Analytical and Monte Carlo comparisons on the optical transport mechanisms of powder phosphors

In indirect medical imaging systems, the optical transfer properties of powder phosphor materials are affected by the intrinsic mechanisms of light extinction. Light extinction (absorption and scattering) can be described either through theoretical models or with Monte Carlo simulation techniques. The purpose of the present paper is to compare both methodologies and assess the association of their optical parameters by examining either the structural (e.g., particle size) or the optical (e.g., light wavelength) intrinsic properties of the phosphor.

Comparisons were carried out for the widely used host Gd₂O₂S for granular phosphors. The parameters considered in Monte Carlo simulation were: (i) a phosphor layer composed of grains with diameter 4-12 μm, (ii) the light wavelength (420 nm, 545 nm and 610 nm), (iii) the refractive index of the phosphor (real part: 2.3, imaginary part: 10^-6 and 10^-5), (iv) the packing density 50%. The results were compared with the Swank analytical model, based in Boltzman diffusion equation, by considering three cases for optical coefficients determination with the help of which the Luminescence Efficiency (LE) and the Modulation Transfer Function (MTF) of the phosphor could be predicted.

It was found that Swank's model seems more appropriate for estimating the MTF values, presenting a deviation less than 5% in all the frequency range. for: wavelength 420 nm, considering (i) grain size 4 μm and thicknesses above 250 μm, (ii) grain size 12 μm and thicknesses above 300 μm, (b) wavelength 545 nm, grain size 8 μm and thicknesses above 160 μm, and (c) wavelength 610 nm, considering (i) grain size 4 μm and thicknesses above 200 μm, (ii) grain size 8 μm and thicknesses above 120 μm and (iii) grain size 12 μm and thicknesses above 80 μm.