Response of photomultiplier tubes to xenon scintillation light

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 (∼ −100 • 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 (32.4±2.9)% at room temperature, we find a relative improvement of (17.9±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 (22.6±2.0)% at low temperature; the DPE increase relative to room temperature, measured here for the first time, was (12.2±3.9)%. Evidence of a small triple photoelectron emission probability (∼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 (9.1±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.

Projected WIMP sensitivity of the LUX-ZEPLIN dark matter experiment

“…S1 and S2 signals are measured in units of phd, an observable that accounts for double photoelectron emission from the PMT photocathode at these wavelengths [24]. The current estimate of g 1 is 11.9%, and both this estimate and g 1;gas are derived from optical simulations based on reflectivity measurements of the LZ PTFE [25][26][27]; measurements of the quantum efficiency, first dynode collection efficiency, and two photoelectron emission probability in a sample of the 3 00 Hamamatsu PMTs to be used in LZ [17]; and a photon absorption length in LXe motivated by the high light yields reported in the literature [28,29]. The electron extraction efficiency, not included in g 1;gas , is extrapolated from [30].…”

“…Photons are detected by 494 Hamamatsu R11410-22 3 00diameter photomultiplier tubes (PMTs), with a demonstrated low level of radioactive contamination [15,16] and high quantum efficiency [17] at the LXe scintillation wavelength of 175 nm [18]. The PMTs are assembled in two arrays viewing the LXe from above and below.…”

XENON1T dark matter data analysis: Signal and background models and statistical inference

“…Besides the intrinsic response of LXe, detector reconstruction effects on the S1 and S2 signals are modeled. More specifically, the spatial dependence of S1 and S2 signals, the single and double photoelectron (PE) emission of the PMT photocathode [32,33], the position reconstruction uncertainty, the reconstruction efficiency, bias, and signal fluctuations, and the acceptance of data selections in analysis are taken into account in the model.…”

“…where p dpe is the probability for the PMT photocathode to emit two photoelectrons when absorbing one photon [32,33], and ϵ ext is the extraction efficiency of the drifted electrons which is assumed to be constant in this study. Note that g 1 and g 2 in [10] correspond to the averages of g 0 1 ðx; y; zÞ and g 0 2 ðx; yÞ, respectively, in Eq.…”

Simulations of events for the LUX-ZEPLIN (LZ) dark matter experiment

“…An example of the single photon response distribution for an LZ TPC PMT is given in Figure 5. The response has been validated against measurements in [42]. The single photoelectron (SPE) peak is visible, followed by the DPE peak at roughly 20% the height [42].…”