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

TL;DR

This paper reports on the characterization of Hamamatsu R11410-21 PMTs for the XENON1T dark matter detector, focusing on quantum efficiency, stability, and radioactivity at cryogenic temperatures, ensuring suitability for sensitive dark matter searches.

Contribution

It provides detailed measurements of PMT performance at low temperatures, introduces a low-radioactivity voltage divider, and demonstrates stable operation in liquid xenon over extended periods.

Findings

Quantum efficiency increased by 10-15% at -110°C at 175 nm.

PMTs showed stable operation and linear response down to 5×10^4 photoelectrons.

Radioactive contamination levels meet XENON1T requirements.

Abstract

We report on the progress in characterization of the Hamamatsu model R11410-21 Photomultiplier tubes (PMTs) for XENON1T dark matter experiment. The absolute quantum efficiency (QE) of the PMT was measured at low temperatures down to -110 $^0$C (a typical the PMT operation temperature in liquid xenon detectors) in a spectral range from 154.5 nm to 400 nm. At -110 $^0$C the absolute QE increased by 10-15\% at 175 nm compared to that measured at room temperature. A new low power consumption, low radioactivity voltage divider for the PMTs is being developed. The measurement results showed that the PMT with the 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 radioactive contamination induced by the PMT and the PMT voltage divider materials satisfies the requirements for XENON1T detector not to exceed a total radioactive contamination in the detector of 0.5 evts/year/1tonn. Most of the PMTs received from the manufacturer showed a high quantum efficiency exceeding 30\%. In the mass production tests the measurements at room temperature showed clear single photoelectron peaks for all PMTs been under study. The optimal operation conditions were found at a gain of 2$\cdot$10$^6$. The operation stability 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 using the XENON1T signal readout components including voltage dividers, cables and feedthroughs. The PMTs tested in liquid xenon demonstrated a stable operation for a time period of more than 5 months.