  The 20kt Liquid Scintillator (LS) JUNO detector is being constructed by the
International Collaboration in China, with the primary goal of addressing the
question of neutrino mass ordering (hierarchy). The main challenge for JUNO is
to achieve a record energy resolution, ~3% at 1MeV of energy released in the
LS, which is required to perform the neutrino mass hierarchy determination.
About 20 000 large 20-inch PMTs with high Photon Detection Efficiency (PDE) and
good photocathode uniformity will ensure an approximately 80% surface coverage
of the JUNO detector. The JUNO collaboration is preparing equipment for the
mass tests of all PMTs using 4 dedicated containers. This approach allows us to
test 144 PMTs in parallel. The primary measurement in the container will be the
PMT response to illumination of its photocathode by a low-intensity uniform
light. Each of the 20 000 PMTs will undergo the container test. Additionally, a
dedicated scanning system was constructed for sampled tests of PMTs that allows
us 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 of
calibrated short light flashes that are placed along the photocathode surface
covering zenith angles from the top of a PMT to its equator. The collection
efficiency of a large PMT is known to be very sensitive to the Earth Magnetic
Field (EMF), therefore, understanding the necessary level of EMF suppression is
crucial for the JUNO Experiment. A dark room with Helmholtz coils compensating
the EMF components is available for these tests at a JUNO facility. The
Hamamatsu R12860 20-inch PMT is a candidate for the JUNO experiment. In this
article the container design and mass-testing method, the scanning setup and
scanning method are briefly described and preliminary results for performance
test of this PMT are reported.
