부산대학교 도서관

We have studied the tube voltage dependence of the modulation transfer function (MTF) of a rare-earth screen-film system. The phosphor of the screen was LaOBr: Tm, whose K-edge energies were 13.5 keV and 38.9 keV. At tube voltages of 50-80 kV, which generate x-ray spectra including the K-edge energy of 38.9 keV, we obtained the line spread images for the front and back emulsions. The line spread functions (LSFs) were determined and we obtained the tube voltage dependence of the MTFs for the front and back sides. As a result, the values of the MTFs for the front and back sides at spatial frequencies of less than 3 mm-1 decreased with the tube voltage. We indicated that, to explain this phenomenon, not only K x rays but coherent and incoherent scattered x-ray photons yielded in the screens were needed to take into consideration as the cause of the decrease of resolution.