Analyses and design of nuclear emulsions for dark matter detection

Abstract

Studies were made on a nuclear emulsion system with 40-nm AgBrI grains for dark matter detection. This system is characterized by its efficient detection of a nuclear recoil to form a latent image center composed of several Ag atoms on the grain and by reduction of an AgBrI grain with the center to form an Ag grain by development, achieving a degree of magnification of ~106. Dark matter detection with this system has two problems: rapid recombination between electrons and positive holes with high concentrations (≥1000 per grain at its maximum) for short times (10–100 fs) in the first step and re-halogenation of once formed latent image centers by halogen molecules formed from positive holes, taking place for long times (up to several days) in the second step of dark matter events. In the framework of detective quantum efficiency (DQE) as a criterion for system evaluation, theoretical and experimental approaches were used to characterize and design emulsions for dark matter detection, in which low-velocity ion beams were used to simulate the behavior of nuclear recoil induced by dark matter events. Instead of chemical sensitization used in conventional photographic emulsions, the immersion of the emulsion layers in an aqueous solution of sodium sulfite (i.e. HA treatment) was most effective for latent image formation by ion beams. Ion beam experiments and photoelectron spectroscopy revealed that HA treatment could overcome both of the problems. Ideas for the reduction of fog in nuclear emulsions for dark matter detection considering DQE are proposed.