Silver clusters of photographic interest Part 5: formation of R centres and P centres by reduction sensitization and light absorption of silver bromide grains

A model of silver clusters with R centres as traps for positive holes and P centres as traps for photoelectrons in silver halide grains was proposed in this series of papers, and provided several subjects for future studies after discussions at several international meetings. Considerations and some experimental examinations on those subjects by use of fine silver bromide grains were made in this study. With the amount of dimethylamine borane (DMAB) used for reduction sensitization, the sensitivity of these grains increased through two steps, as reported in this series of papers. At the beginning of the first step, the radiowave photoconductivity with positive holes as carriers at room temperature began to decrease, while the microwave photoconductivity with photoelectrons as carriers at room temperature began to decrease at the beginning of the second step. Thus, these experiments could clearly confirm the formation of R centres and P centres by reduction sensitization. It was observed by applying electron microscopy to gelatinate shells of the grains that reduction sensitization led to the formation of many silver clusters per grain, being highly contrasted with latent image formation, which led to the formation of only one silver cluster per grain regardless of exposure, owing to the concentration principle. The above-slated silver cluster formation by reduction was therefore not based on the concentration principle, but on the proposed mechanism without contributions from a free electron or a single silver atom to the cluster formation.