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See Article History. Alternative Titles: semiconductor detector, semiconductor radiation detector. Read More on This Topic. When a charged particle loses its energy in a solid rather than a gas, processes similar to ionization and excitation also take place. Learn More in these related Britannica articles:. Get permission to re-use this article. Create citation alert.
Irrespective of the approach, the aim is to transform the energy of gamma-ray photons into an electrical signal. There are usually either inorganic salts or organic substances in the forms of solid crystals. The scintillator detectors convert the gamma-ray photons to the visible photons, and then, the visible photons are converted into an electrical signal. Besides, semiconductor materials such as cadmium telluride CdTe and cadmium zinc telluride CdZnTe are important alternatives for the scintillation detector, known as solid state detectors.
In the detectors, gamma-ray photons caused electron-hole pairs and electrical charge drift to opposite directions using an electric field, then induce signals on electrodes, and directly create electrical current. Gas detectors convert absorbed photon energy into electron-ion pairs that move toward electrodes. The detectors are low-efficiency detectors for counting gamma rays because of their low light interaction chance and their poor capability to quantify the energy of photons.
Furthermore, it is responsible for absorbing and detecting the high-energy photons emitted from the patient and also providing the location of the photon interaction for the photomultiplier tube PMT circuit.
Due to the large and massive scheme of the conventional camera, it is generally difficult to position it as near the organ and to obtain the appropriate view. This complication is exacerbated by the essence of a dead zone of some centimeter at the sides of the camera. In cases of small organ and small animal imaging, the limitation is especially important. Some important characteristics of scintillator crystals under development and currently used to create imaging detectors used with some modification from the references 12 and One of the initial goals of medical imaging is to reach high detection efficiency with maintenance of a proper energy resolution.
Today, research on semiconductor detectors is increasing to improve the energy resolution and intrinsic spatial resolution of the imaging systems. Semiconductor detectors are the main substitute for the scintillation detectors in medical imaging systems. Besides, Ge works at cryogenic temperatures because of its small band gap.
Table 2 shows the physical features of the semiconductors that were commonly used for radiation detection. Besides, the excellent sensitivity of these detectors caused to decrease in statistical noise and to increase in the count rate and possibly imaging at a little time with low administered radiation dose. In addition, semiconductor detector eliminates the need for bulky PMTs and so provides a thin and lightweight camera head with decreased shielding so flexible to design the camera in different shapes; besides, SPECT imaging scanners with semiconductor detector can be built very compact due to direct photon conversion and have not PMTs.
CdTe has been studied as the detector material since s. Its useful properties consist of a wide band gap 1.
Absorption of photons in the material can be computed by the following formula:. For gamma-ray interactions, the linear attenuation coefficient demonstrates the absorption ability of the material per unit thickness. The detection efficiency of the materials versus detector thickness is shown in Figure 2. CdTe detector with various thicknesses at KeV gamma rays has a greater photoelectric attenuation coefficient which gives excellent detection efficiency.
Figure 3 illustrates the efficiency of 1 mm CdTe detector as a function of photon energy. Ultimately, the efficiency generally increases with the atomic number Z.
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Linear attenuation coeffcients of cadmium telluride, thallium-activated sodium iodide, germanium, and silicon materials. The dashed lines are results for the mass energy-absorption coefficient. Cadmium telluride has a higher photoelectric attenuation coeffcient than do the other materials used with permission from the author.
Semiconductor detector - Wikipedia
Detection effciency for keV gamma rays for various thicknesses of cadmium telluride, thallium-activated sodium iodide, germanium, and silicon detectors. The drawn lines are results for the total detection effciency; the dashed lines are results for the photoelectric detection effciency used with permission from the author. The spatial resolution of the detectors was found 2. A main problem of CdTe detectors is polarization effect, which is related to every variations in function of the detector under the biasing condition.
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The other drawbacks include higher cost and operational complexity. Thus, CdTe is appealing for gamma-ray detection applications. Perceiving the charge transport procedure is basic in the improvement of semiconductor detectors. The electrons loss their energies by two processes: ionization and photon creation. Semiconductor detector acts like an ionization chamber.
Detector Research and Development
In the detector, the electron-hole pair is generated instead of the electron-ion pair in the gas detector. Electrons and holes, as negative and positive charges, drift to the anode and cathode electrodes, respectively. Valence band abandoned when the electrons excited into the conduction band, and hence, a hole was created in valence band e. Due to defects and impurities in the detector, charge carrier transport features are different between the electrons and the holes.
Planar configuration of a semiconductor detector. Electron-hole pairs, generated by radiation, are swept toward the appropriate electrode by the electric field used with permission from the author. CdTe semiconductor is the most probable detector for medical imaging devices. The main applications of these detectors have defined for gamma cameras with small field of views that may be applied as appropriate small-organ imaging systems such as cardiac, breast, and brain imaging, sentinel lymph node intraoperative mapping, thyroid or parathyroid, and small animal imaging.
The imaging of dual-radioisotopes will provide two functional images without time difference and position error by a single scan. N-isopropyl-p-[I] Iodoamphetamine IMP of brain phantom a , planar image by cadmium telluride detector module b and by anger-type c , the distance from the phantom surface to collimator surface was 50 mm.
The image of cadmium telluride detector b is superior in contrast and sharpness than of anger-type used with permission from the author. In the recent decade, several epitaxial growth methods were established for thick or thin deposition of the CdTe detectors. To optimize the electrical characteristics and to miniaturize a diverse heterogeneity of CdTe crystals,[ 17 , 45 ] a thick detector deposition was introduced,[ 17 ] so that the detector function has highly improved.
Related Silicon Solid State Devices and Radiation Detection
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