Experimental and theoretical assessment of the performance of Gd2O2S : Tb and La2O2S : Tb phosphors and Gd2O2S : Tb-La2O2S : Tb mixtures for X-ray imaging

The purpose of this work was to investigate and compare the imaging perform ance of Gd2O2S:Tb and La2O2S:Tb phosphors as well as of Gd2O2S:To and La2O2 S:Tb mixtures for use in X-ray imaging detectors (intensifying screens). Ph osphors were supplied in powder form and were used to prepare test screens. Three types of screens were prepared: Gd2O2S:Tb; (Gd50La50) O2S:Tb; and La 2O2S:Tb. Screens were excited by X-rays with tube voltages from 40 to 120 k V and their efficiency (light intensity or light energy flux over exposure) was measured with a photomultiplier and a dosimeter. The light spectrum wa s also measured with a monochromator. From these measurements, the number o f emitted photons per incident X-ray (NEP) and the zero frequency detective quantum efficiency (DQE(0)) of the screens were determined. Additionally, modulation transfer function (MTF) was measured by the square wave response function (SWRF) method. A theoretical model calculating NEP and DQE was al so developed to fit experimental data and predict the performance of Gd2O2S :Tb-La2O2S:Tb mixtures by weight from 10-90 % to 90-10 %. Gd2O2S:Tb screens exhibited highest NEP, DQE, and MTF at tube voltages higher than 55 kV and lower than 45 kV, whereas La2O2S:Tb screens had better NEP, DQE, and MTF w ithin the 45- to 55-kV range. Maximum NEP values were higher than 700 at 10 0-120 kV while maximum DQE(0) was 0.314 at 80 kV. Gd2O2S:Tb screens are mor e efficient for high X-ray tube voltage applications (e.g., abdominal imagi ng, chest radiography, lumbar spine radiography, CT) and for very low volta ge applications (e.g., mammography). Ga2O2S:Tb screens are useful for mediu m-range X-ray voltages (e.g., pediatric radiography).