Lowering the radioactivity of the photomultiplier tubes for the XENON1T dark matter experiment

TL;DR

This paper details the development and screening of ultra-low radioactivity photomultiplier tubes for the XENON1T dark matter detector, significantly reducing background noise for more sensitive dark matter searches.

Contribution

It introduces a new low-radioactivity PMT design and demonstrates its effectiveness through extensive material screening and background measurements for the XENON1T experiment.

Findings

Successful production of ultra-low background PMTs

Reduced radioactive contamination in PMT components

Implications for lower background levels in dark matter detection

Abstract

The low-background, VUV-sensitive 3-inch diameter photomultiplier tube R11410 has been developed by Hamamatsu for dark matter direct detection experiments using liquid xenon as the target material. We present the results from the joint effort between the XENON collaboration and the Hamamatsu company to produce a highly radio-pure photosensor (version R11410-21) for the XENON1T dark matter experiment. After introducing the photosensor and its components, we show the methods and results of the radioactive contamination measurements of the individual materials employed in the photomultiplier production. We then discuss the adopted strategies to reduce the radioactivity of the various PMT versions. Finally, we detail the results from screening 216 tubes with ultra-low background germanium detectors, as well as their implications for the expected electronic and nuclear recoil background of the XENON1T experiment.