Detectors 223
Detectors 223 : Particle Detectors: (4.27) X-Ray Detector: (7) Direct Semiconductor Detectors: Since the 1970s, new semiconductor detectors have been developed (silicon or germanium doped with lithium: Si (Li) or Ge (Li) ). X-ray photons are converted to electron-hole pairs in the semiconductor and are collected to detect the X-rays. When the temperature is low enough (the detector is cooled by Peltier effect or even cooler liquid nitrogen), it is possible to directly determine the X-ray energy spectrum; this method is called energy dispersive X-ray spectroscopy (EDX or EDS); it is often used in smallX-ray fluorescence spectrometers. These detectors are sometimes called "solid state detectors". Detectors based on cadmium telluride (CdTe) and its alloy with zinc,cadmium zinc telluride, have an increased sensitivity, which allows lower doses of X-rays to be used. Practical application in medical imaging started in the 1990s. Currently amorphous selenium is used in commercial large area flat panel X-ray detectors for mammographyand chest radiography. Current research and development is focused around pixel detectors, such as CERN's energy resolving Medipix detector. Note: A standard semiconductor diode, such as a 1N4007, will produce a small amount of current when placed in an X-ray beam. A test device once used by Medical Imaging Service personnel was a small project box that contained several diodes of this type in series, which could be connected to an oscilloscope as a quick diagnostic. Silicon drift detectors (SDDs), produced by conventional semiconductor fabrication, now provide a cost-effective and high resolving power radiation measurement. Unlike conventional X-ray detectors, such as Si (Li) s, they do not need to be cooled with liquid nitrogen
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