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

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.