R&D Status of Nuclear Emulsion for Directional Dark Matter Search

In this study, we are doing R&D for directional dark matter search with nuclear emulsion. First of all, higher resolution nuclear emulsion with fine silver halide crystals was developed in the production facility of emulsion at Nagoya university, and we confirmed that it can detect the expected nuclear recoil tracks. The readout of submicron tracks was required the new technology. We developed the expansion technique, and could readout the signal by shape analysis with optical microscopy. The two dimensional angular resolution is 36 degrees at the original track length of range from 150 nm to 200 nm with optical microscopy. Finally we demonstrated by using recoiled nuclei induced by 14.8 MeV neutron, and confirmed the technique. Moreover, we developed the X-ray microscope system with SPring-8 as final check with higher resolution of selected candidate tracks with optical microscopy. The angular resolution was improved from 31 degree with optical microscopy to 17 degree with X-ray microscopy at the track length of range from 150 nm to 250 nm. We are developing the practical system and planning for start of the test running with prototype detector.