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A: A LED-based fiber calibration system for the ProtoDUNE-Dual Phase (DP) photon detection system (PDS) has been designed and validated. ProtoDUNE-DP is a 6x6x6 m 3 liquid argon time-projection-chamber (LAr TPC) currently being installed at the Neutrino Platform at CERN. The PDS is based on 36 8-inch photomultiplier tubes (PMTs) and will allow triggering on cosmic rays. The system serves as prototype for the PDS of the final DUNE DP far detector in which the PDS also has the function to allow the 3D event reconstruction on non-beam physics. For this purpose an equalized PMT response is desirable to allow using the same threshold definition for all PMT groups, simplifying the determination of the trigger efficiency. The light calibration system (LCS) described in this paper is developed to provide this and to monitor the PMT performance in-situ.

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The Hamamatsu R5912-02Mod photomultiplier tube (PMT) will be used in the DUNE dual-phase module, a 10-kton fiducial volume liquid-argon time-projection chamber, which is one of the four projected far-detector modules of the DUNE long-baseline neutrino experiment. In the DUNE dualphase module, the liquid argon places high pressure on the photo-detectors located at the bottom of the 14-m cryostat. Four Hamamatsu R5912-02Mod PMTs were tested at 4-bar absolute pressure and cryogenic temperature (in liquid nitrogen) for the first time. No mechanical or electrical damage is reported, validating the use of this PMT model in the DUNE dual-phase module and in other large scale cryogenic liquid detectors. The differences observed in their behavior are expected for the change in the PMT operating temperature.