Comparison of wavelength-shifting fiber types and methods of ribbon assembly for the Depth-encoding Anger detector

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We are developing a depth-encoding Anger detector for imaging 511 keV annihilation photons. Our design integrates wavelength-shifting (WLS) fibers onto an Anger-type detector to allow measurement of depth-of-interaction (DOI). As few as 0.3% of blue scintillation photons created in NaI(Tl) contribute to detectable signal at the end of the WLS fiber ribbon. Thus, maximizing signal output from the fibers is important for achieving the best possible DOI measurement. We have investigated the effects of two design options on the signal output from the WLS fibers: fiber geometry and methods of assembling fibers into a ribbon. A blue-to-green WLS fiber (Saint Gobain BCF-91A) was chosen because its absorption spectrum matches well the emission spectrum of NaI(Tl). Its emissions, peaked at 500 nm, can be detected with reasonable quantum efficiency by enhanced-green-response PMTs. These fibers are available in several configurations (round or square cross-section, single or multiple cladding layers) and sizes. Methods for assembling the fibers into a ribbon include the use of mechanical clamps and adhesives. Finally, reflectors can be used to increase the total light output from the ribbon. In this work we compare single-clad 1-mm-diameter round fibers to multi-clad 1-mm-wide square fibers. We also compare cellophane adhesive tape, acrylic-solvent-based cements, and mechanical clamping as possible methods for binding WLS fibers into a ribbon. Our results indicate that square cross-section fibers with multiple layers of cladding outperform single-clad round cross-section fibers. Additionally, aluminized reflectors on certain regions of the fiber ribbon are useful. Placing acrylic mounting components in direct contact with the fibers causes excessive light loss from the ribbon. Solvent-based adhesives, which partially dissolve the cladding material, decrease light output from the ribbon; cellophane adhesive tape or aluminized-face mechanical clamps perform well. In the future, we will extend our investigation to other fiber sizes and cladding configurations.

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IEEE Nuclear Science Symposium and Medical Imaging Conference

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