Development and utilization of ``Plate Changer'' system for neutrino interaction locations in OPERA emulsion target

In the OPERA experiment, so-called Scan Back method is used to locate neutrino interaction vertices in the emulsion target named ECC (Emulsion Cloud Chamber). In Scan Back method, tracks detected in the most downstream emulsion plate in ECCs are followed to upstream plate by plate until it reaches to the interaction points. In order to treat a number of neutrino interactions recorded in OPERA, dedicated systems called Plate Changer has been developed and utilized to neutrino event location in Japan. The details of the system have been described in this report. Until the end of November 2012, 6223 Scan Back trials have been performed using this system, and strongly contributed to the detection of tau neutrino appearance.