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Perspectives on Measurements of Prompt Fission Neutron Spectra
for Fission Induced by Fast Neutrons
Robert C. Haight
Los Alamos National Laboratory
Second International Workshop on Perspectives on Nuclear Data for the Next Decade
CEA Bruyères-le-Chȃtel, France
October 14-17, 2014 LA-UR-14-27963 and LA-UR-14-23021
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• Spontaneous fission (252Cf) • Neutron-induced fission
• Thermal neutron-induced fission • Fast neutron-induced fission
Perspectives on measurements of prompt fission neutron spectra
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Components for neutron-induced PFNS measurements
• Experiments – Neutron source – intense, low background neededs
– Detectors – good neutron identification (psd or ?), good efficiency, “modelable” in MCNP, GEANT, …
– Data acquisition – implementation of new hardware, firmware, software – good resolution, good timing, programmable, capable of handling high counting rates
• Modeling neutron transport as corrections to literature data, and design and analysis of new experiments
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Predictions for PFNS measurement technologies
• Experiments – Neutron source – intense, low background needed -- no new
facilities for this type of measurement (?) :- (
– Detectors – good neutron identification (psd or ?), good efficiency, “modelable” in MCNP, GEANT … -- nothing for greatly advanced capabilities :- ( , :- )
– Data acquisition – implementation of new hardware, firmware, software – good resolution, good timing, programmable, capable of handling high counting rates -- In progress :- )
• Modeling neutron transport as corrections to literature data, and design and analysis of new experiments-- NOW and continuing :- )
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Predictions for PFNS measurements – work to be done
• 239Pu(n,f) – for incident neutron energies > 0.5 MeV and to requested accuracy – Resolve discrepancies for PFNS > 0.5 MeV – probable in 2-3
years – Produce new data for PFNS in range 0.05 to 0.50 MeV -- maybe in
3-4 years
• 235U(n,f) – for incident neutron energies > 0.5 MeV – Data for PFNS > 0.5 MeV – probable in 3-4 years – Produce new data for PFNS in range 0.05 to 0.50 MeV -- maybe in
4-5 years
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Data in the literature: PFNS for 239Pu(n,f) – incident monoenergetic sources
15.4112.8410.708.927.436.195.164.303.582.992.492.071.731.441.201.000.8330.6940.5790.4820.4020.3350.2790.2330.1940.1620.1350.1120.0930.0780.0650.0540.0450.0380.0310.0260.0220.018
0.010483 ThermalThermalThermalThermalThermalThermal 40 keV 0.215 MeV 0.5 1.5 2.5 3.5 10
Belov Werle AbramsonStarostovBoytsovLaitai Conde Knitter Staples 1 Staples 2 Staples 3 Staples 3Nefedov
EXFOR# 40137 20616 20997 40930 40873 30704
year 1968
1971
1977
1983
1983
1985
1965
1975
1995
1995
1995
1995
0.04 0.21 0.50 - 3.5 E-incident (MeV)
10. thermal
15.
5.
1.
0.3
0.1
0.03
Staples Knitter Condé
Out
goin
g en
ergy
(MeV
)
Smith
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Discrepancy in monoenergetic data for high-energy end of PFNS
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Literature data, discrepancies and target accuracies
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239Pu
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38.34 31.95 26.62 22.19 LiG Proton StilbeneMany LiG LiG NE224 BC501 BC501 BC501 BC50118.49 NE102ARecoil NE10215.41 15.4112.84 12.8410.70 10.708.92 8.927.43 7.436.19 6.195.16 5.164.30 4.303.58 3.582.99 2.992.49 2.492.07 2.071.73 1.731.44 1.441.20 1.201.00 1.000.83 0.8330.69 0.6940.58 0.5790.48 0.4820.40 0.4020.33 0.3350.28 0.2790.23 0.2330.19 0.1940.16 0.1620.13 0.1350.11 0.1120.09 0.0930.08 0.0780.06 0.0650.05 0.0540.05 0.0450.04 0.0380.03 0.0310.03 0.0260.02 0.0220.02 0.0180.02 0.01 0.01 0.010483 ThermalThermalThermalThermalThermalThermal 40 keV 0.215 MeV 0.5 1.5 2.5 3.5 10
Belov Werle AbramsonStarostovBoytsovLaitai Conde Knitter Staples 1 Staples 2 Staples 3 Staples 3Nefedov
EXFOR# 40137 20616 20997 40930 40873 30704
year 1968
1971
1977
1983
1983
1985
1965
1975
1995
1995
1995
1995
0.04 0.21 0.50 - 3.5 E-incident (MeV)
10. thermal
15.
5.
1.
0.3
0.1
0.03 Out
goin
g en
ergy
(MeV
) Data in the literature: PFNS for 239Pu(n,f) – incident continuous sources
Previous LANSCE (2) Noda + Chatillon
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Measurements made with “white” neutron source at LANSCE for 239Pu(n,f): CEA-LANL collaboration
S. Noda et al., Phys. Rev. C 83, 034604 (2011)
A. Chatillon et al., Phys. Rev. C 89, 014611 (2014)
Data > ENDF for Eout > 7 MeV Data < ENDF for Eout > 7 MeV
Note: Data for both also for Einc = 1.0 to > 20 MeV 10
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• Correction will reduce data points above 7 MeV but not so much as Noda data because of better time resolution by Chatillon fission chamber
• Major difference with
Noda is in calibration of neutron detector efficiency, which explains why Chatillon < Noda above 7 MeV.
A. Chatillon et al.,Phys. Rev. C89, 014611 (2014)
Chatillon data will also be reduced due to time resolution. Detector calibration difference needs to be included also.
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Los Alamos Neutron Science Center
WNR/LANSCE provides neutrons from 100 keV to 200 MeV for PFNS Studies
Neutron spectrum
30 MeV
Double time-of-flight experiment
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Fission sample and fission counter (LLNL) to contain ~ 100 mg of 239Pu • Parallel-Plate Avalanche Counter (PPAC)
Source – PPAC à Time of flight (1) à Energy of incident neutron
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• 22 6Li-glass scintillation detectors - - or
• 54 liquid scintillation neutron detectors
Chi-Nu array of fast neutron detectors measures prompt neutron spectra emitted in fission
x
Double time-of-flight experiment
21.5 m from WNR source
Fission chamber (PPAC)
Neutron beam
6Li-glass detector array
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Neutron detectors – two types
54 Liquid scintillators – 1.0 m flight path
22 6Li-glass scintillators – 0.4 m flight path
15 PPAC – neutron detector à Time of flight (2) à Energy of outgoing neutron
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Modeling Neutron Transport in PFNS Experiments
Terry Taddeucci
16
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MCNP simulations have been used to investigatesome previous measurements of the PFNS
Two standard papers for 239Pu:
P. Staples et al., Nucl. Phys. A591, 41 (1995)
H.H. Knitter, Atomkernenergie 26, 76 (1975)
Some possible sources of systematic error:
multiple scattering in the target
multiple scattering in the collimation
detector efficiency
background subtraction
Calibrations (TOF, PH, flight path, etc)
these are not necessarily decoupled
previous1.plt: 22 Apr 2014 11:57:02
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Experimental layout for the measurements by Staples et al.
drawing not to scale
staples-expt.plt: 22 Apr 2014 12:41:23
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Experimental layout for the measurements by Knitter
typical for this facility (CBNM, Geel)
M. Coppola and H.H. Knitter, Z. Physik 232, 286 (1970)
knitter-expt.plt: 22 Apr 2014 12:53:17
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How did Knitter and Staples handle target corrections?
Staples:
"Multiple scattering corrections and neutron attenuation corrections
have not been performed because the samples are so small that these
effects can be neglected."
Knitter:
"The result of the fit gave an average fission neutron energy of
= 3/2T = 2.12 ± 0.01 MeV
This result contains a small calculable systematic error, since the
fission neutrons produced in the sample can make secondary interactions
with the sample material. Correction calculations were done in the
manner described in a previous paper [1]."
[1] Kiefhaber,E., D. Thiem: Panel Proceedings Series, p127, IAEA Vienna (1972)
cf. Islam and Knitter, Nucl. Sci. Eng. 50, 108 (1973)
corrections.plt: 22 Apr 2014 16:05:16
U N C L A S S I F I E D
U N C L A S S I F I E D
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Multiple scattering plays a significant role in the 239Pumeasurements of Knitter
0.01 0.1 1 2 5 10 20
MCNP flux at the detector position
neutron energy (MeV)
dN
/dE
(n
/Me
V)
target + shields
target only
Watt
two effects:
n-multiplication
in-scattering
calculations for
239Pu target
knitter-Pu-mcnp.plt: 23 Apr 2013 21:57:26
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Comparison of MCNP calculations to the 239Pumeasurements of Knitter
0.1 1 2 5 10 20
MCNP flux at the detector position
neutron energy (MeV)
dN
/dE
(n
/Me
V)
target + shields
target only
data (0.215 MeV)
Watt
two effects:
n-multiplication
in-scattering
calculations for
239Pu target
knitter-Pu-mcnp-data2.plt: 23 Apr 2014 14:36:40
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Target and collimator effects in the 239Pu dataof Staples et al.
0.1 0.5 1 2 5 10 200.9
1.0
1.1
1.2
1.3
1.4
1.5
MCNP output / input
neutron energy (MeV)
ratio
data range
t
c
p
calculation:
target (t)
collimator (c)
product (p = t × c)
staples-Pu-ratio.plt: 22 Apr 2014 12:42:57
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Target and collimator effects in the 239Pu dataof Knitter
0.1 0.5 1 2 5 10 200.9
1.0
1.1
1.2
1.3
1.4
1.5
MCNP output / input
neutron energy (MeV)
ratio
data range
t
c
p
calculation:
target (t)
collimator (c)
product (p = t × c)
knitter-Pu-ratio.plt: 22 Apr 2014 12:53:40
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Modeling of Our Present Experiments
Terry Taddeucci
17
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The low-energy part of the PFNS is being measuredwith an array of 6Li-glass detectors
active thin target (~100 mg)
many detectors (22)
open geometry
(no shielding)
beam
array-top.plt: 22 Apr 2014 13:00:58
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The Chi-Nu MCNP model accounts for neutron scatteringfrom all nearby objects
top view front view
model space (∆x,∆y,∆z) = 7.5 × 7.6 × 6.9 m3 = 393.3 m3
room2.plt: 22 Apr 2014 13:36:15
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Multiple scattering is a significant problemfor energies < 1 MeV
0.01 0.1 1 10 200.0
0.1
0.2
0.3
0.4
0.5
0.6
neutron energy (MeV)
dN
/dE
(n
/Me
V)
energy spectra derived from TOF (det#03)
1
2
3
41 = Watt
2 = PPAC
3 = + det-frame
4 = + 21 dets
dnde-diff2.plt: 21 Apr 2014 14:22:19
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Multiple scattering effects are more accuratelyrepresented by including the detector response
0.01 0.1 1 10 20
neutron energy (MeV)
co
un
ts/M
eV
det #03
σ × Watt
full model
no dets
no frame
no PPAC
1 calculation =
46 cpu-h
(MCNP-PoliMi)
dnde2.plt: 24 Apr 2014 11:01:35
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A preliminary comparison of simulation and datashows good agreement
0.01 0.1 1 10 20
neutron energy (MeV)
co
un
ts/M
eV
252Cf spectrum
σ × Watt
MCNP
data
239 cpu-h
dnde-det2.plt: 23 Apr 2014 10:00:16
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PFNS for 239Pu(nth,f) –
Is it a good guide for PFNS in fast-neutron-induced fission?
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• Prompt fission neutron spectra have been measured at thermal
for 235U and 235Pu. Reactions at thermal can be dominated by one or only a few resonances
• Do the data at thermal have any relevance to
PFNS for fission induced by higher energy neutrons?
• Zero order analysis – look at average number of neutrons emitted
in fission. If they vary with incident neutron energy, then there could well be a change in the spectra of emitted neutrons
PFNS for 239Pu(nth,f) – is it a good guide for PFNS in fast-neutron-induced fission?
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Are PFNS measured at thermal relevant for higher incident neutrons?
• Nu-bar for 235U(n,f) has no structure structure
Note also the scale:
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Correlate structure in nu-bar for 239Pu(n,f) with fission cross section
• Fission cross section from Weston [NSE 115,164 (1993)] • Subtract a constant (2.82) from nu-bar for clarity of display • Add spins and parities (all positive) from Mughabghab
– 0+ resonance shows no effect in nu-bar – 1+ resonances show varying effects
Nu-bar – 2.82
1+ 1+ 1+ 1+
1+ 1+ 1+ 0+
Probably (n,γ f) process
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Now the good news (maybe)
• Nu-bar at thermal for 239Pu(n,f) is almost the same as for 1-10 keV. Maybe the thermal neutron PFNS is relevant to higher energies
• Q: Is nu-bar at thermal dominated by the 1+ resonance at 0.3 eV ?
0.025 eV (thermal)
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Prospects for PFNS measurements with fission induced by epithermal neutrons
• 239Pu(n,f) – for incident neutron energies in resonance region – not planned but would be interesting physics! – Note: gamma production from fission in resonance region has been
studied. Yes, spectra do depend on incident neutron energy and correlate with variations in nu-bar!
Ref: S. Mosby et al., DANCE collaborations
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Fission total γ-ray energy vs. incident neutron energy for 239Pu(n,f)
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• Fluctuations in prompt fission gamma energy anti-correlated with neutron emission
• More detailed information on 239Pu(n,γf) process (Lynn, 1965) • Qualitative behavior reported by Shackleton in 1972
Mea
n to
tal g
-ene
rgy
(MeV
) bef
ore
corr
ectio
n fo
r det
ecto
r res
pons
e Prom
pt Neutron Yield
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Advanced PFNS measurements
• Correlate PFNS with fission products (Z,A) – difficult – could improve models of fission physics
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Acknowledgments
• This work is performed under the auspices of the U.S. Department of Energy by
Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344
and the Los Alamos National Laboratory under Contract DE-AC52-06NA25396.
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• LANL: H. Y. Lee, T. N. Taddeucci, J. M. O’Donnell, N. Fotiades, M. Devlin, J. L. Ullmann, T. Bredeweg, M.
Jandel, R. O. Nelson, S. A. Wender, D. Neudecker, M. Rising, S. Mosby, S. Sjue, M. White; R. C. Haight
• LLNL: C.-Y. Wu, B. Bucher, R. Henderson
• CEA: T. Ethvignot, T. Granier, A. Chatillon, J. Taieb, B. Laurent, Alix Sardet
Colleagues in PFNS experiments at LANSCE