Fiber Optic Scintillators
Fiber optic scintillators are structurally identical to standard fiber optics. The most important difference is compositional: the core of each fiber is made of a scintillating glass. Single-fiber scintillators can be made in a wide variety of diameters (.05 - 10mm) and lengths (up to 7 meters). Multi-fiber structures, or fused faceplates, are usually constructed with 8-micron fibers, and can be as large as 300mm on a side, containing more than one billion fibers. The thickness of fused faceplates is a function of the energy of the radiation to be converted.
One of the fundamental advantages of the fused structure of individual fibers is that the thickness of the transducer can be increased to optimize absorption without degrading the image due to geometric line spreading (as with conventional polycrystalline screens).
With scintillating cores, each fiber becomes an individual transducer. At some point along the length of each fiber, incident photons are absorbed and re-emitted isotropically as visible light (approximately 550 nm). A small fraction of this light (approximately 10%) propagates along the fiber in both axial directions by way of total internal reflection. The balance of the light is absorbed by dark glass elements (EMA) which are strategically located in the fused structure, or it is scattered and transmitted by immediately adjacent fibers. The optical density of the EMA, weighted by its frequency in the structure, affects the extent of fiber-to-fiber cross-talk.
Total light output is increased through the use of a mirror coating on the input side of the fiber optic. This has the effect of re-directing that portion of the light which propagates back toward the input face. The net result in these fiber optic structures is that virtually all of the incident radiation is absorbed and high resolution is maintained for applications in which the scintillator is direct-coupled or lens-coupled to a sensor.
Thick plates of optically transparent scintillating glass can be used to convert ionizing radiation into visible light. Unlike fiber optic structures, these bulk glass plates do not guide the re-emitted light to the output surface by total internal reflection, and a lens is generally used to collect and couple the light to the sensor. Where lower resolution information is adequate, this structure offers a considerable price advantage because it has not undergone the laborious, low-yield process of multiple re-draws and fusing.
In some cases there is an advantage to combining the conversion efficiency of standard polycrystalline phosphors and the absorbing and light-conducting properties of the fused fiber optic structure. When a thin layer of selected phosphors is deposited directly onto the input face of a fused scintillator screen, a large portion of the incident radiation can be converted and transmitted at high efficiency to the sensor. Those incident photons which pass through the phosphor layer are then absorbed and re-emitted in the scintillating faceplate. This simple structure offers the freedom to choose a phosphor with a desired output, and achieve a boost of additional light and complete absorption from the fiber optic scintillator.