Feasibility Study of Phosphor Screen Containing Nano-Scale Anti-Reflection Layer for Improved Optical Properties in Indirect X-ray Detector.

With increasingly strict regulations regarding patient exposure, research on digital radiography technology has recently focused on indirect methods that can produce high-quality images for a low radiation dose. In particular, medical imaging systems based on indirect methods universally use rare-earth metal phosphors, because of their high atomic number and excellent luminescence efficiency. Thus, various studies aiming to improve the luminescence efficiency of phosphors have been conducted. Despite this research, however, the current luminescence efficiencies are insufficient. Here, we report a basic study aiming to develop a phosphor screen containing a three-quarter-wave optical-thickness layer to improve the light transmission efficiency. Specifically, the fabrication and measurement of a Gd2O2S: Tb phosphor screen containing a single three-quarter-wave optical-thickness layer is presented. The screen is fabricated via a screen-printing and spin-coating method. Based on histograms of the degree of luminescence and the pixel values, we demonstrate that the light transmission efficiency is improved by the three-quarter-wave optical-thickness layer. Note that analysis of the full width at half maximum of the pixel value distribution reveals the possibility of resolution loss when obtaining medical images. Overall, the results of this study confirm that the light transmission efficiency can be improved through use of a single-layer anti-reflection coating. However, because the emission spectrum of the Gd2O2S: Tb screen is in the 480-600-nm band, it is necessary to expand the areas exhibiting the lowest reflectance to the wavelengths at the edge of this band. Thus, further study should be conducted to optimize the optical thickness.