Response of photomultiplier tubes to xenon scintillation light

TL;DR

This study calibrates 35 xenon-sensitive photomultiplier tubes for dark matter detection, revealing temperature-dependent quantum efficiency and double photoelectron emission probabilities crucial for interpreting liquid xenon scintillation data.

Contribution

First comprehensive calibration of Hamamatsu R11410-22 PMTs at low temperature, quantifying DPE and QE variations relevant for dark matter experiments.

Findings

QE improves by ~18% when cooled to -100°C

DPE probability increases by ~12% at low temperature

Triple photoelectron emission observed at ~0.6%

Abstract

We present the precision calibration of 35 Hamamatsu R11410-22 photomultiplier tubes (PMTs) with xenon scintillation light centred near 175 nm. This particular PMT variant was developed specifically for the LUX-ZEPLIN (LZ) dark matter experiment. A room-temperature xenon scintillation cell coupled to a vacuum cryostat was used to study the full-face PMT response at both room and low temperature $\textrm{($\sim$ -100$^\circ$C)}$, in particular to determine the quantum efficiency (QE) and double photoelectron emission (DPE) probability in LZ operating conditions. For our sample with an average QE of $\textrm{(32.4$\pm$2.9)%}$ at room temperature, we find a relative improvement of $\textrm{(17.9$\pm$5.2)%}$ upon cooling (where uncertainty values refer to the sample standard deviation). The mean DPE probability in response to single vacuum ultraviolet (VUV) photons is $\textrm{(22.6$\pm$2.0)%}$ at low temperature; the DPE increase relative to room temperature, measured here for the first time, was $\textrm{(12.2$\pm$3.9)%}$. Evidence of a small triple photoelectron emission probability $\textrm{($\sim$0.6%)}$ has also been observed. Useful correlations are established between these parameters and the QE as measured by the manufacturer. The single VUV photon response is also measured for one ETEL D730/9829QB, a PMT with a more standard bialkali photocathode used in the ZEPLIN-III experiment, for which we obtained a cold DPE fraction of $\textrm{(9.1$\pm$0.1)%}$. Hence, we confirm that this effect is not restricted to the low-temperature bialkali photocathode technology employed by Hamamatsu. This highlights the importance of considering this phenomenon in the interpretation of data from liquid xenon scintillation and electroluminescence detectors, and from many other optical measurements in this wavelength region.