A Theoretical Model of the Performance of Film at Gamma-Photon Energies

Abstract

A performance equation for film, treated as a photon counter, is proposed and discussed. The equation expresses the minimum detectable depth of defect d _min as a function of quantum efficiency Q, grain yield Y, and film density D. The optimum film design is defined as the one that minimizes d _mi„ for a given exposure time, minimizes exposure time for a given d _mi„, and at the same time allows the smallest possible value of d _min to be obtained by permitting long exposure times. This optimum is achieved by the use of an image analyser rather than the unaided eye, and by making Q and Y large, and D small. Since Q is independent of grain size and both 1/Y and density increase with grain size, the optimum is thus achieved always by the use of fine grain film. For the important case of Co ⁶⁰ energies theoretical expressions are then derived, which express Q and Y (as well as film unsharpness U, and intensification factor IF) as a function of various (measurable) physical properties of the emulsion, film base, and screens. Since excellent agreement is obtained when these formulae are compared with experiments, it is suggested that the theory can be used as an aid to optimize film design. The most important conclusions are (1) The highest value of Q is obtained by the use of metal screens (rather than fluorescent screens or no screens); (2) Q varies only slowly with film coating weight so that it will often be preferable to make the silver content lower than is currently practiced to cut costs; (3) film unsharpness can be reduced and Y can be increased by the use of thinner bases or, for a given coating weight, by coating only one side with emulsion; (4) whilst Y is increased simply by increasing the coating weight, a less expensive way is as just described; (5) for a given coating weight Y and U_rare independent of the silver-gelatin ratio, i.e. the emulsion thickness.