Construction and performance evaluation of a meter-scale monolithic scintillator tracker with embedded scatterers
A meter-scale Fiber-Readout mOnolithic and Scatterer-embedded scintillator Tracker (FROST) was constructed and operated in the NINJA experiment to enable muon-track matching between the nuclear emulsion detectors and the downstream muon detector. The detector consists of a 1.32 m $\times$ 1.40 m plastic scintillator plate assembled from optically bonded scatterer-embedded plates, with wavelength-shifting fibers arranged at a pitch of 10 mm. During J-PARC neutrino-beam operation, FROST achieved a data acquisition efficiency of 99.59% and a detection efficiency of 99.946$\pm$0.006% for selected neutrino-induced muons well within the active area. A position reconstruction method based on the light-yield distribution was developed for both single- and multi-particle events. In a neutrino-interaction simulation, muon position resolutions of 1.77 mm and 1.61 mm were achieved in the two orthogonal readout directions, assuming correct muon identification. Using a beam-data sample dominated by single-muon events, upper limits on the position resolution of 2.20 mm and 2.03 mm were obtained for track angles below 20°. These results demonstrate that the FROST concept can be scaled up to a meter-scale active area while maintaining high detection efficiency and a position resolution well below the readout pitch under actual experimental conditions.