The 20kt Liquid Scintillator (LS) JUNO detector is being constructed by theInternational Collaboration in China, with the primary goal of addressing thequestion of neutrino mass ordering (hierarchy). The main challenge for JUNO isto achieve a record energy resolution, ~3% at 1MeV of energy released in theLS, which is required to perform the neutrino mass hierarchy determination.About 20 000 large 20-inch PMTs with high Photon Detection Efficiency (PDE) andgood photocathode uniformity will ensure an approximately 80% surface coverageof the JUNO detector. The JUNO collaboration is preparing equipment for themass tests of all PMTs using 4 dedicated containers. This approach allows us totest 144 PMTs in parallel. The primary measurement in the container will be thePMT response to illumination of its photocathode by a low-intensity uniformlight. Each of the 20 000 PMTs will undergo the container test. Additionally, adedicated scanning system was constructed for sampled tests of PMTs that allowsus to study the variation of the PDE over the entire PMT photocathode surface.The core of the scanning station is a rotating frame with 7 LED sources ofcalibrated short light flashes that are placed along the photocathode surfacecovering zenith angles from the top of a PMT to its equator. The collectionefficiency of a large PMT is known to be very sensitive to the Earth MagneticField (EMF), therefore, understanding the necessary level of EMF suppression iscrucial for the JUNO Experiment. A dark room with Helmholtz coils compensatingthe EMF components is available for these tests at a JUNO facility. TheHamamatsu R12860 20-inch PMT is a candidate for the JUNO experiment. In thisarticle the container design and mass-testing method, the scanning setup andscanning method are briefly described and preliminary results for performancetest of this PMT are reported.
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