Optical photon transport in powdered‐phosphor scintillators. Part II. Calculation of single‐scattering transport parameters

Abstract

Methods:The following transport parameters are calculated: scattering efficiency (Q sct ), absorption efficiency (Q abs ) and the scatter anisotropy (g). Calculations assume spherical phosphor grains and are 20 performed using the analytic method of Mie theory for grain radii of 0.1 to 5.0 µm. The sensitivity of transport parameters to emission wavelength is investigated using an emission spectrum representative of that of Gd 2 O 2 S:Tb. The impact of a grain-size distribution in the screen on the parameters is investigated using a Gaussian size-distribution (σ = 1%, 5% or 10% of mean radius). Results:The Mie theory predictions of transport parameters are shown to be highly sensitive to both grain size and emission wavelength. For a phosphor screen structure with a distribution in grain sizes 30 and a spectrum of emission, only the average trend of Mie theory is likely to be important. This average-behavior is well predicted by the more sophisticated of the geometrical optics models (GODM+) and in approximate agreement for the simplest (GODM). The agreement of MTF predictions with experiment is reasonable, and encouraging given the uncertainties in screen composition. 35 Conclusion:If Mie theory is used for calculating transport parameters for light scattering and absorption in powdered-phosphor screens, care should be taken to average out the fine-structure in the parameter predictions. However, for visible emission wavelengths (λ < 1.0 µm) and grain radii (a > 1.0 µm), geometrical optics models for transport parameters are a reasonable alternative to Mie theory. Their greatest virtue is simplicity and the results presented suggest no substantial loss in 40 predictive accuracy.

Rapid Monte Carlo simulation of detector DQE(f)

“…3 The top reflective support layer was modeled having a specular reflectivity of 0.88. 27 The protection foil was modeled as clear plastic with an index of refraction of 1.5. The photodiode layer had an index of refraction of 1.7 and was assumed to be perfectly absorptive.…”

Optical photon transport in powdered‐phosphor scintillators. Part 1. Multiple‐scattering and validity of the Boltzmann transport equation

“…The question of the accuracy of calculations of the transport crosssections used is largely set aside here, geometrical optics being assumed valid. That assumption is, however, investigated in Part 2 22 in comparison to the more sophisticated approach of Mie-type 65 theory. 18 The effects of x-ray dose-deposition, grain-size distribution and emission spectrum are also examined in Part 2 for examples of real screens, leading to comparison with other authors' experimental measurements.…”

Modelling the transport of optical photons in scintillation detectors for diagnostic and radiotherapy imaging

“…The DOSxyznrc code was used to generate the photon absorption probability distribution function and the optical photon transport was modelled by DETECTII routines (Fasbender et al , 2003; Kausch et al , 1999; Radcliffe et al , 1993). More recently, Poludniowski and Evans modelled the light transport in powdered-phosphor scintillator screens using a combined method based on the Boltzmann transport equations (Poludniowski et al , 2013a; Poludniowski et al , 2013b). The three-dimensional dose distribution was calculated using DOSRZnrc.…”

“…Analytical models and numerical simulations have also been applied to optical modelling (Swindell, 1991; Swindell et al , 1991; Evans et al , 2006; Freed et al , 2010; Poludniowski et al, 2013a; Poludniowski et al, 2013b). …”