Photon Counting with Fiber-Optic Coupled Scintillators

By coupling a scintillation crystal to a photomultiplier with fiber optics, one can place the detector in areas not normally accessible to a crystal-photomultiplier combination. Although fiber optics provide a flexible optical coupling, serious light losses cause about a factor of 10 reduction in the light intensity reaching the photomultiplier. Most of the light loss is due to the small acceptance angle of the fiber optics with transmission losses and crystal aperture losses playing secondary roles. Attention to the design and to the selection of the crystals and fiber optics help to optimize the light collection efficiency. The light losses lower the scintillation intensity from low-energy (10 to 40 keV) radiation into the region where photomultiplier noise becomes important (3.5 - 18 photoelectron equivalents). This noise consists of Cerenkov events, faceplate scintillations, electroluminescence, afterpulses and thermionic emissions. For effective low background counting this noise must be reduced. A combination of pulse-shape discrimination, magnetically limiting the photocathode area and shielding were effective in reducing the background rate from 513±3 counts/minute to 10.0±0.5 counts/minute in the energy range from 6.5 to 40 keV. By coincidence counting, in the same energy region, a background rate of 1.60±0.28 counts/minute was obtained.