Large Area, Low Cost PMTs

1. Large Area, Low Cost PMTs Neutrinos and Arms Control Workshop Paul Hink 6 February 2004 BURLE INDUSTRIES

2. BURLE INDUSTRIES Overview BURLE INDUSTRIES, INC. Conversion Tubes Power Tubes Real Estate BURLE ELECTRO-OPTICS, INC. BURLE INDUSTRIES GmbH BURLE INDUSTRIES UK LIMITED BURLE deMexico Neutrinos and Arms Control Workshop

3. Core Competencies • Conversion Tubes, Lancaster PA • Conventional PMT design and fabrication • Photocathode processing • Image tube design and fabrication • PMT packaging • Electronics: VDN, Miniature HVPS, Front-end electronics • Power Tubes, Lancaster PA • Design and fabrication of vacuum tubes for power generation and switching • Plating and environmental testing • Ceramic-to-Metal joining techniques • BEO, Sturbridge MA • Microchannel plates • Channel multipliers • Fiber optics Neutrinos and Arms Control Workshop

4. PMT Construction/Processing • Electron multiplier is supported by bulb spacers and leads to the stem • Envelope is evacuated through an exhaust tubulation • Cathode processed in-situ with Sb and alkali dispensers • Tip-off of tubulation using flame or electric oven Photocathode Bulb Dynode Structure Sb bead Alkali Channels Stem Exhaust tubulation Neutrinos and Arms Control Workshop

5. Manufacturing • Discrete multiplier fabrication is labor intensive • Materials and processes are critical • Sealing of bulb to stem assembly is semi-automated • Multiple PMTs can be processed simultaneously on an exhaust system • Post-exhaust processing can be done in large batches • Testing is semi-automated Neutrinos and Arms Control Workshop

6. Two inch square flat PMT with dual MCP multiplier. Anodes, 2x2 and 8x8 configurations. Additional configurations available. Bi-alkali cathode on quartz faceplate or cryogenic bi-alkali. Intrinsically low radioactivity Planacon™ MCP-PMTs Neutrinos and Arms Control Workshop

7. MCP-PMT Operation photon Faceplate Photocathode Photoelectron DV ~ 200V Dual MCP DV ~ 2000V Gain ~ 106 DV ~ 200V Anode Neutrinos and Arms Control Workshop

8. MCP-PMT Construction Indium Seal Faceplate Ceramic Insulators MCP Retainer Dual MCP Anode & Pins Cathode is processed separate from multiplier and sealed to body under vacuum Neutrinos and Arms Control Workshop

9. Transfer Cathode Manufacturing • Large UHV chambers required • Parts and materials preparation critical • Movement of parts inside vacuum chamber(s) difficult • Only a few PMTs can be processed simultaneously • Indium sealing is reliable but expensive material costs. • Alternative sealing techniques available. • Can become highly automated, but large capital investments required Neutrinos and Arms Control Workshop

10. photon Faceplate Photocathode Photoelectron DV ~ 10,000V Electron Detector Hybrid Photodetectors • Photo-electron bombarded electron detector • Can use Silicon detector, APD, scintillator + light detector, … • Excellent single pe resolution • Typically requires very high accelerating voltage Neutrinos and Arms Control Workshop

11. Gas Electron Multipliers • Vacuum not required • Low cost envelope • Excellent single pe resolution • Degradation of photocathode/gas in sealed devices needs to be addressed • Solid cathode must be made under vacuum photon Faceplate Photocathode Photoelectron Neutrinos and Arms Control Workshop

12. Large Area PMT Program • Selected for a DOE SBIR to develop a large area PMT • Phase I began in July 2003 and has proceeded well • Focus is to design a low-cost bulb that can be manufactured using automated techniques and allows the use of simple electron-optics • Also investigating low-cost processing techniques Neutrinos and Arms Control Workshop

13. Requirements Neutrinos and Arms Control Workshop

14. Arms Control PMT Assumptions • 4km of water (~400 atmospheres) • 40% coverage on a 10Megaton detector, or ~8 x 104 m2 of photocathode area • Requires ~ 400,000 PMTs having 2000 cm2 of projected area (~20” diameter) • Production of 40M PMTs for 1 array, $6B at $200 per PMT ($0.10 / cm2) • Maintain performance of existing PMTs, including QE, Dark Counts, and Timing Neutrinos and Arms Control Workshop

15. Traditional PMT Approach • Vacuum Envelope is critical to the performance of the PMT • Envelope must withstand 400 atmospheres unless a separate pressure vessel is used, which will add cost. • Window must be made out of low cost glass, resistant to ultra-pure H2O, low strain (low expansion Borosilicate) • Remainder of the vacuum envelope can be made of glass or appropriate metal or composite. • Vacuum envelope requires electrical feed-throughs for bias, signal, and processing of photocathode. Neutrinos and Arms Control Workshop

16. Traditional PMT Sealing • Vacuum Envelope can be sealed using a flame. However, annealing temperatures are too high to be done with the multiplier and cathode processing materials in the PMT. • Vacuum envelope can be welded together if appropriate flanges have been attached to the glass sections. Residual strain a problem. • Low temperature glass process yields high strength bonds, but tight flatness tolerance on seal surfaces. • Vacuum feed-throughs must be inserted and annealed. Minimum number of feed-throughs is desired • Exhaust tubulation typically has some residual strain and will be a weak point. Could use a metal tubulation Neutrinos and Arms Control Workshop

17. Multiplier Selection • Standard discrete dynode structure is hard to automate and labor intensive. • Spherical bulb, while strongest, is not ideal for good timing performance. Electron Optics design work required • Silicon detector or APD has the fewest feed-throughs required • Micro-channel plate multiplier is simple and also has fewer feed-throughs. In high volume may approach the cost of silicon detectors. Neutrinos and Arms Control Workshop

18. Glass Hemisphere Photoelectron APD Focus Element Leads Tubulation Example of Spherical PMT • Schott produces hemispheres of size needed • Sealed with low temperature process • 18mm wall good to >4km depth • Silicon electron detector preferred Neutrinos and Arms Control Workshop

19. Example Fabrication Flow • Glass hemispheres are formed out of a melt • Sealing surfaces are machined to high tolerance • Feed-throughs are installed in one hemisphere • Both hemispheres are annealed • Multiplier/Detector, electron optics, and process materials are installed on rear hemisphere • Low temperature glass-to-glass seal of two halves • Tube is exhausted and processed • Tubulation is tipped-off • PMT is aged and tested Neutrinos and Arms Control Workshop

20. Design Challenges • High precision machining of the hemisphere seal surfaces (few microns) • Installation of electrical feed-throughs and tubulation • Annealing the rear hemisphere to remove all strain • Exhaust and processing of tube needs to be automated with no handling of PMTs between processes. Neutrinos and Arms Control Workshop

21. Feasibility of this Design • Requires significant glass manufacturing capabilities. At 28kg per bulb, three 1Gton arrays, and 15 years to manufacture, ~8 large glass lines (100,000 kg/day/line) would be required. • Raw glass cost could be ~$15 per bulb. • Hemispheres could be pressed/molded out of the melt or vacuum formed out of float glass. • Precision machining and grinding of seal surfaces would need to be highly automated • All basic technologies are developed, but requires development of automated processing Neutrinos and Arms Control Workshop

22. Alternative Design • Alternative designs could provide lower manufacturing costs, but have some technology developments associated with them. • Transfer design could be lowest cost per unit, but may have higher capital requirements. • The pressure requirement drives all designs. Neutrinos and Arms Control Workshop

23. Conclusions • PMTs for < $0.10/cm2 photocathode area are not limited by existing technology • Manufacturing and processing techniques require significant R&D • Can learn from picture tube and semiconductor industries • Questions about business strategies in responding to this opportunity. • Partnering opportunities • Government involvement • Concerns about limited lifetime of project Neutrinos and Arms Control Workshop