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 5.6cm diameter R8778 PMT, used in the LUX dark matter experiment, has yielded a positive detection of four common radioactive isotopes: ^2^3^8U, ^2^3^2Th, ^4^0K, and ^6^0Co. Screening of LUX materials has rendered backgrounds from other detector materials subdominant to the R8778 contribution. A prototype Hamamatsu 7.6cm diameter R11410 MOD PMT has also been screened, with benchmark isotope counts measured at <0.4^2^3^8U/<0.3^2^3^2Th/<8.3^4^0K/2.0+/-0.2 ^6^0Co mBq/PMT. This represents a large reduction, equal to a change of x124^2^3^8U/x19^2^3^2Th/x18^4^0K per PMT, between R8778 and R11410 MOD, concurrent with a doubling of the photocathode surface area (4.5-6.4cm diameter). ^6^0Co 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 x125 after further material selection for ^6^0Co reduction, and nuclear recoil backgrounds by a factor of x136. 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.