragment TOF channel-300 -200 -100 0 100 200 300
200
400
600
800(b)
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Raw experimental data: neutron - - quanta separation method
100 200 300 400 500
50
100
150
200
250
300Neutrons
- quanta
Total Integral [arb. units]
Part
ial In
teg
ral [a
rb. u
nit
s]
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.
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Raw experimental data:total prompt neutron time-of flight spectrum
1000 1500 2000 2500 3000 35001
10
100
1000
10000
(b)
(a)
Background
Co
un
ts
Neutron TOF Channel
850 875 900 925 950
500
1000
1500
2000
2500
3000
3500
Co
un
ts
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
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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
1 2 3 4 5 6 7 8 9 10
0.03
0.06
0.09
0.12
0.15
0.18
0.21
n(E
n ,
) [
neu
tro
n /
fiss
ion
/ sr
/ M
eV]
Neutron energy, En [MeV] Neutron energy, En [MeV]
= 00
( light fragments )
n(E
n ,
) [
neu
tro
n /
fiss
ion
/ sr
/ M
eV]
1 2 3 4 5 6 7 8 9 10
0.02
0.04
0.06
0.08
0.10
0.12 = 180
0
( heavy fragments )
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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.
0.01 0.1 1 10
0.8
1.0
1.2
1.4
1.6
1.8
2.0
Heavy fragments
Light fragments
This experiment LANL model
This experiment LANL model
Rat
io t
o M
axw
ellia
n T
= 0
.75*
<E
c.m
.>
Neutron energy, Ec.m.
[MeV]
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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 shouldbe 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
0 18 36 54 72 90 108 126 144 162 180
0.2
0.4
0.6
0.8
Model calculation
Skarsvag (1963)
neutron detector N1 neutron detector N2 average
n(
) [ne
utro
n / f
issi
on /
sr]
[degree]0 18 36 54 72 90 108 126 144 162 180
0.9
1.0
1.1
1.2
anisotropy A2 = 0
anisotropy A2 = 0.04
neutron detector N1 neutron detector N2 average
n()
exp /
n()
mod
el
[degree]
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Results (all registered events):angular distribution of the average prompt neutron emission
energy in the lab. system
0 18 36 54 72 90 108 126 144 162 180
1.4
1.6
1.8
2.0
2.2
2.4
2.6
2.8
Model calculation
Skarsvag (1963)
neutron detector N1 neutron detector N2 average
Ave
rag
e n
eutr
on
en
erg
y, <
En(
)> [
MeV
]
[degree]
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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
1 2 3 4 5 6 7 8 9 101
10
100
N(00) / N(90
0)
N(1800) / N(90
0)
Model calculation
This experiment
Rat
io
Neutron energy, En [Ì ýÂ] [MeV]
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Results (all registered events): total prompt neutron spectra in the laboratory system
1 2 3 4 5 6 7 8 9 10
0.2
0.4
0.6
0.8
reference spectrum 235U Model calculation ENDF/B-VII
n(E
n),
[ n
eutr
on
/ fi
ssio
n /
MeV
]
Neutron energy, En [MeV]
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Results (coincident fission fragments):average prompt neutron multiplicity vs fragment mass
80 100 120 140 1600.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
Nishio (small-angle geometry) Maslin (large liquid detector) Maslin tot
Mueler (2E-2V - method) Present work Present work tot
Nu
mb
er o
f n
eutr
on
s, (
m)
Pre-neutron fragment mass, m [a.m.u.]
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Results (coincident fission fragments):average prompt neutron multiplicity vs TKE
120 140 160 180 200
1
2
3
4
Nishio Maslin Present data
tot
Present data L
Present data H
Num
ber
of n
eutr
ons,
tot(T
KE
)
Pre-neutron fragment TKE [MeV]
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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 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 233U(nth , f).
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Thank you very much for your attention
