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Gamma Insensitive, Highly Borated Microchannel Plates for Neutron Imaging

This concept paper explores the development, design, and supply of specialized neutron-sensitive Microchannel Plates (MCPs) and MCP collimators by NOVA Scientific, Inc. Potential applications for these compact MCPs include high-resolution neutron imaging, moderate-resolution imaging, MCP collimators to reduce scattering, and the detection of Special Nuclear Materials (SNM).

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Gamma Insensitive, Highly Borated Microchannel Plates for Neutron Imaging

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  1. Gamma Insensitive, Highly Borated Microchannel Plates for Neutron Imaging Bruce Feller, Paul White, Brian White Nova Scientific, Inc. Sturbridge, MA USA www.novascientific.com NOVAScientific, Inc. WCNR-8

  2. NOVA Scientific, Inc. Neutron-sensitive MCPs: detectors and collimators Concept paper Univ. Leicester space scientists, early 1990’s. Li in low-noise Philips MCP glass “a problem” in orbit. “On the other hand…” Nova first implemented, then extended the concept of Li, B, Gd doping, brought to attention of neutron community. Numerous glass engineering ‘landmines’ to overcome: 10B or Gd-loaded MCPs – over 100 manufacturing steps. IP coverage. Develops, designs and supplies • Specialized MCP detectors and novel variants (Microsphere, MicroFiber plates). MCP collimators (spinoff). For… • Research, industrial markets, government applications

  3. Potential Applications of Compact Neutron-Sensitive Electron Multipliers and Glasses • High Resolution Neutron Imaging (~10 μm) • Moderate resolution imaging(~100 μm) • MCP collimators to reduce scattering • Potential for detection of SNM

  4. Neutron Imaging Microchannel Plate Formats (courtesy Photonis/Burle)

  5. MicroChannel Plate Operation

  6. 10B incorporated into bulk glass Alpha and 7Li particles create secondary electrons in channel Secondaries amplified to pulse-counting levels (105-106 e-) Low background (<.04 cm-2s-1) Direct neutron detection: no photon conversion, films, scintillators Neutron  e- 7Li e- e- e- e- 10B Secondary Electrons coating to release e- MCP Neutron Detection with 10B

  7. Front Surface vs. Bulk Detection Input Neutron Flux, η Neutron Reaction Plane (Can occur at any depth) Output Electron Flux

  8. Thermal Neutron Detection Efficiency with 10B

  9. Neutron-Sensitive Solid-State Detection with Gd • Gd incorporated into bulk glass • High cross-section for thermals • Conversion electrons fire channel • n + 155Gd --> 156Gd + γ + conversion electrons Q-value (7.9 MeV), and n + 157Gd --> 158Gd + γ + conversion electrons Q-value (8.5 MeV)

  10. MCP neutron sensitivity, Gd vs. 10B

  11. Gd-doped MCPs – conversion e-’sinitiate avalanches

  12. Gd gamma ray emission

  13. 2nd stacked multifiber bundle, which is fused, cut into disks and polished 2nd draw Final MCP MCP manufacturing Glass mixture with 10B2O3 or natGd2O3 added 1st multifiber bundle 1st draw Chemical etch of core glass

  14. High Resolution Neutron Imaging • B or/(and) Gd doped MCPs with electronic readout • Electronic readouts employed can include: • Cross-delay line, Cross-strip (Sensor Sciences), • Medipix (CERN-Cal collaboration) • Conventional phosphor screen/CCD

  15. Neutron-Sensitive MCP Imaging – 25mmearly NIST image (phosphor/CCD)

  16. NIST-Sponsored High-Resolution MCP Neutron Imaging • SBIR Ph. 1 - recent tests with cross-strip Sensor Sciences readout yield ~15 μm rms

  17. Next Generation High-Resolution Neutron Imaging MCP, NIST Ph.II SBIR • Goal ~10 µm spatial resolution • Up to 50% thermal neutron sensitivity • Gamma background suppression • High count rate > 1 MHz • MCP collimator between object and detector for lower scatter • but...’physics brick wall’ at ~10 microns, set by MCP structure and reaction particle ranges.

  18. Gamma noise mitigation • Gamma background hurts SNR • MCP gamma detection QE now 1-2%, due to high-Z components, mainly Pb, Ba • 3-4x Pb reduction: glass now lower Z • 10-5 reduction: electronic discrimination, add shielding, collimation • “3He (or other) or bust”: can a solid state detector ever rival a gas tube?

  19. Next Generation Glasses – “low-Z” • Higher boron, NVN-7 glass (lead-free) • Novel glass MCP system allows for high efficiency, low-noise system • Clear advantage for low flux neutron detection, but demonstration for high-resolution imaging TBD. Electron Multiplier Glass High Boron, Low-Z NVN-7 Glass

  20. NVN-7 Microchannel Plate • Finished microchannel plate blank, not processed yet for electron multiplication. • High mag photograph showing both electron multiplier glass and high-boron NVN-7 glass (core glass removed)

  21. Superclad Construction • Unique three glass system • First superclad MCP built, now in evaluation

  22. 10B capture gamma ray energy 478 keV Coincidence detection with neutron (MCP pulse), 478 keV gamma within ~ 10 ns gate = valid neutron event Others rejected (bkgd gammas, noise, etc) LaBr3 Gamma Detector LaBr3 Gamma Detector Nova / Sensor Sciences: Electronic neutron-gamma discrimination: 478 keV γ Neutron  e- 7Li e- e- e- e- 10B Secondary Electrons coating to release e-

  23. Image representation of neutron-gamma (478 keV) coincidence timing.“First light” MNRC data, w/25 ns window. Sensor Sciences collaboration. Right side Outside Coincidence Gating time window Coincidencegating can be veryeffective!!!! Left side Inside Coincidence Gating time window

  24. Next-Generation Neutron-SensitiveElectron Multipliers • For Lower Cost, Large Area (>> 100 cm2) • Utilizes 10B/Gd Incorporation, also glass • Microsphere, Microreticulated, MicroFiber • All Demonstrated to Detect Neutrons • Development Focus on MicroFiber Approach

  25. MicroSphere Plates (MSPs) • DOE earlier program • Neutron sensitive spheres • Fused into large area • Low resolution imaging

  26. MicroFiber Plates (MFPs) • Neutron sensitive fibers • Same operation as MCP • Large Area, shapeable

  27. Oriented MicroFiber Plates • CAD modeling showing cutaway view and top-down view

  28. Oriented MicroFiber Plates • More uniform response than standard MFP • Improved gain through fiber spacing control • Monte Carlo model for gain • Moderate resolution imaging (200-400μm) • Potential high-volume weaving into large sheets (>8x10”, etc.)

  29. Modeled MFP Detection Efficiency.025 eV neutrons

  30. Neutron Collimators • Gd-doped MCP glass • Very high scattered thermal neutron absorption (up to 100%) • Identical MCP processing, but simpler (a spinoff): no need for activated electron multiplier processing

  31. MCP “walls” absorb neutrons MCP glass contains absorbing material

  32. y D P x LMCP Core glass absorbs neutrons Walls are made of low absorbing glass Core glass contains absorbing material

  33. UPCOMING EFFORT NEUTRON-GAMMA DISCRIMINATION: • Goal to rival gas detectors, ~10-6 gamma-neutron ratio • Refine gamma coincidence technique - several ns timing window. • Even deeper gamma rejection potential using <ns PSD: but may not be practical – or necessary. HIGH RESOLUTION NEUTRON IMAGING: • NIST Ph. II SBIR, goal of 10 micron spatial resolution with improved gamma rejection. MICROFIBER PLATE: Higher gain targeted, >104 for high SNR pulse-counting. • Modify fiber cross-section for higher gain, add surface coating • Gamma rejection techniques adapted • Fiber weaving with manufacturing precision, large thin sheets (~3 mm). • Larger format vacuum housing design.

  34. Acknowledgements NIST (DoC), DoD, DoE, – SBIR programs Sensor Sciences O. Siegmund A. Martin UCal-Berkeley A. Tremsin NIST M. Arif D. Hussey D. Jacobson G. Downing D. Mildner

  35. Some Recent Papers • “The Theory of Compact and Efficient Circular Pore MCP Neutron Collimators,” Nuclear Instruments and Methods A (2006), in press. • “Neutron Collimation with MCPs: Calibration of Existing Technology and Near Future Possibilities,” IEEE Transactions Nuclear Science 54 (2005). • “Efficiency Optimization of Microchannel Plate (MCP) Neutron Imaging Detectors. I. Square Channels with 10B Doping,” Nuclear Instruments and Methods in Physics Research, A 539 (2005) 278-311. • “The Efficiency of Thermal Neutron Detection and Collimation with Microchannel Plates of Square and Circular Geometry,” IEEE Nuclear Science Symposium, Rome Italy, October 2004. • “Very Compact High Performance Microchannel Plate Neutron Collimators,” IEEE Transactions Nuclear Science 51 (N3) 2004 1020-1024. • “Large Area Microchannel Plate Detector with Amorphous Silicon Pixel Array Readout for Fast Neutron Radiography,” Nuclear Instruments Methods, Vol. A 500, pp 351-361, 2003. • “Neutron Detection and Characterization Using Electron Cascade Multipliers,” DOE Spallation Neutron Source Detector Group, University of Indiana, May 2003. www.novascientific.com

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