Images on developed photographic emulsions can be significantly intensified by making the image silver radioactive and exposing a second emulsion to this radiation. Because the print, an autoradiograph, reproduces the original with significant increases in density and contrast, the method can be used to enhance underexposed emulsions and to increase the limits of photographic detection. This paper reports a new, practical chemical method for activating the silver, using thiourea labeled with sulfur-35, a process convenient for use in photographic laboratories. Speed increases of 13 and contrast increases of 18 were achieved with Kodak Plus-X film in this initial phase.
Barbara S. Askins, "Photographic image intensification by autoradiography," Appl. Opt. 15, 2860-2865 (1976)