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We report on the progress in characterization of the Hamamatsu modelR11410-21 Photomultiplier tubes (PMTs) for XENON1T dark matter experiment. Theabsolute quantum efficiency (QE) of the PMT was measured at low temperaturesdown to -110 $^0$C (a typical the PMT operation temperature in liquid xenondetectors) in a spectral range from 154.5 nm to 400 nm. At -110 $^0$C theabsolute QE increased by 10-15\% at 175 nm compared to that measured at roomtemperature. A new low power consumption, low radioactivity voltage divider forthe PMTs is being developed. The measurement results showed that the PMT withthe current version of the divider demonstrated a linear response (within 5\%)down to 5$\cdot$10$^4$ photoelectrons at a rate of 200 Hz. The radioactivecontamination induced by the PMT and the PMT voltage divider materialssatisfies the requirements for XENON1T detector not to exceed a totalradioactive contamination in the detector of 0.5 evts/year/1tonn. Most of thePMTs received from the manufacturer showed a high quantum efficiency exceeding30\%. In the mass production tests the measurements at room temperature showedclear single photoelectron peaks for all PMTs been under study. The optimaloperation conditions were found at a gain of 2$\cdot$10$^6$. The operationstability for most of the PMTs was also demonstrated at a temperature of -100$^0$C. A dedicated setup was built for testing the PMTs in liquid xenon usingthe XENON1T signal readout components including voltage dividers, cables andfeedthroughs. The PMTs tested in liquid xenon demonstrated a stable operationfor a time period of more than 5 months.

Progress in characterization of the Photomultiplier Tubes for XENON1T Dark Matter Experiment