3 He Neutron Detection Alternatives for Radiation Portal Monitors

1. 3He Neutron Detection Alternatives for Radiation Portal Monitors Richard Kouzes Ken Conlin, James Ely, Luke Erikson, Azaree Lintereur, Emily Mace, Edward Siciliano, Daniel Stephens, David Stromswold, Renee Van Ginhoven, Mitch Woodring Pacific Northwest National Laboratory Work Supported by DOE, DOD, DHS, PNNL IAEA 3He Workshop March 22-24, 2011 PNNL-SA-77910

2. The 3He Problem • National security and science applications have driven up demand for 3He for neutron detection • Currently, 3He comes solely from the processing of tritium • No significant production of new tritium • Production of tritium solely for 3He need is cost prohibitive • Reserves of 3He have been consumed • Projected 3He Supply ~10-20 kL/y (U.S.& Russia) • Demand for 3He was ~65 kL/y – now reduced • Nothing matches all of the capabilities of 3He • An alternative is needed now 2 3He Tubes 2

3. 3He Applications • 3He is a rare isotope with important uses in: • Neutron detection • science • national security • safeguards • oil/gas exploration • Industrial applications • Low-temperature physics • Lung imaging • Missile guidance • Laser research • Fusion 3

4. 3He Characteristics • 3He is excellent for neutron detection • Large thermal neutron capture cross-section • Inert gas • Good gamma ray rejection 4

5. 3He Demand Forecast: FY09 Projected demand ~65 kL/y - Projected Supply ~10-20 kL/y Supply 5 Data From Steve Fetter, OSTP

6. 3He Demand Forecast: FY1 6 Plot From Julie Bentz, National Security Staff

7. Border Security Examples • Over 1400 RPM systems deployed in US • About 3000 RPM systems deployed worldwide • Neutron and gamma ray detection 7

8. Alarms and “Nuisance” Alarms • Few sources of Neutron Alarms (~1/10,000) • Troxler gauges, well logging sources, nuclear fuel, yellowcake • Nuisance alarms: large gamma ray sources and “ship effect” • Gamma Ray Nuisance Alarms (~1/100) • agricultural products like fertilizer • kitty litter • ceramic glazed materials • aircraft parts and counter weights • propane tanks • road salt • welding rods • ore and rock • smoke detectors • camera lenses • televisions • medical radioisotopes Troxler Gauge 8

9. Requirements for Neutron Detection for National Security • Plutonium emits detectable quantities of neutrons • Neutron background arises from cosmic ray produced secondaries and is a very low rate (~1000 times smaller than gamma ray background) • Neutron alarms initiate a special Operating Procedure • Fast and slow neutron detection required with flat response • Absolute efficiency per panel: єabs = 0.11% or 2.5 cps/ng 252Cf • Gamma ray discrimination of better than 10-6 • Maintain neutron detection efficiency in presence of gamma rays: gamma absolute rejection ratio (0.9 < GARRn < 1.1) • Meet all ANSI N42.35/N42.38 requirements 9

10. Requirements for Alternative Neutron Detection for National Security • Physically fit in the volume currently occupied by the neutron detection assembly in existing systems • Electronics compatible with existing system • Thermal and fast neutron detection • Non-responsive to gamma rays • Rugged, reliable, and accurate • Safe • Inexpensive • Readily available commercially now 10

11. Alternative Neutron Detectors • Proportional Counter Alternatives • BF3 filled proportional counters • Boron-lined proportional counters • Scintillator-based Alternatives • Coated wavelength shifting fibers/paddles • Scintillating glass fibers loaded with 6Li • Crystalline: LiI(Eu), LiF(W), Li3La2(BO3)3(Cr) • Liquid scintillator • Semiconductor Neutron Detectors in Development • Gallium arsenide, perforated semiconductor, boron carbide, boron nitride, pillar-structured detectors • High efficiency, but limited in size • Other: doped glasses, Li-foil ion chamber, Li phosphate nanoparticles, fast neutron detectors 11

12. Existing Commercial Alternative Neutron Detectors • Proportional Counter Alternatives • BF3 filled proportional counters • Boron-lined proportional counters • Scintillator-based Alternatives • Plastic fiber/paddle light-guides coated with ZnS scintillator and 6Li neutron absorber • Scintillating glass fibers loaded with 6Li • Systems from 9 vendors tested 12

13. Boron-based Detectors “Straw tube” designs (Proportional Technology) Boron lined (Reuter Stokes) BF3 (LND) Multi-chamber boron lined approaches LND Centronic

14. BF3 Proportional Counters • Neutrons captured by the 10B (>90%) yields α + 7Li • Gas pressure must be low (0.5 to 1.0 atm.) to operate at reasonable voltages (2000-2500 V) • Cross-section ~70% that of 3He • Advantages • Inexpensive direct replacement for 3He • Better gamma-neutron separation than 3He • Disadvantages • BF3 is toxic, difficult to purify, degrades over time, and is corrosive to the gas enclosure • Subject to strict DOT shipping regulations • Requires the use of multiple tubes to meet capability • Requires changes to electronics 14

15. Boron-Lined Proportional Counters • Disadvantages • Counting efficiency is lower than that of either 3He or BF3 • More variation in pulse height • Requires the use of multiple tube assembly to meet efficiency requirement 15 • Similar detection mechanism to BF3 (yields α + 7Li) • Boron in matrix on walls; more signal amplitude spread • Advantages • New prototypes promise needed efficiency • Better gamma-neutron separation than 3He • Direct tube replacement for 3He • Only minor electronics changes

16. ZnS + 6Li-coated Light-guide Detectors • Paddles or fibers coated with ZnS scintillator mixed with 6Li • Advantage • Comparable performance to 3He tube(s) • Disadvantages • Gamma-ray discrimination as tested required improvement for fiber version • Possible significant change to electronics Coated Paddles (Symetrica) Coated Fibers (IAT) Coated Paddles (SAIC)

17. 6Li Loaded Glass Fibers • 6Li-enriched lithium silicate glass fibers doped with cerium (Bliss et al. 1995, PNNL) • Neutron capture on 6Li produces charged particles that cause Ce ions to fluoresce (observed by photomultiplier tubes) • Advantages • Comparable performance to one 3He tube • Fibers can be formed into different shapes • Disadvantages • Less gamma-ray discrimination than 3He • Possible significant change to electronics 17

18. PNNL Neutron Detector Testing • Measurements of neutron efficiency have been carried out at PNNL for standard deployable RPM systems. • Testing of alternatives: • 3He at pressures of 1.0, 2.0, 2.5 and 3 atmospheres • BF3 filled proportional counter tubes • Boron-lined proportional counters • ZnS-6Li coated plastic fibers/paddles • Glass fibers loaded with 6Li 18

19. BF3 Results • Detection efficiency (cps/ng) for shielded source • Uncertainty primarily due to uncertainty in source activity ASP spec RPM spec ANSI N42.35

20. Boron-lined Neutron Detection • Modeled with MCNP • Good qualitative agreement with data

21. Boron-LinedGamma Discrimination • Insensitive to 60Co gamma rays (~10-8) • Good neutron efficiency with gamma ray discriminating threshold

22. ZnS + 6Li-coated Fiber Signal • Neutron and gamma pulse from IAT system • Differences in pulse shape allow for pulse-shape discrimination Neutron Pulse Gamma Pulse

23. All options will require hardware and software modifications Summary of Technology Testing Meets Requirement Does Not Meet Requirement

24. Conclusions • Applications for 3He are diverse • Demand is greater than supply • The national security need for an alternative is immediate • Four alternative neutron detection technologies have been tested • Alternatives for RPM systems can meet the technical requirements for national security applications

25. Support Work supported by: DOE NNSA DoD DHS DNDO PNNL Thank you!

26. Backup 26

27. 3He Supply • 3He not currently extracted from natural supplies • Primordial abundance of 3He:4He is 1:10000 • 1.4 ppm by volume atmospheric He • 0.2 ppm by volume natural-gas He (fission product) • Lunar sources • By-product of nuclear weapons program • Tritium was produced for nuclear weapons in reactors • Tritium production in U.S. ended in 1988 since weapon needs met through reductions in weapon stockpile, recycle • Tritium production restarted in U.S. in 2007 only to support smaller stockpile • Tritium decays with 12.4-year half-life to to 3He • Separated 3He made available by DOE SC/NP Isotope Program • U.S. accumulated 200,000 liters of 3He by the end of 1990s • Decay produces ~8000 liters/year of 3He in U.S. 27

28. Estimate of Supply and Demand 28 Data from Steve Fetter, OSTP

29. 3HeFive Year Usage: All Applications Data from Linde Electronics and Specialty Gases From Ron Cooper, ORNL

30. 3He Demand – AAAS Study • Neutron Scattering: 120,000 liters over the next five years • Homeland Security: • Historically large • 1000 – 2000 liters / year for 5 years • Dropping to zero once alternative technologies become available • Medical Imaging: 2000 liters / year • Cryogenics: 2500 – 3000 liters / year • Oil and gas exploration: 2000 liters / year • DOE “emergency response assets”: few 1000 liters / year • Other fields: each require a few hundred liters / year 30

31. Comparative Results GRR = Gamma Ray Rejection GARRn = Gamma Absolute Rejection Ratio BT = Better Than PC = proportional counter MTPC = multi-tube (or multi-chamber) proportional counter