The T2K fine-grained detectors

The T2K Fine-Grained Detectors (FGDs) are segmented scintillator detectors that form part of the Tracker for T2K's ND280 near detector. The FGDs are composed of square scintillator bars glued together to form tracking planes. Each scintillator bar has a co-extruded titanium dioxide coating and a central hole that contains a wavelength-shifting fiber. These fibers carry scintillator light to Hamamatsu Multi-Pixel Photon Counters at one end of each fiber, with the far end of the fiber mirrored to increase the light yield. Photosensor readout is by waveform digitization. The FGDs serve as the primary target mass for neutrino interactions in the ND280's Tracker. The first FGD was fully assembled and partially instrumented in the fall of 2008. It was tested in the M11 secondary beamline at TRIUMF. Preliminary analysis of the beam test results indicate that the performance is as expected, and there is good agreement between measurements and GEANT4 based simulations.