Dark matter event in nuclear emulsions: Comparative analysis by time-resolved photoconductivity of silver halide grains

Abstract

Dark matter (DM) detection by new nuclear emulsions composed of 40 nm AgBrI grains (Lindhard–Scharff theory) has been compared with minimum ionizing particle (MIP) detection and radiation therapy (RT) by conventional nuclear emulsions composed of 200 nm AgBrI grains (Bethe-Bloch theory), and with picture-taking by color films composed of 500–1000 nm AgBrI grains (band-to-band transition), and characterized by its uniqueness: latent image formation in the presence of electron–hole pairs with extremely high concentration (∼1000/grain) and insufficient space for chemical sensitization centers on a grain (∼10/grain). While AgBrI grains in nuclear emulsions for RT are also associated with electron–hole pairs with high concentration, they have space available for chemical sensitizations. Then, we have studied the effect of these characteristics on latent image formation by analyzing the behaviors of electrons and positive holes with variation of their concentrations in 200 nm AgBr grains by means of time-resolved microwave and radio-frequency photoconductivity methods. It has been predicted from the results of the above-stated experiments that DM detection and RT by nuclear emulsions are disturbed by direct recombination and by reaction between Agn and Br2 owing to high activity of positive holes on the contrary to MIP detection by nuclear emulsions and picture-taking by color films, both of which are solely disturbed by indirect recombination. On the basis of the obtained results, discussions are made on ideas to enhance the latent image formation in the presence of electron–hole pairs with high concentration.