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X-ray detection with sctintillators

Scintillation detectors are optically transparent solid or fluid materials that respond to ionizing radiation with emitting visible light. The scintillators with crystal structure
have an electron band structure containing a restricted band, where due to extra components (activators) excitable states can be created (luminescence centers).


Fig. 16. Possible states of a scintillation crystal

As response to external ionizing excitation the electrons in the valence band can be either excited to the conduction band or to the luminescence centers. From the luminescence centers electrons get de-excitated to the valence band and a photon is emitted with the energy corresponding to the energy level differences. The emitted photon can be efficiently detected through the photoelectric effect by using a photomultiplier tube (PMT) or a semiconductor photon detector 7. By the extra component the wavelength of the emitted photon can be optimized to the maximum detective efficiency of the photon detector. The most commonly used scintillator type is the NaI(Tl), but several crystalline materials with similar properties can be applied (CsI(Na), ZnS(Ag), LiI(Eu) etc.). Organic materials may also provide excitable states that enables the detection of ionizing radiation. Organic scintillators may be in any form (liquid, gas, solid) broadening the range of applications.
The latest CT's apply Gd2O2S (gadolinium-oxysulphide=GOS) cheramic as scintillation detectors offering many advantages over other detector materials:

  1. The high atomic number of Gd (64) and the high density of Gd2O2S , namely 7.32 g/cm2 the detection efficiency is high even in small sizes for X-rays given by the high specific energy loss
  2. The maximum of the emitted light is 546 nm, perfectly suiting semiconductor detectors
  3. The Gd2O2S scintillator detectors allows for an extremely fast detection, increasing the speed of X-ray examinations.

This latter is especially important for CT examinations.

The Ga-based scintillator has lower detective efficiency compared to CsI-types, but due to the higher absorption layer thickness can be lowered. The amplitude of the light impulses are in correlation with the flux of the X-ray beam. The photons acquired like that are either used for exposing a film or for producing electrons through the photoelectric effect. The image on the film is provided by the transformation of silver-bromide grains. The detectors consisting of pixellated sensors can utilize the secondary photons emitted in the scintillator for image formation.
The most advanced pixellated detectors perform the light-electron conversion with field-effect transistors (TFT= thin film transistor) with high detection efficiency. Behind every scintillation tile plenty of tiny TFT pixels are located generating electic charges proportional to the incoming photons, that is further processed by a read-out electronics for further processing. The great advantage of multipixel detectors consisting of a scintillator tile and TFT elements is that it produces a signal proportional to the X-ray flux very rapidly. This latter aspect is especially important at fast imaging techniques like cardial imaging or when patient dose should be kept very low.

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