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Alternatives to 3 He for Neutron Detection

Alternatives to 3 He for Neutron Detection. James Ely 1 Edward Siciliano 1 , Richard Kouzes 1 , Martyn Swinhoe 2 1. Pacific Northwest National Laboratory 2. Los Alamos National Laboratory IAEA Workshop March 22-24, 2011. PNNL-SA- xxxxx. Research Project in Alternatives.

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Alternatives to 3 He for Neutron Detection

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

  6. Cross-sections of Neutron Detector Material • Relatively small cross-sections for fast neutron detection via elastic scattering

  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 = 0.764 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 = 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 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 Tubes 10

  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 Tubes 12

  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 Products 14

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

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

  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 (100-500 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-linedproportional 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

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