Submicron track readout in fine-grained nuclear emulsions using optical microscopy

In this paper, we deal with the problem of reading out submicron tracks in a fine-grained nuclear emulsion using optical microscopy for directional dark matter searches. We present a technique for automatically reading out the submicron track under an optical microscope that consists of expansion of the emulsion film and image processing. This expansion technique is suitable for reading out tracks in a large volume of emulsion film, because it can reduce the scanning load compared to conventional techniques. After noting the difference in shape between a track and a noise, we address the problem of estimating the incident direction of the dark matter. The technique is based on an elliptical fit to each grain in the optical image. The effectiveness of a track detection system that consists of both the methods is confirmed using low-velocity Kr ion tracks. We successfully recognize tracks with lengths as short as 150 nm and with an angular resolution of 0.66 radians. The system allows us to extract track signals in directional dark matter search experiments using nuclear emulsions.