Track formation in nuclear emulsion plates for cosmic-ray imaging with stabilized Ag nanoparticles

In cosmic-ray imaging system with nuclear emulsions, silver halide (AgX) grains capture muon-created electrons to form a latent image composed of small Ag nanoparticles (latent image centers), which could be as small as three atoms. Since it needs such a long time as several months for the grains to capture sufficient number of muon-created electrons for imaging, the stabilization of latent image is inherently important. There are two kinds of latent image fading; the first- and second-order reactions. The former is the oxidation of latent image centers by oxidants, which are not formed by cosmic-ray irradiation, and the latter is the re-halogenation of latent image centers by irradiation-created $X_2$. It was revealed that the latter became to be predominant over the former within several days after irradiation as the volume ratio of gelatin in emulsion layers decreased, indicating that gelatin is very effective to depress this type of latent image fading. Following the previous paper, we have extended our study on the property and behavior of new additives by means of photoelectron spectroscopy to prove the mechanism of the latent image fading with the first-order reaction, according to which a latent image center with electric charge of +1/2 degrades via the repetition of the ionic process (detachment of an Ag${}^{+}$ion) and the electronic process (losing of an electron) one after the other, its electric charge changing to -1/2 and back. The mechanism thus proved has provided several ideas to depress latent image fading in nuclear emulsions for cosmic-ray imaging.