We report the results of a characterization study of several types ofcryogenic photomultipliers manufactured by Hamamatsu Photonics and intended foroperation 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-inchR11410-20 tubes originally designed for operation in liquid Xe. These types ofPMT are proposed for installation into the future two-phase cryogenic avalanchedetector that is developed in the Laboratory of Cosmology and Particle Physicsof the Novosibirsk State University jointly with the Budker Institute ofNuclear Physics. Eight R11065 PMTs and seven R11410-20 tubes were tested andall demonstrated excellent performance in liquid Ar in terms of gain andrelative single electron efficiency. All 3-inch PMTs showed a maximal gain inliquid 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-inchphotomultipliers and thus their single electron energy resolution and relativeefficiency is much worse than that of 3-inch tubes. From 21 2-inch PMTs only 12tubes were selected with acceptable, i.e. higher than 75%, relative singleelectron efficiency and the maximal gain higher than 5x10^6. However, thesePMTs are very attractive because they are the only compact type of tubes thatcan operate in liquid Ar.
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