The use of film methods in small-angle X-ray scattering

Various sources of random deviations occurring in the determination of X-ray intensities with the aid of film methods are discussed and corresponding mathematical expressions are derived. From a comparison with experimental values of the standard deviations obtained from measurements on homogeneously exposed films, it follows that the deviations are primarily caused by the variations of the X-ray photon flux due to the stochastic nature of the X-ray emission process. The accuracy attainable with most X-ray films is found not to be limited by saturation of the number of excited AgBr particles occurring at increasing exposures, but at low exposures film methods are at a disadvantage because of the presence of fog. If, however, small signals superimposed on a high sample background have to be measured, relatively high exposures are required; in this case film registration is found to be much faster than methods involving step-scanning counters, and in general little slower than registration with the aid of a position-sensitive counter. It is also shown that the dynamic range of the film method may be extended without sacrificing accuracy by resorting to slower films.