Scintillating Glass Fiber-Optic Neutron Sensors
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*Pacific Northwest Laboratory, Box 999, Richland, WA 99352 "**LawrenceLivermore National Laboratory, Box 808, Livermore, CA 94551 ABSTRACT Pacific Northwest Laboratory (PNL) has fabricated cerium-activated lithium silicate scintillating fibers via a hot-downdraw process. These fibers, which, as produced, typically have a transmission length (e-1 length) of greater than 2 meters, are found to undergo aging when subjected to room air. The aging, which is complete in a few weeks, reduces the transmission length to the order of 0.5 meter. Because of the high alkali content of the glass (on the order of 2030 mole % lithia), we have attributed this aging to aqueous corrosion at the polymer cladding/glass interface. Changes in transmission with chemical treatment of the surface support the corrosion model. Fiber transmission performance has been preserved by modifying the hot-downdraw to a double crucible to produce glass-on-glass waveguides. INTRODUCTION The development of neutron-sensitive scintillating fibers was initiated to produce large-area, fast, solid-state, thermal-neutron detector packages which are insensitive to gamma rays. Based on experience and literature data,t it was determined that cerium-activated lithium silicate glass fiber waveguides would be the best candidate for the active part of the detector packages. Because such fibers are not available commercially, a fiber fabrication facility was developed. As recently as 1993, cerium-activated lithium silicate fibers were described as having "...useful lengths of fiber [limited] to
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