Development of Welded Fiber-optic Faceplate Assemblies for Portable High-flux API D-T Neutron Generators

Oak Ridge National Laboratory (ORNL) is utilizing associated particle imaging (API) D-T neutron generators to image thick objects using portable equipment. Starfire Industries and Adelphi Technology have both worked with the DOE via the SBIR contracting mechanism to develop these portable, high-flux API D-T neutron generators. They feature a scintillator mechanically affixed to the inside of a viewport and a custom readout on the other side of the viewport to make sense of the scintillation light from each alpha event. The accelerator part of the neutron generator is at high vacuum, so the viewport is part of the vacuum boundary.Previous designs used the Thermo Fisher Scientific API-120 equipped with a Schott 75C-6 fiber-optic faceplate. Schott hermetically sealed the 75C-6 to a flange and then Thermo laser welded the assembly into the accelerator tube. The fabrication process, from creating the faceplate assembly to installing it into a neutron generator and then baking it out to remove contaminates had an unacceptably high failure rate. Simple monolithic glass viewports or flange mounted viewports could be a workable solution, but the latter are relatively heavy and ORNL has a strong preference to permanently install (welded) fiber-optic faceplate assemblies instead. This proposal addresses that need by seeking to develop a robust process for reliably fabricating fiber-optic faceplate assemblies with the required size, optical properties, and final assembly/welding/bakeout tolerance. The Phase I work will focus primarily on developing the glass-to-metal joint for the fiber-optic faceplate assemblies. Depending on the materials of construction and physical arrangement, our baseline approach will be to use a vitreous glass solder in combination with a compliant metal layer to accommodate differential thermal expansion. If the metal component of the fiber-optic faceplate assembly can be made of an alloy with a low coefficient of thermal expansion (CTE) and compatible metallurgy, we may investigate a joint in which the glass component is bonded directly to the metal component. Our goal will be to fabricate full-scale prototype fiber-optic faceplate assemblies by the end of Phase I that can be trial fit to the Starfire Industries and Adelphi Technology portable high-flux API D-T neutron generators.