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

Abstract.

The purpose of this work was to investigate and compare the imaging performance of Gd₂O₂S:Tb and La₂O₂S:Tb phosphors as well as of Gd₂O₂S:Tb and La₂O₂S:Tb mixtures for use in X-ray imaging detectors (intensifying screens). Phosphors were supplied in powder form and were used to prepare test screens. Three types of screens were prepared: Gd₂O₂S:Tb; (Gd₅₀La₅₀)O₂S:Tb; and La₂O₂S:Tb. Screens were excited by X-rays with tube voltages from 40 to 120 kV and their efficiency (light intensity or light energy flux over exposure) was measured with a photomultiplier and a dosimeter. The light spectrum was also measured with a monochromator. From these measurements, the number of 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 also 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 within the 45- to 55-kV range. Maximum NEP values were higher than 700 at 100–120 kV while maximum DQE(0) was 0.314 at 80 kV. Gd₂O₂S:Tb screens are more efficient for high X-ray tube voltage applications (e.g., abdominal imaging, chest radiography, lumbar spine radiography, CT) and for very low voltage applications (e.g., mammography). La₂O₂S:Tb screens are useful for medium-range X-ray voltages (e.g., pediatric radiography).