Alternatives to 3He for Neutron Detection

James Ely1

Edward Siciliano1, Richard Kouzes1, Martyn Swinhoe2

1. Pacific Northwest National Laboratory2. Los Alamos National Laboratory

IAEA WorkshopMarch 22-24, 2011

PNNL-SA-xxxxx

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Research Project in AlternativesDOE NNSA Office of Non-Proliferation (NA-22)

Project initiated in FY2009

Focus on commercially available technologies For use in portal monitor applications

Provide same neutron detection capability as 3He-basedProvide same level of gamma discriminationFit in existing detector footprint

Testing of commercial or near commercial modulesTest neutron detection capability and gamma discriminationSeveral technologies appear viableContinue testing of longer term reliability and durability

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Research Project in Alternatives

Focus changed in FY2011 Research into safeguards applications; primarily multiplicity countersResearch optimized configurations for existing materials

Use available promising technologiesModel and simulate to optimize moderator and detector

Maximize detection of coincidence eventsMinimize die-away time

Current multiplicity designs uses 3He at high pressure; significant challenge to identify suitable replacement

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Example Multiplicity Counter

Canberra Large Epi-Thermal Multiplicity Counter (LEMC)126 3He tubes at 10 atm (1 inch dia. By 30 inches long)

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Cross-sections of Neutron Detector Material

Cross-section inversely proportional to neutron energy – need moderator to slow neutrons to thermal energies

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10

100

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1000000

1.E-08

1.E-07

1.E-06

1.E-05

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1.E-01

1.E+00

1.E+01

Rea

ctio

n C

ross

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(bar

ns)

Neutron Energy (MeV)

Elementary Cross-Section Data

3He(n,tot) 6Li(n,tot)

10B(n,tot) 157Gd(n,tot)

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Cross-sections of Neutron Detector Material

Relatively small cross-sections for fast neutron detection via elastic scattering

0.1

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1.E-07

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n C

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Neutron Energy (MeV)

Elementary Cross-Section Data

3He(n,tot) 3He(n,el)

4He(n,el)

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Alternative Neutron TechnologyCommercially available technologies tested 1. BF3 filled proportional tubes2. Boron-lined proportional tubes3. Scintillating glass fibers loaded with 6Li 4. Non-scintillating fibers coated with scintillator and 6Li

Multiplicity Counters Most promising alternatives

Boron-10 basedLithium-6 based

Less attractiveGadolinium-based: reaction products harder to detect and discriminate from other gammasFast neutron detection: small cross sectionsFission reactions: requires fissionable material

Neutron-Capture Kinematics for 3He &10B

n + 3He p + 3H (triton “t”) sT (thermal) = 5330 b, sT ~ 1/KEn, Q = 0.764 MeV Using KEp + KEt = Q, => KEp = 573 keV & KEt = 191 keV

n + 10B 4He (alpha “a”) + 7LisT (thermal) = 3840 b, sT ~ 1/KEn

~ 6% to g.s. with Q = 2.792 MeV => KE a = 1.777 MeV & KELi = 1.015 MeV

~ 94% to 7Li* with Q = 2.310 MeV=> KE a = 1.470 MeV & KELi = 0.840 MeV

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Assuming Thermal Neutrons: the Lab ~ Center of Mass, and the final-state total KE in Lab ~ Q value. Equating momenta gives values below.

Evaluation Method used for 3He & BF3

Modeling and Simulation using MCNP

“Reaction Rate” Method Defined as MCNP5 or MCNPX Tally Type 4 (Cell-Averaged Flux) with the Tally Multiplier Option for Reactions

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Accuracy of Reaction-Rate Method for Simulating Total Counts in 3He Tubes

0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0 3.21.5

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2.5

3

3.5

f(x) = NaN ln(x) NaNR² = NaN Logarithmic Fits to 3He Tube Data Compared to Model Predictions

One-Tube AverageLogarithmic (One-Tube Average)

Pressure in AtmospheresCou

nts

per s

econ

d pe

r nan

ogra

m 2

52C

f

10

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Considerations for BF3 Proportional Tubes

Thermal cross-section is 72% of 3HeReaction products are higher energy than for 3He

Better gamma discrimination

High voltage requirements for BF3 proportional tubesIncreases rapidly as pressure increasesMax pressure ~ 1 atm to keep HV below 2-3 kV

→ to replace 3 atm 3He tube, will need ~ 3 tubes of BF3 at ~ 1atm (same size)

Accuracy of Reaction-Rate Method for Simulating Total Counts in BF3 Tubes

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Evaluation Methods for Boron-Lined Tube

“Surface Current” Method:Available Only with MCNPX Beta 2.7b or newerDefined as Tally Type 1 (Surface-Averaged Current) with the Neutron Capture Ion Algorithm (NCIA) on for the Physics options

“Pulse-Height” Method:Also available Only with MCNPX Beta 2.7b or newerDefined as Tally Type 8 (w/out special treatment FT8 PHL “anti-coincidence” option) Also must have the NCIA on for the Physics options

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Currents Vs. Pulse-Heights for B-Lined Tube Reaction Products

0.05 0.25 0.45 0.65 0.85 1.05 1.25 1.45 1.65 1.850.0E+00

2.0E-07

4.0E-07

6.0E-07

8.0E-07

1.0E-06

1.2E-06

1.4E-06

1.6E-06

1.8E-06

2.0E-06Alpha Current Into Gas Alpha PH in GasLi7 Current Into Gas Li PH in GasTotal Current Into Gas Reaction KE Values

Energy Bins (MeV)

Cou

nts

per e

mitt

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erut

ron

per 1

0keV

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Measured Response of GE Reuter Stokes Prototype Multi-Tube Detector System

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Efficiency of B-Lined Tube Vs. Lining Thickness

0.0 2.0 4.0 6.0 8.0 10.0 12.0 14.0 16.0 18.0 20.00.01%

0.03%

0.05%

0.07%

0.09%

0.11%

B10-Lined Tube 1 atm. BF3 (96% B10)

Lining Thickness in Micro-Meters

Cou

nts

per e

mitt

ed N

eutro

n

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Considerations for the Boron-Lined Tube

Use regular proportional gas and pressureP-10 or similar, less than 1 atm, HV < 1000V

Increase surface area to increase efficiencyAbout ½ as efficient (best case) as BF3 for same size tubeFor portal applications, needed 3 BF3 tubes to be equivalent to a single 3He tube at 3 atm, therefore, would need ~ 6 boron-lined tubes for equivalent capabilityBut not enough room in current footprint, vendors went to smaller (and more) tubes to increase the surface areaStraw tubes is one approach to maximize surface area

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Neutron-Capture Kinematics for 6Li

n + 6Li 4He (alpha “a”) + 3H (triton “t”)

sT (thermal) = 940 b, sT ~ 1/Ken, Q = 4.78 MeV => KE a = 2.05 MeV & KEt = 2.73 MeV

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Assuming Thermal Neutrons: the Lab ~ Center of Mass, and the final-state total KE in Lab ~ Q value. Equating momenta gives values below.

Lithium-6 Zinc Sulfide (Ag) Coated Material

Reaction products from 6Li generate scintillation light in the ZnS(Ag)

Matrix of 6LiF crystals, ZnS and binderZnS is opaque to scintillation light (thin layers only)

Light transferred in wavelength shifting materialFibers – wavelength shifted light moves down fibers using total internal reflectionWavelength shifting light guides

Collect light with photomultiplier tube

Complicated mechanism allows for gamma-insensitivity via pulse shape discrimination

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Lithium-6 Zinc Sulfide (Ag) Coated Material

Pulses from gammas significantly different than from neutrons

Plot from LANL paper (2000 INMM conference proceedings)

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Lithium-6 Zinc Sulfide (Ag) Coated Material

Lithium in ZnS matrixThicker layers than boron lining (100-500 µm)

Limited by ZnS opaqueness

Estimate of amount of 6Li neededUse layers of 6Li matrix, with wavelength shifting material

Perhaps 10x thicker per layer than optimal boronBut cross section is 4x less than 10B

→ Need multiple layers, perhaps 5-10 to be equivalent to a single 3He tube in portal application

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Considerations for Multiplicity Counter

Canberra Large Epi-Thermal Multiplicity Counter (LEMC)126 3He tubes at 10 atm (1 inch dia. By 30 inches long)

BF3 estimate from portal workEfficiency -- will need ~ 10 for each 3He or 1260 tubesDie-away time considerations?

New concept for boron – layered wire chambers?

Lithium coated material estimateWill need ~ 10 layers for each 3He row – 30 layers

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Lithium Coated FibersLANL system Neutron Capture Counter for Residues (NCCR)

3 detectors shown (12 total) with 20 layers of LiF/ZnS and wavelength shifting fibersGood die away time (<5 µsec)

Multiplicity Counter Application

Currently building up MCNP models to characterize technologies

BF3 and boron-lined proportional tubes and 6Li coated wavelength shifting materialsStarting from the LANL MNCP model of the Epi-thermal Neutron Multiplicity Counter (ENMC)

Challenging to replace high pressure 3HeBoron

Straw tubes or other approach to increase surface areaBut still need to minimize die-away time

LithiumWill need many layers

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Initial Model: ENMC with 3He at 10 atm

Efficiency 0.66; die-away time 23 µsecConsistent to LANL values (0.65 and 22)

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y = 1.46E-01e-4.37E-02x

R² = 9.90E-01

1.E-06

1.E-05

1.E-04

1.E-03

1.E-02

1.E-01

1.E+00

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t Effi

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Time (micro seconds)

ENMC (10 Atm 3He): Die-Away Time, 252Cf

Total

E-Bin 0 to 0.25 eV

E-Bin 0.25 ev to 20.0 MeV

Exp.Fit to Total

Initial Model: ENMC with 3He at 1 atm

Efficiency 0.44; die-away time 90 µsecNot huge drop in efficiency, but significant in die-away time

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y = 2.68E-02e-1.11E-02x

R² = 9.71E-01

1.E-06

1.E-05

1.E-04

1.E-03

1.E-02

1.E-01

1.E+00

0 10 20 30 40 50 60 70 80 90 100 110

Coun

t Effi

cien

cy

Time (micro seconds)

ENMC (1Atm 3He): Die-Away Time, 252Cf

Total

E-Bin 0 to 0.25 eV

E-Bin 0.25 ev to 20.0 MeV

Exp.Fit to Total

Initial Model: ENMC with BF3 at 1 atm

Efficiency 0.38; die-away time 120 µsecEfficiency ~2 less than 3He, but die-away time 6x longer

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y = 1.82E-02e-8.31E-03x

R² = 9.46E-01

1.E-06

1.E-05

1.E-04

1.E-03

1.E-02

1.E-01

1.E+00

0 10 20 30 40 50 60 70 80 90 100 110

Coun

t Effi

cien

cy

Time (micro seconds)

ENMC (1Atm 96% 10B): Die-Away Time, 252Cf

Total

E-Bin 0 to 0.25 eV

E-Bin 0.25 ev to 20.0 MeV

Exp.Fit to Total

AcknowledgementsSupport from:

DOE NA-22 Office of Non-Proliferation and Verification, Research and Development

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