Nuclear emulsion and its scanning
Recently, the OPERA experiment has reported a 4.2sigma evidence for nu_mu -> nu_tau oscillations in appearance mode. This experiment was performed by using 9 million nuclear emulsion films, of which dimension is 4×5 inches, since nuclear emulsion was the almost only detector to identify tau neutrino interactions. Nuclear emulsion is a true 3-dimensional tracking detector with sub-micron spatial resolution. However only a percent of nuclear emulsion was scanned in the OPERA experiment due to the slow speed of the nuclear emulsion read-out systems. Now we developed a new generation of nuclear emulsion scanning system ‘HTS’. One of the goal of HTS is 2 orders of magnitude faster than previous systems. The nuclear emulsion films of the 2nd flight of GRAINE project, which is a balloon borne high resolution gamma-ray telescope with nuclear emulsion, will be scanned by HTS as commissioning this May.