Improved quality tests of R11410-21 photomultiplier tubes for the XENONnT experiment

TL;DR

This paper presents an enhanced testing procedure for R11410-21 photomultiplier tubes used in the XENONnT dark matter experiment, leading to better performance and reliability by detecting issues like light emission and vacuum degradation.

Contribution

The study introduces new testing methods and upgraded facilities for PMT quality assurance, reducing rejections and improving performance for XENONnT.

Findings

Rejection of 26 out of 368 tested PMTs due to issues.

Development of tests for light emission and vacuum leaks.

Expected improved PMT performance in XENONnT.

Abstract

Photomultiplier tubes (PMTs) are often used in low-background particle physics experiments, which rely on an excellent response to single-photon signals and stable long-term operation. In particular, the Hamamatsu R11410 model is the light sensor of choice for liquid xenon dark matter experiments, including XENONnT. The same PMT model was also used for the predecessor, XENON1T, where issues affecting its long-term operation were observed. Here, we report on an improved PMT testing procedure which ensures optimal performance in XENONnT. Using both new and upgraded facilities, we tested 368 new PMTs in a cryogenic xenon environment. We developed new tests targeted at the detection of light emission and the degradation of the PMT vacuum through small leaks, which can lead to spurious signals known as afterpulses, both of which were observed in XENON1T. We exclude the use of 26 of the 368 tested PMTs and categorise the remainder according to their performance. Given that we have improved the testing procedure, yet we rejected fewer PMTs, we expect significantly better PMT performance in XENONnT.