Development of new-type nuclear emulsion for a balloon-borne emulsion gamma-ray telescope experiment

This study reports a new-type of nuclear emulsion that improves the track-finding efficiency of charged particle detection. The emulsion is applied to the GRAINE project, a balloon-borne experiment that observes cosmic γ-rays through an emulsion γ-ray telescope. The new emulsion film dramatically improves the detection efficiency for γ-rays. The nuclear emulsion gel and films for the second GRAINE balloon-borne experiment (GRAINE-2015) were fully self-produced by ourselves. New handling methods for the novel emulsion film have also been developed. Over time, the stored films gradually become desensitized to minimum ionizing particles, but the original sensitivity can be restored by a resetting process. Moreover, the fading of latent images can be arrested by a drying process. To sensitize the new-type films and avoid their fading, emulsion preprocessing was applied immediately prior to GRAINE-2015. A balloon flight with the emulsion γ-ray telescope was successfully completed in Australia on 12th May 2015. By scanning with automated optical microscopes and analyzing the penetrated tracks, we confirmed the high track-finding efficiency (97%) of the mounted films. The analysis of γ-ray event detection, aims at detecting Vela pulsar, is in progress.