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Description
The next-generation liquid xenon experiment XLZD will probe the WIMP parameter space down to the irreducible neutrino background. It also aims to achieve excellent sensitivity to neutrinoless double beta decay of $^{136}$Xe, as well as to neutrinos from the atmosphere, the Sun, and supernovae. Achieving such sensitivities requires a detailed understanding and mitigation of background contributions.
In particular for the search for the neutrinoless double beta decay of $^{136}$Xe, special care must be taken to minimize the activity of $^{214}$Bi, a daughter isotope of $^{226}$Ra. The main contribution to this background arises from detector materials, primarily the PMTs and the cryostat. Significant effort is therefore devoted to the selection and development of low-radioactivity PMTs, with a strong focus on reducing $^{226}$Ra contamination.
This work presents the screening results of 16 Hamamatsu R17317 PMTs with low-activity stems. The measurements were performed using the Gator facility, a low-background, high-purity germanium (HPGe) gamma-ray spectrometer operated by the University of Zurich at the Gran Sasso National Laboratory (LNGS).
A $^{226}$Ra activity of (0.21 $\pm$ 0.02) mBq/PMT was measured, corresponding to a reduction by more than a factor of two compared to the PMTs employed in XENONnT.