Persons

The conversion stack is the most technologically complicated optical element of the X-ray sensitive panels used in the digital X-ray technology, which causes an increased attention to the optimization of the stack design, both in terms of the technical parameters of the panel and in terms of its process ability. The computer modeling of optical stacks of the X-ray radiation conversion, using various methods of immersion, bleaching and the technologically conditioned surface has been presented. A mathematical model of the optical transducer stack, which is a set of the functional layers of various thicknesses, has been developed. The main elements that can be a part of the conversion stack have been considered: a scintillator; an optically transparent adhesive for the scintillator; fiber optic plate; an optically transparent adhesive for a fiber optic plate; protective antireflection coating of the photo-reading device; the passivation layer of the photo-reading device. The variants of the construction of the investigated stacks, which has made it possible to compare the experimental data on the work of traditional methods of stacking for X-ray diffraction and to assess the adequacy of the used model, have been proposed. A mathematical model of an optical converter stack, geometrically representing a set of various thicknesses, has been developed. The top layer is exposed to X-rays. The lower layer is a photosensitive cell. The proposed model permits to affectively study the parameters of the optical stack to change the parameters of its components and, thus, to formulate the recommendations on the parameters of adhesives and protective coatings.

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There is no single concept, in terms of optimizing the diagnostic characteristics of the X-ray detector, to sel ect the photocell size. The reduction of the photocell size allows an improvement of the diagnostic quality, in its turn, providing a sufficient level of the contrast and the distinguishability of objects requires an increase of illumination, also depending on the area, which is not always possible due to the dose loading limitations. The model of the process of converting the X-ray radiation into an image, taking into account the effect of the photocell size on the transformation result, has been proposed. During the optimization process the parameters of the photocell the influence of all converter stack elements was taken into account, including the thickness of the oscillator in order to balance the brightness and contrast, the losses in the passage of the image through the protective fiber optic plate to account for the overall noise spectrum, the filling factor of the photocell, which has a significant effect upon image blurring. The model, representing the linear calculation of the frequency-contrast characteristic and the quantum efficiency at different values of the photocell size, has been obtained. Such dependence makes it possible to determine the photocell optimum size from the point of view of achieving certain values of the frequency-contrast characteristic. It has been shown that in accordance with the technical requirements of the project the maximum permissible size of the photocell is 140 μm, and its optimum value lies in the range fr om 60 to 70 μm. In determining this range the dependences, obtained for the simulation of the detected quantum efficiency, also, have been used. The proposed model permits with the high extent of reliability to specify the key parameter of the photo-reading unit of the X-ray-sensitive panel - the photocell size.

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