Radiation Transfer In Medical X-Ray Intensifying Screens

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An experimental technique has been developed for characterizing the complex light absorption and transmission (radiative transfer) properties of any phosphor-binder composite in terms of the mean free photon scattering length (Xs), the mean free path before photon capture (A ), and the average cosine of the scattering angle <cos * >. These properties are determinedcfor arbitrary phosphor crystal size, phosphor and binder refractive indices, phosphor-binder volume ratio, phosphor emission wavelength, and binder absorption spectrum. The technique requires the measurement of total reflectance and transmission, using an integrating sphere, of a self-supporting screen series differing only in thickness. The experimental data is then used to find As,A , and <cos IP > by means of a nonlinear least squares fit to the 'enhanced' "6-flux" radiative transfer model developed by Richards and Mudgett. The radiation transfer data can then be used in Monte Carlo methods or Swank's neutron diffusion theory models to help predict speed, MTF and noise properties of practical X-ray screens constructed from the same phosphor-binder composite. We have found that, for a given binder and binder-phosphor volume ratio, scattering (A a) is primarily determined by mechanical factors such as phosphor crystal size, crystal habit, and packing density. Absorption (A ) is primarily determined by the phosphor's emission wavelength relative to the binder's spectral absorption. We also confirm Swank's results that light scatter within the screen reduces image spread and increases image sharpness.