An Ultra-Low Background PMT for Liquid Xenon Detectors

Abstract: Results are presented from radioactivity screening of two models of photomultiplier tubes designed for use in current and future liquid xenon experiments. The Hamamatsu 5.6 cm diameter R8778 PMT, used in the LUX dark matter experiment, has yielded a positive detection of four common radioactive isotopes: 238U, 232Th, 40K, and 60Co. Screening of LUX materials has rendered backgrounds from other detector materials subdominant to the R8778 contribution. A prototype Hamamatsu 7.6 cm diameter R11410 MOD PMT has also been screened, with benchmark isotope counts measured at <0.4 238U / <0.3 232Th / <8.3 40K / 2.0+-0.2 60Co mBq/PMT. This represents a large reduction, equal to a change of \times 1/24 238U / \times 1/9 232Th / \times 1/8 40K per PMT, between R8778 and R11410 MOD, concurrent with a doubling of the photocathode surface area (4.5 cm to 6.4 cm diameter). 60Co measurements are comparable between the PMTs, but can be significantly reduced in future R11410 MOD units through further material selection. Assuming PMT activity equal to the measured 90% upper limits, Monte Carlo estimates indicate that replacement of R8778 PMTs with R11410 MOD PMTs will change LUX PMT electron recoil background contributions by a factor of \times1/25 after further material selection for 60Co reduction, and nuclear recoil backgrounds by a factor of \times 1/36. The strong reduction in backgrounds below the measured R8778 levels makes the R11410 MOD a very competitive technology for use in large-scale liquid xenon detectors.

• The paper reveals that the R11410 MOD PMT dramatically reduces radioactivity, achieving up to 1/24 reduction for 238U and 1/9 for 232Th compared to the R8778.
• It employs precise radioactivity screening using a germanium detector and Geant4 Monte Carlo simulations at SOLO to quantify isotope levels accurately.
• These advancements enable LXe detectors like LUX and LZ to maintain ultra-low backgrounds, enhancing sensitivity toward dark matter detection.

Ultra-Low Background PMTs in Liquid Xenon Detectors

Introduction

The paper "An Ultra-Low Background PMT for Liquid Xenon Detectors" presents crucial findings on photomultiplier tubes (PMTs) screening, focusing on reducing radioactive background necessary for improved performance in liquid xenon (LXe) detectors. The research examines the radiopurity of two distinct PMT models developed by Hamamatsu: the R8778 utilized in the Large Underground Xenon (LUX) experiment, and the newer R11410~MOD, a promising candidate for the upcoming LUX-ZEPLIN (LZ) detector.

This study underscores the significance of minimizing radioactive emissions from PMTs, particularly when they are positioned close to the active detecting region, potentially contributing undesirable backgrounds that could obscure dark matter signals.

Photomultiplier Tube Characteristics

The tested Hamamatsu PMTs, R8778 and R11410~MOD, are specifically optimized for LXe applications due to their high quantum efficiency at the 178 nm scintillation wavelength characteristic of xenon, coupled with excellent single-photon resolution capabilities. The distinction between these models primarily lies in their photocathode dimensions: the R11410~MOD features a 6.4 cm diameter, effectively doubling the surface area compared to the 4.5 cm R8778, while substantially reducing radioactivity levels, a pivotal advancement for scaling up detector applications.

Figure 1: The Hamamatsu R8778 PMT, used in the LUX experiment (left), and the R11410~MOD PMT, a candidate for use in the LZ detector (right).

Radioactivity Screening Methodology

At the heart of this research is the rigorous radioactivity screening conducted at the Soudan Underground Laboratory's Soudan Low-Background Counting Facility (SOLO). SOLO's germanium detector setup is detailed, serving as the primary tool for accurately measuring radioactive isotope content. Through precision calibration using Geant4 Monte Carlo simulations, PMT batches underwent intensive testing to isolate and quantify isotopes such as $^{238}$U, $^{232}$Th, $^{40}$K, and $^{60}$Co.

Figure 2: The open SOLO chamber, demonstrating the robust shielding and the high-purity germanium detector used for radioactivity measurements.

The R8778 PMTs demonstrated significant isotope presence, with counts notably reduced in the R11410~MOD due to enhanced material selection and radiopurity efforts. The findings from SOLO highlight a dramatic reduction, nearly by factors $\times\frac{1}{24}$ for $^{238}$U and $\times\frac{1}{9}$ for $^{232}$Th, between the older and newer PMT models.

Figure 3: Comparison of calibration Monte Carlo output with data from a $^{60}$Co source, illustrating the methodology for determining radioactivity.

Implications for PMT Background in LXe Detectors

An analysis of neutron backgrounds deriving from ($\alpha$,n) reactions and spontaneous fission indicate a substantial reduction potential in next-generation detectors. Specifically, transitioning from R8778 to R11410~MOD PMTs in LUX yields a predicted background decrease by factors of $\times\frac{1}{25}$ for electron recoil (ER) and $\times\frac{1}{36}$ for nuclear recoil (NR) after optimizing material selection further to reduce $^{60}$Co activity.

Figure 4: SOLO PMT spectra showcasing the comparative radioactivity levels between the R8778 and the considerably less active R11410~MOD.

Prospects for Future Detector Development

Subsequent utilizations in the significantly larger LZ detectors could allow background levels from PMTs, even those as radiopure as the R11410~MOD, to fall beneath the inherent neutrino scattering background constraints. Here lies no practical need to pursue further PMT radioactivity reduction as experimental performance approaches the fundamental neutrino interaction limits.

Figure 5: Expected neutrino scattering backgrounds for liquid xenon detectors, with PMT contributions depicted as a negligible factor.

Conclusion

The results herald the R11410~MOD as a transformative PMT choice for future implementations of liquid xenon detectors, including LUX and LZ. The ultra-low background properties fulfill critical reduction thresholds, offering prolonged operational periods free of background interference and enhancing detection sensitivity. The paper invites further innovation in PMT design while setting a formidable benchmark for radiopurity critical to the evolution of dark matter experiments.

By systematically advancing material and design processes, this work anticipates the superior integration of radiopure PMTs in upcoming high-mass xenon detectors, potentially achieving unparalleled acuity in dark matter inquiry.