n e Fluxes BC Rasco JINPAORNLUTK UTK September 28 2016 1 Nuclear Reactors 2 How do they work Basically big fancy steam engines Most reactors use 235 U as a fuel But 238 U ID: 534847
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Slide1
Total Absorption Spectroscopy and Its Influence on Decay Heat and Predicted Reactor ne Fluxes
B.C. RascoJINPA/ORNL/UTKUTK September 28, 2016
1Slide2
Nuclear Reactors2
How do they work?
Basically big fancy steam engines.
Most reactors use
235
U as a fuel.
But
238
U, 239Pu, and 241Pu fuels become important, more so as the reactor runs longer.Each of these different fuels are called a reactor fuel type.
TRIGA Research Reactor at
Argonne National LaboratorySlide3
Nuclear Fission - 235U
3
What are the direct fission decay products?
More than 800 nuclei per fuel type.Slide4
Nuclear Fission - 235U
4
What happens to the fission decay products?They
b
decay and
b
neutron decay back to stability.
b
Decay and
b Neutron Decay
to StabilitySlide5
Nuclear Fission - Decay Heat
5
Fuel Rod
X
Y
The fission products do not travel far in the fuel rod.
They do not leave the fuel rod.
They heat up the fuel rod which then heats the water.
The fuel rood must dissipate this heat.
Neutrons can escape fuel rods and then get absorbed by other fuel rods continuing fission, but neutrons also can heat the water directly.
In addition to the fission energy...The fission products are not stable nuclei,
they continue to emit energy by
b
,
g
, , and
neutrons via
b
decay and
b
delayed neutron decay. This is called decay heat.
During
b
decay
and
b
delayed
neutron decay
there are many
g rays emitted.All antineutrinos are generated during the decay heat portion of the reactor fuel cycle.
g
n
n
n
b
Fuel Rod
n
WaterSlide6
Nuclear Fission -
b decay6
Here is a simple b
decay example.
During
b (
and
b
-neutron) decay often there are g rays emitted.All antineutrinos are generated during the decay heat portion of the reactor fuel cycle.Slide7
Nuclear Fission -
b decay7
Simulated b
and antineutrino spectrum for
137
Cs decay.
In each decay the
b
- and antineutrino share the Qb. How they share energy depends on the forbiddeness of the b decay.
T. Langford, private communication
AntiSlide8
Nuclear Fission -
b decay8
b
decays are not all peaches and cream like
137
Cs.
This is the ENSDF for
137
I.
The
b
-neutron decays are not shown, so things are even more complicated than shown.
And this decay scheme is incomplete.
(More about this later)Slide9
Nuclear Fission -
b neutron decay9
For some nuclei, including 137
I, there is enough energy to create and eject neutrons.
Eventually making
136
Xe and a neutron.
136
Xe
S
n
=4002 keV
+ free neutron
High density of levels!
Not showing 6 pages
of
g
decays here.
Lower energy
b
+ antineutrinoSlide10
Reactor Decay Heat
Nuclear Fuel Cycle*
To improve our knowledge on the energy release (
g
,
b
,
n
, n) from fission products produced during the nuclear fuel cycle:
operation safety (loss-of-cooling accident), operation efficiency,
reactor material science, and nuclear waste transportation and storage.
While reactor is on,
approximately 7% of total reactor energy is in decay heat.
Important
b
decays are ranked from priority 1-3 in [*] and these rankings are referenced in this presentation.
100% of antineutrinos are emitted by decay heat.
Nuclear Structure
To determine true
b
-decay intensities. This helps to identify their origin and contributes to the verification and improvements of
b
-strength predictions.
Astrophysical Interest:
b
half-life extrapolations and
b
-neutron decay branching fractions for r-process analysis.
*T
. Yoshida and A. L. Nichols, Assessment of
Fission Product
Decay Data for Decay Heat Calculations: A report by the Working Party on International Evaluation Co-operation of the Nuclear Energy Agency Nuclear Science Committee (Nuclear Energy Agency, Organization for Economic Co-operation and Development, Paris, France, 2007), ISBN 9789264990340.
10Slide11
Precision
Antineutrino Reactor Measurements~5.5% overall deficiency of antineutrinos
Daya Bay:
An
et al.
, PRL
116
, 061801 (2016
)Ratio of measured to expected ~0.946(22) Antineutrino Reactor Anomaly Similar
deficit reported by RENO and Double Chooz
11
Inverse
b
decay has a
1.8 MeV threshold,
i.e.
no antineutrinos below 1.8 MeV are detected.
At these energies the cross section increases quadratically so these antineutrino experiments are more likely to detect higher energy antineutrinos.
The extracted antineutrino spectrum
and its correlation matrix.
Antineutrino Reactor
ShoulderSlide12
Precision Antineutrino Reactor Measurements
Gando, A. et al. arXiv:1309.6805 [hep-ex](KamLand white paper to look for sterile neutrinos)Reactor Anomaly: Mention
et al., PRD 83, 2011
Daya
Bay:
An
et al.
, PRL
116, 061801 (2016)
Antineutrino Reactor Shoulder
Ratio of the extracted reactor antineutrino spectrum to the Huber-Mueller prediction
. The
solid red band represents the square roots of the diagonal elements of the prediction covariance matrix, which included reactor and Huber-Mueller model uncertainties.
Similar
shoulder
reported
by
RENO and Double Chooz
Reactor Anomaly possible indication of physics beyond the standard model,
such as sterile neutrinos.
Ratio
12
~5.5% overall deficiency of antineutrinos
Ratio of measured to expected ~0.946(22)
Antineutrino Reactor Anomaly Slide13
What is "Expected"
Conversion vs Summation MethodConversion method is an integral
b measurement by fuel type.
b
energies were measured by fuel type
(
235
U,
238U, 239Pu, 241Pu) and then converted to an antineutrino spectrum. But...Measuring all of the bs aggregately by fuel type does not allow precise prediction of antineutrino energies, a Z effective must be used to calculate
an effective Fermi function.Also the b spectrum depends on the individual decay "forbiddenness",
i.e. allowed, first forbidden, unique first forbidden, second forbidden etc. and converting this to an antineutrino spectrum is not straight forward.
Limitations based on the shape factor for each type of decay and by weak magnetism corrections.
Current comparison with experiment is based on a version of this method
Hayes
,
et al.
, PRL
112
, 202501 (2014)
13Slide14
What is "Expected"
Conversion vs Summation MethodSummation method converts each
b decay produced during fission. There are a lot of b decays, over 800 for each fuel type!
But there are a few dominant decays for the anomaly and the shoulder, the top 20 for each fuel type is shown at right.
Limitation is based on accuracy of the nuclear data.
A. A. Sonzogni, T. D. Johnson, and E. A.
McCutchan
, PRC
91, 011301(R) (2015)
Is there some reason to suspect the fidelity of the nuclear data?
14Slide15
How is b Decay Measured?
15HPGe Detector
Measure b
directly?
If there are associated
g
rays then you can not tell which level is fed.
(unless you measure them also)
Remember b energy is shared with antineutrino, so it is not mono-energetic.Measure g rays directly?If Q
b is known and the number of nuclei present is known and all the g rays are detected then one can determine which level is fed.
But how confident should one be that all
g
s are detected?
137
Cs sourceSlide16
Efficiency Is Key
4-
g Decay
5.8 MeV
3.5MeV
1.7 MeV
GS
2.3 MeV
g
1.8 MeV
g
1 MeV
g
700 keV
700 keV
g
100%
b
-
100% Efficient
MTAS
75% MTAS
Solid
Angle
50% MTAS Solid Angle
25% MTAS Solid Angle
5% MTAS Solid Angle
16Slide17
Efficiency Is Key
Pandemonium 4-g Decay17
5.8 MeV
3.5MeV
1.7 MeV
GS
Continuum
g
s
2.3 - 4 MeV
1.8 MeV
g
1 MeV
g
700 keV
700 keV
g
100%
b
-
7.5 MeV
Which
g
s are going to be detected?
So which states look like they are fed?
States below 3.5 MeV will be identified but states above will look like a background.
None of the 2.3-4 MeV
g
will make a peak in a high precision detector.
Can these types of decays be accurately measured?
Apparent feedingsSlide18
Pandemonium Effect
N-RICH PARENT (Z,N)
DAUGHTER (Z+1, N-1)
β
- transitions
- transitions
Greenwood
et al.
,1997,
Algora
et al.
, 2010,
Zakari
-Issoufou
et al.
, 2015
Pandemonium Effect
For high-precision, low efficiency detectors, the combination of low efficiency with a high density of states fed by
b
decay means many multi-
g
decays will be misinterpreted as direct
b
feeding to lower energy levels.
Hardy et al, PL B 71,1977
Pandemonium Effect General Trends in Average Energy by Particle Type:
18
Can the Pandemonium Effect be overcome?Slide19
The Modular Total Absorption Spectrometer - MTAS
Outer Ring
Middle Ring
Inner Ring
Center Module
MTAS: 18 - 8”x 7”x 21” (20cm x 17.8cm x 53.3cm) hexagon NaI(Tl) modules
Organized in 3 Rings of 6 modules each (Inner, Middle, and Outer)
1 - Center module, same dimensions but with a 2.5” diameter hole
Over 1 ton of NaI(Tl)!
Over 5 tons of lead shielding + neutron shielding
Other total absorption spectrometers include the
TAS at
ISOLDE, Lucrecia,
TAS
at
GSI (now at UML), SuN
(
MSU), DTAS (Valencia, Jyvaskyla).
19Slide20
What and How MTAS Detects
g
s in MTAS
b
s in MTAS
Neutrons in MTAS
20
m
s in MTASSlide21
What MTAS Detects -
g
Rays
g
s in MTAS
MTAS
Peak
Efficiency
MTAS Center Module
Only Peak
Efficiency
MTAS Inner
Ring Only
Peak Efficiency
21
Single
g
-ray
e
fficiency of various MTAS regions and comparison with a high-efficiency HPGe Array.
Gammasphere Peak Efficiency
Part of
g
-ray energy detectedSlide22
What MTAS Detects -
g
Rays
g
s in MTAS
Center Module
Center Module + Inner Ring
MTAS Total
22
142
La [Q
b
= 4509(6) keV, T
1/2
= 91.1(5) min]
Total Energy Detected (keV)
Counts per keV
Increased Efficiency Adding Successive Rings
142
La
b
decay
(MTAS Data)Slide23
What MTAS Detects - g Rays
g
s in MTAS
Simulated MTAS response to a 2850
keV
g
-
ray
+ allowed
b
spectrum for
137
Xe
Total MTAS
Center Module
Inner Ring
Middle Ring
Outer Ring
Includes nonlinear light
production in NaI crystals.
23Slide24
What MTAS Detects - g Rays
g
s in MTAS
Segmentation is
powerful.
Can see dominant decay paths
from
various
energy levels
.
MTAS Center vs Total Energy
24
137
Xe
b
decay
(MTAS Data)
B.C
. Rasco,
et. al.
,
JPS Conf. Proc. 6, 107 (2015)Slide25
What MTAS Detects - bs
b
s in MTAS
Simulated Total
MTAS response to
b
s from
92
Rb ground state to
92
Sr ground state
decay (Q
b
= 8095
keV)
For
92
Rb decay to the
92
Sr ground state
~
55%
of the
b
s
trigger the
silicon detectors
and
leave energy in MTAS
25Slide26
What MTAS Detects - Neutrons
Thermal Neutron Capture Peak
(From background cycles)
137
I
Q
b
=
6027 keV, Qb-n= 2002 keV
Neutrons in MTAS
MTAS Raw Data
Sum
Simulated Neutrons (25 keV bins)
26Slide27
MTAS and ms
27Turned down voltage on PMTs and calibrated on 6.8 MeV neutron capture peak.
Looked for large energy deposit (> 100 MeV) in center and inner ring with no exit from MTAS, then looked at next event.
m
s in MTASSlide28
MTAS and
ms28
Turned down voltage on PMTs and calibrated on 6.8 MeV neutron capture peak.
Looked for large energy deposit (> 100 MeV) in center and inner ring
with no exit from MTAS, then looked at next event.
(Only high energy events (> 15 MeV) shown here)
m
s in MTASSlide29
MTAS Muon Spectrum versus Michel
Spectrumwith Detector Resolution (sE= 0.04 * Energy)
MTAS Energy Spectrum of Delayed Events
Neutron Capture Peak
(from
127
I-
m
-
and/or
23Na-m-atoms?)
Michel Spectrum
Smeared Michel Spectrum
MTAS Data
Energy/10 (MeV)
m
s in MTAS
Counts per BinSlide30
Data ACQ
cycle logistic
M/
Δ
M~600
MTAS
Experiments and Pure Beams at Oak Ridge National Laboratory
Batch or Dan
30Slide31
Mass A
2 Silicon
b
-
Detectors
(~96% solid angle coverage)
Shielded
Tape Box
HPGe Detector
To Monitor Implantation
31
MTAS -
b
Detectors and Tape SystemSlide32
Nuclear Structure 2016
Rate for MTAS (most exposed single module)
without shielding
~16000 Hz
(test stand near a lab with some activated materials)
with four Pb blankets and paraffin
~600 Hz
with “Pb house”, Pb blankets and paraffin shielding ~160 Hz
(now two SWX-227A layers instead of paraffin bricks)
1 Pb blanket
4 Pb blankets
4 Pb blankets + paraffin
no blanket
The weight of (mostly Pb) shielding for this setup is
~12 000 pounds
(with about 1” layer of solid lead and ~0.75” lead in lead wool blankets)
MTAS - Shielding and BackgroundSlide33
1
ν
1
ν
1
ν
ν
ν
ν
ν
2
ν
2
ν
2
ν
ν
ν
ν
1
2
1
1
MTAS
- lower mass fission peak
( 39 decays measured )
January
2012,
March, October-December
2015
, January
2016
Priority “
1
”
(
6
nuclei
)
and “
2
”
(
4
)
for decay heat simulation established by a Nuclear Energy Agency in 2007.
The same activities have priority for “anomaly” analysis
. Most important nuclei (
13
) for reactor high-energy
ν
according to
Sonzogni
et al.
,
PRC 2015
and Dwyer-Langford PRL 2015
33Slide34
2
ν
2
2
2
1
1
1
1
1
ν
ν
ν
ν
ν
ν
3
ν
1
1
Reactor high-energy
ν
:
8
decays
Priority 1
,
2
,
3
:
12
decays
MTAS
- higher mass fission peak
( 38 decays measured )
January
2012,
March, October-December
2015
, January
2016
34Slide35
MTAS *Validation* -
137
Xe (Priority 1)
MTAS Data
Sum of Fit Simulated Data
Ground State Feeding
Individual
Simulated
Decay Paths
137
Xe MTAS Data
Simulated ENSDF
137
Xe Data
A slight increase in feeding to the higher energy levels
Otherwise our feeding intensities are in agreement with the current ENSDF evaluation.
Q
β
=
4162.4
(
3
) keV
T
1/2
=
3.818
(
13
)
m
35Slide36
92Rb (Priority 2)
ENSDF Ground State Feeding: 95.2±.7%
(up from 50±18% in 2007)and 87.5±2.5%**A.-A. Zakari-Issoufou
et al,
PRL
115
, 102503
MTAS Ground State Feeding: 91±3%Our uncertainty mainly from ground state b simulation.Ground state feeding insensitive to exact decay pattern from higher states.
When we vary the decay paths the b-feeding to the non-ground state levels changes minimally.
Simulated Individual Decay Paths
MTAS Data
Sum
36
b
-Feeding Intensity
GS off scale at 91±3%
B.C. Rasco,
et al.
, PRL
117
, 092501 (2016)
Q
b
= 8095 (6) keV T
1/2
= 4.48 (3) sSlide37
b
-Feeding Intensity
Q
β
=
7
325
(9) keV T1/2= 1.684 (14) sSimulated ENSDF DataMTAS Data
Simulated Individual Decay Paths
MTAS Data
Sum
MTAS Ground State feeding:
44±2%
(down from
56±5
% in ENSDF)
MTAS
1
st
E
xcited
State feeding:
<0.5%
(down from
7.2±1.2%
in ENSDF)
MTAS
b
feeding to levels below 2 MeV:
14
±1%
(down from 20% in ENSDF)
Average
g
energy
1.7 MeV
(up from 900 keV in ENSDF)
B.C. Rasco,
et al.
, PRL
117
, 092501 (2016)
37
142Cs (Priority 3)Slide38
Changes to the 142Cs anti-νe Flux
Antineutrino spectrum for
142Cs MTAS assuming allowed b decay spectrum.
Antineutrino spectrum for
142
Cs ENSDF
Antineutrino Detection Threshold
(= Inverse
b Decay Threshold)
Fraction Change Below 1.8 MeV (Not Detectable): 0.11
to
0.23
(3
)
Fraction Change Above 5 MeV: 0.20
to
0.14
(1
)
B.C. Rasco,
et al.
, PRL
117
, 092501 (2016)
38Slide39
Changes to the νe Shoulder
ENDF/B-VII.1 & ENSDF Calculation
Ratio of antineutrino production for new MTAS Data / Previous Data for modified
92
Rb,
96gs
Y, and
142
Cs calculated by fuel type.Daya Bay: An et al., PRL 116, 061801 (2016)
Unity line in this graph will change shape in a similar manner as above.
For the summation calculation this increases the shoulder ratio by about 0.02 at 6 MeV.
235
U
238
U
239
Pu
241
Pu
B.C. Rasco,
et al.
, PRL
117
, 092501 (2016)
39Slide40
Additional EvaluationsA. Fijałkowska, Ph.D. Dissertation, and in Preparation
40Slide41
Additional Measurements
A. Fijałkowska, Ph.D. Dissertation, University of Warsaw
41Slide42
Changes to g Decay Heat versus Time
Ratio of average g energy (Decay Heat) as a function of time for new measurements to ENDF for 235U fuel.
A. Fijałkowska, Ph.D. Dissertation, University of Warsaw
42Slide43
SummaryWe have measured 22 Priority 1, 2, and 3
b decays that are relevant for nuclear decay heat and antineutrino anomaly.Evaluated cases (~10) average g decay heat increases by up to ~3%
over the first 1000 seconds. We have measured 21 b
-decays that are relevant for high energy antineutrino (antineutrino shoulder) produced in nuclear reactors.
Evaluated cases (~10) alter the neutrino anomaly ratio from 0.95(2) to 0.97(2) ratio is getting close to 1.00 which means no anomaly.
(Which makes high energy physics sad)
Antineutrino shoulder grows from 0.10 to 0.12
(Which makes high energy physics happy)
Nuclear Structure implications are a work in progress.43Slide44
CollaboratorsK.P.
Rykaczewski, B.C. Rasco, D. Stracener, N. Brewer, C.J. Gross, J. MattaOak Ridge National LaboratoryA. Fijałkowska,
M. Karny, K. Miernik, M. Wolińska-CichockaUniversity of WarszawaK.C. Goetz , R.K. Grzywacz, S. Paulauskas, M. Madurga, T. King
University of Tennessee
E
.
Zganjar, J.C
.
Blackmon Louisiana State UniversityJ.C. BatchelderUniversity of California, BerkeleyM.M. RajabaliTennessee Technological University J.A. Winger
Mississippi State University J. Hamilton, et al.
Vanderbilt UniversityThis work was supported by the US DOE by award no. DE-FG02- 96ER40978
and by US DOE, Office of Nuclear Physics
44Slide45
Thank You45Slide46
Backup Slides46Slide47
Nuclear Fission - 239Pu
47Slide48
Motivation - Top Contributors to the Shoulder
D.A. Dwyer and T.J. Langford, PRL
114, 012502 (2015)
~14
~1
Used older ENSDF
92
Rb ground state
feeding (50±18
%, which was increased to
95.2±.7
%).
92
Rb now most dominant contributor to "shoulder".
Is there some reason to suspect the accuracy of other nuclear data?
48Slide49
0- → 0+ Allowed Shape?
A. A. Sonzogni, T. D. Johnson, and E. A.
McCutchan, PRC 91, 011301(R) (2015)
D.L
. Fang and
B.A
. Brown, Phys. Rev. C 91, 025503 (2015)
A more general discussion is given in
49Slide50
Efficiency Is Key
3
g
decay
4
g
decay
100% Efficient
MTAS75% MTAS
Solid Angle50% MTAS Solid Angle25% MTAS Solid Angle
5% MTAS Solid Angle
5.8 MeV
3.5MeV
1.7 MeV
GS
2.3 MeV
g
1.8 MeV
g
1 MeV
g
700 keV
700 keV
g
100%
b
-
5.8 MeV
2.8MeV
1.0 MeV
GS
3.0 MeV
g
1.8 MeV
g
1 MeV
g
100%
b
-
100% Efficient
MTAS
75% MTAS
Solid
Angle
50% MTAS Solid Angle
25% MTAS Solid Angle
5% MTAS Solid Angle
50Slide51
What MTAS Detects - g-Rays
g
s in MTAS
Segmentation is
powerful.
Can get relative decay path probabilities.
137
Xe Center Module if Total in 2850 keV peak
137
Xe Inner Ring if Total in 2850 keV peak
B.C.
Rasco,
et al.
, JPS Conf. Proc.
6
, 030018 (2015)
51Slide52
What MTAS Detects - Scattered Neutrons
59 keV
59 keV x2
202 keV
MTAS-Total
MTAS-Central Module
MTAS-Inner Ring
MTAS-Middle Ring
MTAS-Outer Ring
127
I(n,n’
g
)
127
I: 59, 202, 618, 628, 1044 keV
23
Na(n,n’
g
)
23
Na: 440, 2076 keV
Neutrons in MTAS
52Slide53
MTAS Muon Spectrum versus Michel Muon Spectrum
with Detector Resolution (
sE= 0.04 * Energy)
MTAS Time versus Energy Spectrum of Delayed Events
m
s in MTAS
1 and 2 Neutron Capture Peaks
(from
127
I-
m
-
and/or
23
Na-
m
-
atoms?)Slide54
96gsY (Priority 2)
b
-Feeding Intensity
GS off scale at
95.5±2.0
%
Q
β
= 7103 (6) keV T1/2
= 5.34 (5) sENSDF Ground State Feeding: 95.5±.5%
MTAS Ground State Feeding: 95.5±2.0%Uncertainty mainly from
ground state
b
simulation
Simulated Individual Decay Paths
No
96m
Y (1140 keV, (8
+
),
T
1/2
=
9
.
6
(
2
)
s
) measured.
1.5 MeV 0+ → 0+
E0
decay
, hard to detect with
1 mm silicon beta detectors.
Future experiments will have an array of
b
detectors to choose from.
MTAS Data
Sum
54Slide55
Changes to the anti-νe Shoulder JEFF Based
JEFF
& ENSDF Based Calculation
ENDF/B-VII.1
& ENSDF Calculation
Ratio of antineutrino production for new MTAS Data / Previous Data for modified
92
Rb,
96gsY, and 142Cs calculated by fuel type.
Daya Bay: An et al.
, PRL 116, 061801 (2016)
Unity line in this graph will change shape in a similar manner as above.
For the summation calculation this increases the shoulder ratio by about 0.02 at 6 MeV.
235
U
238
U
239
Pu
241
Pu
235
U
238
U
239
Pu
241
Pu
Rasco,
et al.
, PRL
117
, 092501 (2016)
55