Photon detector system timing performance in the DUNE 35-ton prototype liquid argon time projection chamber.

The 35-ton prototype for the Deep Underground Neutrino Experimentfar detector was a single-phase liquid argon time projection chamberwith an integrated photon detector system, all situated inside a membrane cryostat. The detector took cosmic-raydata for six weeks during the period of February 1, 2016 to March 12, 2016. The performance of the photon detection system was checked with these data. An installed photon detector was demonstrated to measure the arrival times of cosmic-raymuons with a resolution better than 32 ns, limited by the timing of the trigger system. A measurement of the timing resolution using closely-spaced calibration pulses yielded a resolution of 15 ns for pulses at a level of 6 photo-electrons. Scintillation light from cosmic-raymuons was observed to be attenuated with increasing distance with a characteristic lengthof $155 \pm 28$ cm.