An Ultra-Low Background PMT for Liquid Xenon Detectors

TL;DR

This paper evaluates ultra-low background photomultiplier tubes (PMTs) for liquid xenon detectors, demonstrating significant reductions in radioactive isotopes and background noise, thus enhancing the sensitivity of dark matter experiments.

Contribution

It introduces a new low-background PMT model with substantially reduced radioactivity levels compared to previous models, suitable for large-scale liquid xenon detectors.

Findings

R11410 MOD PMT shows 8-24 times lower radioactivity than R8778

Replacing R8778 with R11410 MOD reduces background by factors of 25 (electron recoil) and 36 (nuclear recoil)

The new PMT maintains larger photocathode area with lower radioactivity levels.

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.