Dark matter event in nuclear emulsions Ⅱ: Ionic relaxation and recombination

The previous paper has indicated that it is important to depress both direct recombination and reverse reaction between ionic relaxation products, i.e., latent image centers (Agn) and Br2 for dark matter (DM) detection by a super-fine-grained nuclear emulsion named NIT. This paper describes the results of the analyses on the reverse reaction in the NIT containing a typical halogen acceptor Na2SO3 (SS), which is used to make the NIT sensitive. The electronic structure of the NIT layer containing SS as measured by means of photo-electron yield spectroscopy in air revealed that Ag(SO3)23− was formed in emulsions and could become to be Ag(SO3)22−by giving an electron to Br2. Although SS could not stop the reverse reaction, SS made it so slow that SS could make the NIT with SS available for DM detection when the irradiated NIT layers were kept at low temperature (−20 °C to −50 °C) in order to bring about further slowdown of the reaction rate. It is thought that Ag(SO3)22− took a place of Br2 for the reverse reaction and decreased its rate, since the diffusion of Ag(SO3)22−, which is electrically charged and large in size, is slower than that of Br2 owing to attractive interaction between Ag(SO3)22− and many ionized substituents in gelatin. Discussions are made on ideas to depress further the reverse reaction.