Shrinkage of Photographic Emulsions for Nuclear Research

Abstract

THE photographic emulsions used for the recording of ionizing particles contain a very high ratio of silver halide to gelatin. This results in a considerable shrinkage of the emulsion at processing, due to the washing out of the silver halide. In order to be able to interpret correctly events occurring in the emulsion, or to measure accurately the lengths of tracks which have a large angle of dip, it is important to know the exact relationship between the orientation of the track in the processed emulsion and the path of the particle while passing through the plate. This relationship is usually given by the so-called shrinkage factor S, which is the ratio of the thickness of the emulsion at exposure to that after processing ; it can be determined either directly, by measuring the thicknesses of processed and unprocessed strips of emulsion, or by observations on tracks of known angle of dip¹ or known length². The usual way to correct for shrinkage is to multiply the vertical projection of the track-length by the shrinkage factor. This is based on the assumption that the shrinkage affects all tracks in a uniform manner ; but this is not immediately obvious, owing to the complicated phenomena which occur at processing. Taking into account that the photographic emulsion technique is becoming a method of high precision, it was thought worth while to study the shrinkage problem in greater detail.