Abstract: Cubic silver bromide emulsions which were sensitized internally with (NH₄)₂ IrCl₆ + K₂PtCl₅, (NH₄)₃ IrCl₆, and Na₂S₂O₃, respectively, were prepared. The emulsions were fogged by thiourea dioxide. It was found that fogging centers were formed in the interior of emulsion grain and grain surface, and fogging centers were formed preferentially in the interior of emulsion grain. It is suggested that fogging agent molecules collided with silver ions on the grain surface forming single isolated silver atom, which subsequently loses electrons to the conduction band of the crystal. Therefore, it is conclused that electron of fogging agent molecules are transfered to conduction band of silver halide crystal such as photo-electron in the conduction band of tht crystal, electrons of conduction band are trapped by sensitivity centers and lattice disorder forming fogging centers.

Key words: sensitivity center, fogging agent, fogging center