1. Alternatives to 3He for Neutron Detection
James Ely1
Edward Siciliano1, Richard Kouzes1, Martyn Swinhoe2
1. Pacific Northwest National Laboratory
2. Los Alamos National Laboratory
IAEA Workshop
March 22-24, 2011
Neutron detection is an essential aspect of interdiction of radiological threats for national security purposes since plutonium, a material used for nuclear weapons, is a significant source of fission neutrons. Radiation portal monitoring (RPM) systems, of which there are thousands deployed for national security and non-proliferation purposes, currently use 3He gas-filled proportional counters for detecting neutrons. The US supply of 3He comes almost entirely from the decay of tritium used in nuclear weapons in the US and Russia. Due to the large increase in use of 3He for national security, the supply has dwindled, and can no longer meet the demand. Consequently, a replacement technology for neutron detection is required in the very near future.
In addition to alarming on the presence of actual neutron sources, national security applications also have a strict requirement for limiting neutron false alarms produced by a detector. This constrains any possible replacement neutron detection technology not to generate false neutron counts in the presence of a large gamma ray-only source. Of the currently available neutron detection technologies, BF3-filled proportional detectors, boron-lined proportional detectors, 6Li-loaded scintillating glass fiber, or non-scintillating coated plastic fiber detectors are the possible replacements for 3He detector technology—if they are proven to have appropriate capabilities. This presentation discusses these potential alternative neutron detection technologies for deployed RPM systems; and outlines what is needed for testing these alternatives and modeling to optimize their efficiency by varying moderator geometry.
PNNL-SA-xxxxx

2. Research Project in Alternatives
DOE 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-based
Provide same level of gamma discrimination
Fit in existing detector footprint
Testing of commercial or near commercial modules
Test neutron detection capability and gamma discrimination
Several technologies appear viable
Continue testing of longer term reliability and durability

3. Research Project in Alternatives
Focus changed in FY2011
Research into safeguards applications; primarily multiplicity counters
Research optimized configurations for existing materials
Use available promising technologies
Model and simulate to optimize moderator and detector
Maximize detection of coincidence events
Minimize die-away time
Current multiplicity designs uses 3He at high pressure; significant challenge to identify suitable replacement

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

5. Cross-sections of Neutron Detector Material
Place holder to describe what we’re after
Cross-section inversely proportional to neutron energy – need moderator to slow neutrons to thermal energies 5

6. Cross-sections of Neutron Detector Material
Place holder to describe what we’re after
Relatively small cross-sections for fast neutron detection via elastic scattering 6

7. Alternative Neutron Technology
Commercially available technologies tested
BF3 filled proportional tubes
Boron-lined proportional tubes
Scintillating glass fibers loaded with 6Li
Non-scintillating fibers coated with scintillator and 6Li
Multiplicity Counters
Most promising alternatives
Boron-10 based
Lithium-6 based
Less attractive
Gadolinium-based: reaction products harder to detect and discriminate from other gammas
Fast neutron detection: small cross sections
Fission reactions: requires fissionable material

8. Neutron-Capture Kinematics for 3He &10B
Assuming Thermal Neutrons: the Lab ~ Center of Mass, and the final-state total KE in Lab ~ Q value. Equating momenta gives values below.
n + 3He  p + 3H (triton “t”)
sT (thermal) = 5330 b, sT ~ 1/KEn, Q = MeV
Using KEp + KEt = Q, => KEp = 573 keV & KEt = 191 keV
n + 10B  4He (alpha “a”) + 7Li
sT (thermal) = 3840 b, sT ~ 1/KEn
~ 6% to g.s. with Q = MeV
=> KE a = MeV & KELi = MeV
~ 94% to 7Li* with Q = MeV
=> KE a = MeV & KELi = MeV 8

9. 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 9

10. Accuracy of Reaction-Rate Method for Simulating Total Counts in 3He Tubes10

11. Considerations for BF3 Proportional Tubes
Thermal cross-section is 72% of 3He
Reaction products are higher energy than for 3He
Better gamma discrimination
High voltage requirements for BF3 proportional tubes
Increases rapidly as pressure increases
Max 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)

12. Accuracy of Reaction-Rate Method for Simulating Total Counts in BF3 Tubes12

13. Evaluation Methods for Boron-Lined Tube
“Surface Current” Method:
Available Only with MCNPX Beta 2.7b or newer
Defined 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 newer
Defined as Tally Type 8 (w/out special treatment FT8 PHL “anti-coincidence” option)
Also must have the NCIA on for the Physics options 13

14. Currents Vs. Pulse-Heights for B-Lined Tube Reaction Products14

15. Measured Response of GE Reuter Stokes Prototype Multi-Tube Detector System15

16. Efficiency of B-Lined Tube Vs. Lining Thickness16

17. Considerations for the Boron-Lined Tube
Use regular proportional gas and pressure
P-10 or similar, less than 1 atm, HV < 1000V
Increase surface area to increase efficiency
About ½ as efficient (best case) as BF3 for same size tube
For 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 capability
But not enough room in current footprint, vendors went to smaller (and more) tubes to increase the surface area
Straw tubes is one approach to maximize surface area 17

18. Neutron-Capture Kinematics for 6Li
Assuming Thermal Neutrons: the Lab ~ Center of Mass, and the final-state total KE in Lab ~ Q value. Equating momenta gives values below.
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 18

19. Lithium-6 Zinc Sulfide (Ag) Coated Material
Reaction products from 6Li generate scintillation light in the ZnS(Ag)
Matrix of 6LiF crystals, ZnS and binder
ZnS is opaque to scintillation light (thin layers only)
Light transferred in wavelength shifting material
Fibers – wavelength shifted light moves down fibers using total internal reflection
Wavelength shifting light guides
Collect light with photomultiplier tube
Complicated mechanism allows for gamma-insensitivity via pulse shape discrimination 19

20. Lithium-6 Zinc Sulfide (Ag) Coated Material
Pulses from gammas significantly different than from neutrons
Plot from LANL paper (2000 INMM conference proceedings) 20

21. Lithium-6 Zinc Sulfide (Ag) Coated Material
Lithium in ZnS matrix
Thicker layers than boron lining ( µm)
Limited by ZnS opaqueness
Estimate of amount of 6Li needed
Use layers of 6Li matrix, with wavelength shifting material
Perhaps 10x thicker per layer than optimal boron
But cross section is 4x less than 10B
→ Need multiple layers, perhaps 5-10 to be equivalent to a single 3He tube in portal application 21

22. 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 work
Efficiency -- will need ~ 10 for each 3He or 1260 tubes
Die-away time considerations?
New concept for boron – layered wire chambers?
Lithium coated material estimate
Will need ~ 10 layers for each 3He row – 30 layers

23. Lithium Coated Fibers
LANL system Neutron Capture Counter for Residues (NCCR)
3 detectors shown (12 total) with 20 layers of LiF/ZnS and wavelength shifting fibers
Good die away time (<5 µsec)

24. Multiplicity Counter Application
Currently building up MCNP models to characterize technologies
BF3 and boron-lined proportional tubes and 6Li coated wavelength shifting materials
Starting from the LANL MNCP model of the Epi-thermal Neutron Multiplicity Counter (ENMC)
Challenging to replace high pressure 3He
Boron
Straw tubes or other approach to increase surface area
But still need to minimize die-away time
Lithium
Will need many layers 24

25. Initial Model: ENMC with 3He at 10 atm
Efficiency 0.66; die-away time 23 µsec
Consistent to LANL values (0.65 and 22) 25

26. Initial Model: ENMC with 3He at 1 atm
Efficiency 0.44; die-away time 90 µsec
Not huge drop in efficiency, but significant in die-away time 26

27. Initial Model: ENMC with BF3 at 1 atm
Efficiency 0.38; die-away time 120 µsec
Efficiency ~2 less than 3He, but die-away time 6x longer 27

28. Acknowledgements Support from:
DOE NA-22 Office of Non-Proliferation and Verification, Research and Development 28