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Optimization of phosphor screens for charge coupled device based detectors and 7–34 keV x-rays

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2 Author(s)
Wang, P.-C. ; Department of Chemical Engineering, Materials Science and Mining Engineering, Columbia University, New York, New York 10027 ; Cargill, G.S.

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Phosphor screens which convert x-ray images to visible light images are key components in two-dimensional charge coupled device (CCD) based detector systems used for x-ray diffraction. Some experimental and theoretical aspects of phosphor screen performance are described in this article. The efficiencies of x-ray-to-light conversion were measured using a CCD camera for transmission phosphor screens fabricated from two different phosphor powders, Y2O2S:Eu (P22R) and Gd2O2S:Tb (P43), for screen mass thicknesses of 3–50 mg/cm2 and for x-ray energies of 7–34 keV. A model was developed and evaluated for the dependence of the emitted light brightness on screen thickness and x-ray energy. Inputs to the model are x-ray absorption coefficients available from published compilations, and light attenuation versus thickness data, which were determined experimentally for the phosphors and found to be dominated by scattering rather than absorption. The angular distribution of emitted light, measured for one of the phosphor screens, was found to be nearly Lambertian. Broadening of image features in the x-ray-to-visible-light conversion by phosphors for 19.6 keV x-rays was found to increase approximately linearly with phosphor screen thicknesses in the range of 30–160 μm, but with a minimum width of 110 μm for P22R phosphor and 70 μm for P43 phosphor. In the range of 7–15 keV, maximum brightness was obtained for P43 phosphor screens of about 10 mg/cm2 mass thickness (60 μm). For P22R screens, the thickness for maximum brightness increased from about 8 mg/cm2 (50 μm) for 7 keV to more than 46 mg/cm2 (210 μm) for 15 keV. For 7 keV the maximum brightnesses for P22R and P43 phosphors were about the same. For 10 keV the maximum brightness for P43 phosphor was about 60% greater than the maximum brightness for P22R phosphor sample- s tested. For 15 keV the maximum brightness for P43 phosphor was again about 60% greater than that for the P22R samples tested. In the range of 20–34 keV, maximum brightness would occur for thicknesses greater than 46 mg/cm2 (210 μm) for P22R phosphor and greater than 40 mg/cm2 (160 μm) for P43 phosphor. Comparing the brightness for 90 μm thickness for the two phosphors, P43 was about 30% brighter for 20 keV, 20% brighter for 24 keV, and 10% brighter for both 29 and 34 keV. © 1997 American Institute of Physics.

Published in:

Journal of Applied Physics  (Volume:81 ,  Issue: 3 )