  We report the results of a characterization study of several types of
cryogenic photomultipliers manufactured by Hamamatsu Photonics and intended for
operation in liquid Ar conditions, namely: compact 2-inch R6041-506MOD tubes,
3-inch R11065-10 and R11065-MOD tubes for operation in liquid Ar and 3-inch
R11410-20 tubes originally designed for operation in liquid Xe. These types of
PMT are proposed for installation into the future two-phase cryogenic avalanche
detector that is developed in the Laboratory of Cosmology and Particle Physics
of the Novosibirsk State University jointly with the Budker Institute of
Nuclear Physics. Eight R11065 PMTs and seven R11410-20 tubes were tested and
all demonstrated excellent performance in liquid Ar in terms of gain and
relative single electron efficiency. All 3-inch PMTs showed a maximal gain in
liquid Ar above 5x10^6 and relative single electron efficiency higher than 95%.
Compact R6041-506MOD tubes have dynode system different from that of the 3-inch
photomultipliers and thus their single electron energy resolution and relative
efficiency is much worse than that of 3-inch tubes. From 21 2-inch PMTs only 12
tubes were selected with acceptable, i.e. higher than 75%, relative single
electron efficiency and the maximal gain higher than 5x10^6. However, these
PMTs are very attractive because they are the only compact type of tubes that
can operate in liquid Ar.
