Latent image stabilization in nuclear emulsions for cosmic-ray imaging

Abstract Latent image stability is one of the most important characteristics of nuclear emulsions for cosmic-ray imaging. The emulsions studied are composed of AgBrI grains (about 200 nm in diameter) with a benzothiazolium compound (Additive Ⅰ) as a new latent-image stabilizer, and characterized by means of photoelectron yield spectroscopy in air (PYSA). The results thus observed are discussed in the light of the mechanism of latent image formation. A latent image center is composed of a small Ag cluster, and situated at a site with electric charge of +1/2, thus being expressed as Agn +1∕2 . The growth processes for a latent image center are the trapping of an electron by Ag n−1 +1∕2 to form Ag n−1 −1∕2 (electronic process), and the attachment of an interstitial silver ion to Ag n−1 −1∕2 to form Ag n +1∕2 (ionic process). It is thus considered that the first step of a latent image fading is its discharge of a silver ion to form Ag n−1 −1∕2 , and that the second step is the oxidation of Ag n−1 −1∕2 to form Ag n−1 +1∕2 . It is proposed and has been confirmed by means of PYSA that Additive Ⅰ opens its ring to disclose –SH and depresses the first step of latent image fading by forming on a latent image center barely soluble silver salt with tolerance to oxidation. Discussions have been made on latent image stabilization in nuclear emulsions for cosmic-ray imaging.