Radioluminescence : A simple model for fluorescent layers

• The aim of this thesis is to present a simple model for the radiation to light conversion processes in fluorescent layers as an aid in future developments and applications. Optimisation between sensitivity and spatial resolution for fluorescent layers in digital radiology is a delicate task where the extrinsic efficiency for various phosphors needs to be established for varying parameters. The extrinsic efficiency of a fluorescent layer can be expressed as the ratio of the light energy per unit area at the screen surface to the incident xray energy fluence. Particle size is a critical factor in determining the value of the extrinsic efficiency, but in most models it is not treated as an independent variable. Based on the definition of a light extinction factor (ξ), a model is proposed such that, knowing the intrinsic efficiency η, the particle size and the thickness of a certain make of screen, the extrinsic efficiency can be calculated for an extended range of particle sizes and / or screen thicknesses. The light extinction factor ξ is an optical parameter determined from experimental data on extrinsic efficiency. The proposed model is compared to established methods. Further experiments have confirmed the validity of the model. Monte-Carlo simulations have been utilised to refine the calculations of energy imparted to the phosphor by taking into account the escape of scattered and K-radiation generated in the screen and interface effects at the surfaces. The luminance was measured for a set of in-house manufactured screens of varying thicknesses and particle sizes. Utilising the proposed model, the corresponding calculated values deviated ± 14 %.within the studied range.

Nyckelord

Extrinsic efficiency

extinction factor

radioluminescence

modelling

Monte Carlo

optical attenuation

optimisation

particle size

phosphor

scintillator

MEDICINE

MEDICIN

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