K-characteristic photon absorption from intensifying screens and other materials: theoretical calculations and...

When an extended object is irradiated with primary x rays, K-characteristic radiation is produced throughout the object. At a given location, the ratio of the K-characteristic radiation received to the primary radiation is not well understood. Knowledge of this ratio is important in the consideration of image detector design, as K-characteristic radiation generated within or proximal to the imaging detector may be considered as secondary radiation and as a specialized form of scatter to primary ratio, will reduce image contrast. This article models the production of K-characteristic radiation and calculates the K-characteristic to primary exposure and absorbed energy ratios for a number of sample materials, geometries, and detector types. Calculations have been experimentally verified for tin and gadolinium oxysulphide sample materials in some geometries using an ionization chamber detector. It was found from calculation that the magnitude of the K/p exposure and absorbed energy ratio were closely related to the ratio of the field size to the detector-to-sample distance when the detector-to-sample distances are small compared to the source-to-detector distance. It was also found that the K/p exposure and absorbed energy ratios were maximal and constant when the field size to detector-to-sample ratio was greater than 20:1 for an intensifying screen detector and greater than 100:1 for ionization chamber measurements. Calculation, also confirmed experimentally, indicated that significant ratios of K/p absorbed energy could be detected using intensifying screens proximal to fluorescing sample material.