Modeling the YAGUAR Reactor Neutron Field and Detector Count Rates in the Direct a nn Measurement
Model of the gamma ray-induced out-gassing in the nn-experiment at YAGUAR B. Crawford for DIANNA
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Transcript of Model of the gamma ray-induced out-gassing in the nn-experiment at YAGUAR B. Crawford for DIANNA
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Model of the gamma ray-induced out-gassing in the nn-experiment at YAGUAR
B. Crawford for DIANNA
May 26, 2009
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aCSB = (app – ann)
Use aCSB to test theory. But the magnitude and sign of aCSB are uncertain!
n-scattering at YAGUAR
app = (-17.3 ± 0.8) fm ann = (-18.5 ± 0.3) fm (-d capture, n-d breakup)
ann = (-16.27 ± 0.40) fm (n-d breakup)
Nagels et al. Nagels et al. NUCL. PHY BNUCL. PHY B 147147 (1979) 189. (1979) 189.
Howell et al. Howell et al. PHYS LETT BPHYS LETT B 444444 (1998) 252. (1998) 252.
GonzGonzáález Trotter et al. lez Trotter et al. PHYS REV LETT PHYS REV LETT 8383 (1999) 3788. (1999) 3788.
Huhn et al. Huhn et al. PHYS REV C PHYS REV C 6363 (2001) 014003. (2001) 014003.
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• Motivation
ann measurements disagree within experimental uncertainty
ann ’s lack of precision does not constrain theory
• Experimental goal
Make the first directdirect measurement of ann (related to the strength of attraction between two neutrons) to a precision of 3%
n-scattering at YAGUAR
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n-scattering at YAGUAR
Pulsed reactor with high instantaneous fluxAnnular design with open through-channel (nn-cavity)90% enriched 235U-salt/water solutionEnergy per pulse – 30 MJPulse duration – 0.9 msFluency – 1.7x1015 /cm2
Flux – 0.8x1018 /cm2/sNeutron density – 1x1013 /cm3
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Vacuum testing of upper section of neutron channel.
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• ann determined from detector counts• Expect ND ~ 150 counts/pulse• ~10 pulses achieves required statistics
effav
avnnD Vv
FaN
02.096.0
22
dttF avav
0
2 vvav
Sharapov, ISINN-13 Report E3-2006-7, p. 130
n-scattering at YAGUAR
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Monte Carlo modeling of neutron background
Neutron speed
Source of background
Number of neutrons per pulse
Fast (>0.5eV)
Initial and delayed
~10
Thermal
(<0.5eV)
Back wall ~10
Collimators/walls <10
Residual gas P(H2)~10-7 <1
P(N2)~10-6 <1
Total 20—40
A. Yu. Muzichka, et al., Nucl. Phys. A 789 (2007)
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n-4He 27MJ Maxwellian*(1/v) to=5.26
Time of Flight (ms)2 4 6 8 10 12
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n-n measurement
nn 31MJ Maxwell*DetEff to=5.26ms
t(ms)2 4 6 8 10 12
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n-n measurement
(Poor) fit shown here is Maxwellian x
nn 31MJ Maxwell*DetEff to=5.26ms
t(ms)2 4 6 8 10 12
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n-n measurement
(Poor) fit shown here is Maxwellian x
Detector count rate N ~ x40 too high
nn 31MJ Maxwell*DetEff to=5.26ms
t(ms)2 4 6 8 10 12
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Not wall background
N ~ E2
The n’s “target” varies with reactor power
n-scattering at YAGUAR
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Radiation Induced
Desorption of H2 or
H2O
Explains N ~ E2
Image courtesy of Arno Shindlmayr, Universitat Paderborn
n-scattering at YAGUAR
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nn data fit well by Maxwellian x x n
n (E) for H2
nn 31MJ Maxwell*DetEff*Sigma(H2) to=5.26ms
Time of Flight (ms)2 4 6 8 10 12
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nn data fit well by Maxwellian x x n
n (E) for H2O
nn 31MJ Maxwell*DetEff*Sigma(H2O) to=5.26ms
Time of Flight (ms)2 4 6 8 10 12
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n-scattering at YAGUAR
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MR/hr 105.2 Gy103.6 63 pulseD
MR/hr cml/- torr105.4 211 sqr
Dobrozemsky, NIM 118 (1974) 1 - 37
n-scattering at YAGUAR
312 cmmolecules/ 103~ gasn
Desorption rateunbaked Al
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effgasngasD TVnN
n-scattering at YAGUAR
6
3
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105.4
ms 9.0
cm 1131
b 80
sn/cm 108.0
counts 4200
2
eff
nH
D
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N
313DataH cmmolecules/ 104.1~
2n
312kyDobrozehms cmmolecules/ 103~gas
n
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n-scattering at YAGUAR
Desorption induced by photons, electrons, ions is an ongoing research effortCharacterized by desorption yield, (molecules/particle) Values span many orders of magnitude
ParticleEnergyMaterialAngle of incidenceSurface treatment (polishing, baking, irradiating, coatings…)
Alon 00eV1 1010~
Alon e 00eV3 1010~
stainlessson K 1010~
23
-11
43
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03.02H
N
N
n-scattering at YAGUAR
16gas
313gas
17
217
3
2
106.1
/cm104.1
109.4
s /cm107.0
cm 1131
cm 754
N
n
N
V
A
Implied value from nn measurement
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n-scattering at YAGUAR
Molvic et al., desorption yield ~ electronic energy loss in layer near surface
K+ ions on Stainless steel68-1000keV80-88o from normal
Molvic, PRL 98 (2007) 1 - 4
ionmolecules/ 1010~ 43
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n-scattering at YAGUAR
A simple Model to relate desorption yield to energy deposit Treat each point along ion trajectory as an e- source Uniform energy deposit along ion trackExponential conversion of energy deposit to number of desorbed molecules with respect to depth in target, z=Rcos()
zo
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190
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z
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n-scattering at YAGUAR
972keV K+ ions in Stainless Steel(90o)=15,000 [Molvic]Range 3914 eV/Ang [TRIM]
=750Ang
At z= desrob 9500 molecules797keV electronic energy loss
84eV/molecule
Energy Deposit in YAGUAR…
eV 84
in deposit ~ E
z
ez
1000,15
Data: Molvic, PRL 98 (2007) 1 – 4 Bieniosek, PR ST-AB 10 (2007) 1—5
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n-scattering at YAGUAR
GEANT4 simulation of gamma/electron transportGammas incident on 2-mm thick Al slabDetect energy deposit in 0.1-m thick slabs per incident gamma
z
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n-scattering at YAGUAR
Assume Al ~ SSto 2 SS
Energy deposit per gamma in last 750-1500Ang Al in YAGUAR
0.3—0.7 eV/
Desorption yield for H2 from Al in YAGUAR if baked~0.004 – 0.008
Correcting by factor of ~10 for baked vs. unbaked*
~0.04 – 0.08
Result from nn-experiment unbaked Al~0.03
*A.G. Mathewson, CERN-ISR-VA/76-5 (1976)
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n-scattering at YAGUAR
Effect of baking stainless steel and Al (~ 6x improvement)Irradiation by Ar ions, 1018 /cm2 (> 100x improvement)
Mathewson, CERN-ISR-VA/76-5 (1976)
baking
Ar ions
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n-scattering at YAGUAR
Signal to noise in current experiment 1:40Need to reduce desorption by ~400New coatings suggest improvements of greater than 300!
Mahner, PR ST-AB 8 (2005) 1—9
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Conclusion
Initial nn measurements imply radiation-induced desorption of H2 and/or H2O in nn-collision cavity.
Model relating electronic energy deposit along depth in target to desorption yield approximates recent results of K+ ions in stainless steel.
Results from this model are consistent with implied desorption from nn experiment.
n-scattering at YAGUAR
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n-scattering at YAGUAR
Possibility of fitting combination ofMaxwellian*(nH2) and Maxwellian
Assume 20% from desorbed H2
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n-scattering at YAGUAR
Effect of baking stainless steel (~ 6x improvement)
Mathewson, CERN-ISR-VA/76-5 (1976)
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3He detector with 375 mTorr for n-4He measurements
3He detector with 375 Torr for n-n measurements
n-scattering at YAGUAR
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3He detector with 375 mTorr for n-4He measurements
detector efficiency goes as 1/v
National Nuclear Data Center, Brookhaven National Lab
n-scattering at YAGUAR
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DetEff(t) = 1-Exp(-0.34177*t)
Time of Flight (ms)1 10
Thi
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n-scattering at YAGUAR
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Fast vs. Thermal TOF spectra
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“Back Wall” Background
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• Computer modeling
Characteristics of neutron field
Detector count rate sensitivity to neutron field characteristics
Neutron background experiment
n-scattering at YAGUAR
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Background Modeling Tests
• Neutron flux measured as a function of depth in underground channel.
• Neutron flux modeled with MCNPX
• Thermal neutron flux agrees with model (3He ion. detectors)
• Fast neutron flux also agrees with modeling
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sig-H2(b) = 0.07456t^2+3.159t+17.434
Time of Flight (ms)1 3 5 7 9 11
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sigH2O (t) = 0.0133t^3-0.5798t^2+11.04t+8.367
ToF (ms2 4 6 8 10 12 14
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n-4He 27MJ Pure Maxwellian to=5.26ms
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n-4He 27MJ Pure Maxwellian to=5.26ms
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Radiation-induced
desorption?
Image courtesy of Arno Shindlmayr, Universitat Paderborn
n-scattering at YAGUAR
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Radiation-induced
desorption?
Image courtesy of Arno Shindlmayr, Universitat Paderborn
n-scattering at YAGUAR
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Thermal Neutron vs. depth
Open circles = measuredClosed circles = modeled
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Fast Neutrons vs. depth
Open circles = measuredClosed circles = modeled
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Sulfur outgassing?
Image courtesy of Arno Shindlmayr, Universitat Paderborn
n-scattering at YAGUAR