Use of X-Ray Film for comparing Radioactivities

Abstract

ALTHOUGH the initial discovery of radioactivity resulted from its action on a photographic plate, electronic instruments have to-day become accepted as the first choice for detecting, and more particularly for measuring, radioactivity. It has, however, been shown that photographic emulsions can, with suitable methods for internal calibration, be used for giving reasonably accurate comparisons of radioactivity¹. The present communication is concerned with a direct comparison of the accuracy obtained with a counter and with X-ray film, when used for comparing the radiation per unit area from a number of different sources. These were prepared by converting images from a photographic step-wedge quantitatively into silver iodide in the presence of iodine-131². The relative quantities of silver, and hence of radioactivity, in the sources were compared : (a) by direct chemical analysis ; (b) by a Geiger–Müller counter using a count of at least ten thousand for each specimen ; (c) by making a series of autoradiographs with accurately determined exposure times and deriving the relative activity of each source from plots of density against log-exposure time ; (d) by using visual density matching to determine (from the same autoradiographs) the exposure time needed by each source to give a standard (but not measured) density. The quantities in the weaker sources were then expressed as percentages of the strongest source.