Use of Plastic Optical Fibers for Charged Particle Tracking in High Energy Physics
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Experimentally these studies are made with large, sophisticated particle detectors which are designed to detect and measure as many of the final collision products as accurately as possible. An integral part of nearly all particle detectors are the tracking detectors. Generally speaking, tracking detectors measure the traces of ionization (the "tracks") left behind by charged particles in some detector medium, and use these traces to deduce important properties of the particle, such as direction, velocity and momentum. Over the years the experimental methods used for charged particle tracking have changed as dramatically as our understanding of the physics. Rutherford detected alpha particles scattering off a nucleus by visually detecting the flash of light emitted by the alpha passing through a zinc sulfide screen. In the succeeding decades particle detection techniques have included the bubble chamber, optical spark chambers, multi-wire proportional chambers and silicon strip detectors. As the field has progressed, so have the demands on detectors. In particular, throughout the history of the field, both the rate at which detectors must operate and the complexity of the collisions they study continues to increase. It is the desire to study complex events at very high rates that has motivated the development of the scintillating fiber tracking detector. SCINTILLATING FIBER DETECTORS A scintillating fiber (SciFi) detector combines the old technology of scintillating plastics with the new technology of fiber optics. Fig. 1 shows a schematic view of a generic SciFi detector for a colliding beam experiment. Plastic optical fibers doped with scintillating dyes are precisely placed on support cylinders which surround the point where two beams collide. Charged particles which 27 Mat. Res. Soc. Symp. Proc. Vol. 348. 01994 Materials Research Society
Clear Fber Waveguides
SupportCylinders
Photodetectors
Figure 1. Schematic view of a generic scintillating fiber tracking detector. are produced in the collision pass through the fibers and deposit energy, which is converted into scintillation light. A fraction of that light is optically trapped in the fiber and travels to the end of the cylinder, where the doped fiber is mated to a clear optical fiber, which in turn pipes the light over some distance to a photodetector. The photodetector responds to the light input with an electronic output which can be discriminated, or digitized, and read out by a computer. From the patterns of hit fibers the paths of the charged particles which passed through the detector can be reconstructed. A variety of technically demanding challenges must be met in order for this technique to work well as a particle detector. The location of the active fibers must be precisely known. The scintillating dyes should produce enough light to be detected while maintaining a low level of selfabsorption. Fiber-to-fiber connections need to have optical transmissions of near 100%, the clear fiber must transmit light over large distances and the photodetector
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