Effects of iridium doping in cubic and octahedral AgBr grains on the latent image formation process

Effects of iridium doping on the differences in photographic behavior betwe en cubic and octahedral AgBr grains have been investigated. The experimenta l AgBr grains were prepared without spectral or chemical sensitization. The iridium ions were incorporated into the major shell region of each grain ( shell/core = 36/1) with different concentrations of iridium (0 similar to 1 0(-5) mol Ir/mol Ag). Regardless of the iridium doping amount, it was obser ved that latent images of cubic grains are almost all located at the surfac e, while in octahedral grains some latent images are also located inside th e grains. These features are explained by relating the latent image positio n to the potential difference across the space charge layer in octahedral a nd cubic grains. Undoped octahedral grains exhibited reduced effectiveness in latent image formation than undoped cubic grains. Introduction of iridiu m doping (with a concentration of 10(-6) mol Ir/mol Ag) remarkably eliminat es this reduced effectiveness observed for octahedral grains. In addition, iridium doping gives the octahedral grains a higher sensitivity increase du ring delayed latent image formation, compared to cubic grains. Such behavio r is discussed in terms of a greater number of electrons being temporarily trapped by iridium centers.