<title>Measurement Of The X-Ray-Induced Light Photons Emitted From Radiographic CaW0<formula><inf><roman>4</roman></inf></formula> Intensifying Screens</title>

Abstract

Abstract For calcium tungstate intensifying screens employed in film-screen imaging systems, Coltman found that approximately 1000 light photons of average energy 2.7 eV were produced for each 50 keV x-ray absorbed. Of this number, he found that only about 55% are emitted from the output side of a 109 mg/cm2 screen. We have developed a method based on counting single photons to determine this number for various thicknesses of calcium tungstate screens. Monoenergetic x rays in the energy range from 17-69 keV produce upon absorption, a shower of individual photon pulses which are detected by a low noise photomultiplier. After amplification and discrimination against the noise background of the phototube, the resultant pulses are counted in a 70 MHz sealer or a 150 MHz counter. The detection system has a pulse resolving time of less than 15 ns. The data are then corrected for the quantum efficiency of the detector and normalized to the number of absorbed x rays which is determined in a separate experiment. For calcium tungstate screens with thicknesses of 30, 50, 86, and 123 mg/cm2 , the average numbers of light photons emitted per absorbed x ray are measured for 8 x-ray ener­ gies between 17- and 69-keV. The values for 50 keV are less than the values found by Coltman. Studies of the causes of this discrepancy are in progress.IntroductionRecent improvements in the performance of video camera tubes and optical systems have revived interest in a fairly old approach to photoejlectronic imaging for diagnostic radiology: an intensifying screen optically coupled to a video camera tube. ' ' As shown in Figure 1, x rays which pass through the patient interact with a radiographic intensifying screen generating a shower of light photons which are focussed on a video camera tube. Video camera tubes are available currently with resolutions of more than 2000 TV lines (TVL/RHr and even 8-10,000 TVL/RH can no longer be considered exotic. * ' In addition, there has been substantial progress in improving/the quantum detection efficiency of the photosensors. New heterojunction surfaces,as used in the saticon tube for example,have quantum efficiencies of nearly 100%. Furthermore, optical systems have be­ come more compact and efficient and have object areas up to 500 mm in diameter, numerical apertures (N.A.)/as low as .12, and resolutions of up to 7 line pairs per mm across an appreciable portion of the total field.