Resetting of nuclear emulsions by erasing noises formed during storage: Mechanism and application

Abstract

The resetting of a nuclear emulsion is one of the most important technologies for the start of its new era and was realized in the OPERA experiment. Since the OPERA experiment was carried out several months after the production of its nuclear emulsion layers, AgBrI grains in the layers were exposed to ionizing particles before experiments, and formed latent image centers as noises on their surfaces. The technology to reset the emulsion layers was needed and achieved by the refresh treatment, which erased the latent image centers by enhancing their fading under the condition with high humidity at high temperature in the presence of 5-methyl-benzotriazole (5-MBT). While the treatment is being applied to other experiments with nuclear emulsions, its mechanism has not yet been made clear. Taking into account the fact that 5-MBT played a central role for the effect of the treatment, this study has been undertaken to reveal the mechanism of the treatment by focusing on the behavior of 5-MBT. It has been revealed that the irreversible phase transition of 5-MBT for the formation of epitaxial silver salt nanoparticles of 5-MBT anions at the corners of the AgBrI grains during the treatment enhances the detachment of a silver ion from a latent image center on a site with electric charge of +1/2, thus making the center to bear the electric charge of −1/2 and unstable against oxidation. Ideas have been proposed to make the treatment available for other experiments with nuclear emulsions.