| Summary: | A first experimental test of tracking relativistic charged particles by `drifting' Cherenkov photons in a water-based optical time-projection chamber (OTPC) at the Fermilab Test Beam Facility is described. By measuring the relative time-of-arrival and (z,&phis;) coordinates of individual photons, we show spatial and angular resolutions on the charged particle track of 15 mm and 60 mrad, respectively, over a track length of 40 cm. The OTPC consists of a 77 cm long, 40~kg cylindrical water mass instrumented with a combination of commercial 5.1x 5.1 cm2 micro-channel plate photo-multiplier tubes (MCP-PMT) and 6.7 x6.7 cm2 mirrors. Using planar MCP-PMTs with an anode of 50O microstrips, it is feasible to resolve the time-of-arrival of a single photon to ≤100 ps and its detected position to a few~mm. The MCP-PMTs are installed in two columns along the OTPC cylinder in a small-angle stereo configuration. A mirror is mounted opposing each MCP-PMT on the far side of the detector cylinder, which effectively doubles the photo-detection efficiency and provides a time-resolved image of the Cherenkov light on the opposing wall. A 180-channel data acquisition system digitizes the MCP-PMT signals using the PSEC4 waveform sampling chip operating at 10 Gigasamples-per-second. The detector was installed on the Fermilab MCenter test-beam in a location where the primary flux is multi-GeV muons. Approximately 80 Cherenkov photons are detected for a through-going muon track in an event duration of 2 ns.
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