A simple model for estimating the particle size dependence of absolute efficiency of fluorescent screens

The absolute efficiency of a phosphor screen is the ratio of the light ener gy per unit area at the screen surface to the incident x-ray energy fluence . Particle size is a critical factor in determining the absolute efficiency , but in most models its influence is not accounted for. To allow derivatio n of the particle size dependence, a model is proposed that describes the o ptical properties of the screen by means of a single parameter, the light e xtinction factor, xi, and assumes that the intrinsic efficiency (light ener gy/energy imparted to the phosphor material) is independent of particle siz e. The value of xi depends on the type of screen (phosphor, reflective back ing, coating and binder) and has to be determined from measurements on at l east two screens with known particle size and thickness. The absolute effic iency can then be calculated for an extended range of particle sizes and/or screen thicknesses. To test the model, experimental data from the literatu re were used to derive values of xi for screens of La2O2S:Tb, LaOBr:Tm and ZnCdS:Ag. The extinction factor was found to vary between -6 and +20%. The non-physical negative value for xi, found from one set of experiments on La 2O2S:Tb screens, may be explained as resulting from a lack of accurate know ledge of the actual tube potential, influencing calculated values of the en ergy imparted to the screen. The results are promising but further well-con trolled experiments (including improved dosimetric calculations to account for escape of K-radiation from the screen) are needed to confirm the model.