Radiobiological aspects of supralinear photographic emulsions

Nuclear emulsions of the Ilford K series, exposed to X-rays from 15 to 150 kVp and systematically underdeveloped, exhibit supralinear blackness--exposure curves ranging from 1- to 4-hit in character, and whose shape is the same at all excitation voltages. Processing can be so adjusted that the curve shape is well represented by the alpha--beta model, and matches an experimental dose--response curve obtained with hamster cells after irradiation with gamma rays. The effective grain size of these emulsions is 1.3 micron when normalised to unit density material for comparison with the radiation sensitive elements of biological cells. With fractionated exposures, response curves which mimic Elkind recovery are produced.