Tile Calorimeter

ATLAS Tile Calorimeter: WLS Fibers

Wave-Length Shifting (WLS) optical fibers are used to collect the light produced in the tiles. The plastic in these fibers have been doped with special dyes that absorb the predomoinantly blue light from the scintilltor and reemit green light. A fraction of this green light (about 6%) is captured in the fiber by internal reflection at the core-cladding boundary. The light is transmitted through the fiber to a photo-multiplyer tube (PMT). To enhance the light yield, the other end of the fiber is polished and coated with Aluminum to make a mirror surface.

Tests of Mirrors on Fibers Received During Sept. ATLAS Week

The reflectivity of the mirror surface was measured on a sample of fibers removed from the prototype Module0 after two years to check for deterioration with age. The fibers are stimulated with a uv light source. The light leaving the unmirrored end is recorded using a PMT as the light source is scanned along the length of the fiber. The mirrored end is then cut off at a 45 degree angle and painted black to spoil any reflection and the fiber is scanned again. The difference between the two scans is a measure of the reflected light. By comparing this with the light expected for a perfect mirror, predicted by extrapolating the light generated in the unmirrored fiber, the mirror reflectivity can be deduced.

Postscript files showing the mirror reflectivity analysis

Text files, Formatted 5F10.1, Containing
Source position(mm), Average and Sigma of PMT current for mirrored fiber, Average and Sigma of PMT current for bare fiber
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