Model of the gamma ray-induced out-gassing in the nn-experiment at YAGUAR B. Crawford for DIANNA

Model of the gamma ray-induced out-gassing in the nn-experiment at YAGUAR

B. Crawford for DIANNA

May 26, 2009

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.

• 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%



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

Vacuum testing of upper section of neutron channel.

• 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


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)

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

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n-n measurement

(Poor) fit shown here is Maxwellian x


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n-n measurement

(Poor) fit shown here is Maxwellian x

Detector count rate N ~ x40 too high


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Not wall background

N ~ E2

The n’s “target” varies with reactor power


Radiation Induced

Desorption of H2 or

H2O

Explains N ~ E2

Image courtesy of Arno Shindlmayr, Universitat Paderborn


nn data fit well by Maxwellian x x n

n (E) for H2

nn 31MJ Maxwell*DetEff*Sigma(H2) to=5.26ms


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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


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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


312 cmmolecules/ 103~ gasn

Desorption rateunbaked Al

effgasngasD TVnN


6

3

218

105.4

ms 9.0

cm 1131

b 80

sn/cm 108.0

counts 4200

2

eff

nH

D

T

V

N

313DataH cmmolecules/ 104.1~

2n

312kyDobrozehms cmmolecules/ 103~gas

n


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

03.02H

N

N


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


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


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

ez

190

de

z

zo

0

90

R

z


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


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


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)


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


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

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.



Possibility of fitting combination ofMaxwellian*(nH2) and Maxwellian

Assume 20% from desorbed H2


Effect of baking stainless steel (~ 6x improvement)

Mathewson, CERN-ISR-VA/76-5 (1976)

3He detector with 375 mTorr for n-4He measurements

3He detector with 375 Torr for n-n measurements


3He detector with 375 mTorr for n-4He measurements

detector efficiency goes as 1/v

National Nuclear Data Center, Brookhaven National Lab


DetEff(t) = 1-Exp(-0.34177*t)

Time of Flight (ms)1 10

Thi

ckD

etec

tor

Effi

cien

cy(0

.5ba

r3H

e)

0.1

1


Fast vs. Thermal TOF spectra

“Back Wall” Background

• Computer modeling

Characteristics of neutron field

Detector count rate sensitivity to neutron field characteristics

Neutron background experiment


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

sig-H2(b) = 0.07456t^2+3.159t+17.434


Sig

ma

(b/p

roto

n)

20

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28

32

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48

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60

64


sigH2O (t) = 0.0133t^3-0.5798t^2+11.04t+8.367

ToF (ms2 4 6 8 10 12 14

Sig

ma

H2

O(b

/pro

ton

)

30

35

40

45

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55

60

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75

80

85


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?



Radiation-induced

desorption?



Thermal Neutron vs. depth

Open circles = measuredClosed circles = modeled

Fast Neutrons vs. depth

Open circles = measuredClosed circles = modeled

Sulfur outgassing?



Model of the gamma ray-induced out-gassing in the nn-experiment at YAGUAR B. Crawford for DIANNA

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