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Register a new userImproved quality tests of R11410-21 photomultiplier tubes for the XENONnT experiment
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Photomultiplier tubes (PMTs) are often used in low-background particle physics experiments, which rely on an excellent response to single-photon signals and stable long-term operation. In particular, the Hamamatsu R11410 model is the light sensor of choice for liquid xenon dark matter experiments, including XENONnT. The same PMT model was also used for the predecessor, XENON1T, where issues affecting its long-term operation were observed. Here, we report on an improved PMT testing procedure which ensures optimal performance in XENONnT. Using both new and upgraded facilities, we tested 368 new PMTs in a cryogenic xenon environment. We developed new tests targeted at the detection of light emission and the degradation of the PMT vacuum through small leaks, which can lead to spurious signals known as afterpulses, both of which were observed in XENON1T. We exclude the use of 26 of the 368 tested PMTs and categorise the remainder according to their performance. Given that we have improved the testing procedure, yet we rejected fewer PMTs, we expect significantly better PMT performance in XENONnT.
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Thin flexible sheets of high-permeability FINEMET foils encased in thin plastic layers have been used to shield various types of 20-cm-diameter photomultiplier tubes from ambient magnetic fields. In the presence of the Earths magnetic field this type of shielding is shown to increase the collection efficiency of photoelectrons and can improve the uniformity of response of these photomultiplier tubes.
The detector for the MiniBooNE experiment at the Fermi National Accelerator Laboratory employs 1520 8 inch Hamamatsu models R1408 and R5912 photomultiplier tubes with custom-designed bases. Tests were performed to determine the dark rate, charge and timing resolutions, double-pulsing rate, and desired operating voltage for each tube, so that the tubes could be sorted for optimal placement in the detector. Seven phototubes were tested to find the angular dependence of their response. After the Super-K phototube implosion accident, an analysis was performed to determine the risk of a similar accident with MiniBooNE.
XENONnT is a dark matter direct detection experiment, utilizing 5.9 t of instrumented liquid xenon, located at the INFN Laboratori Nazionali del Gran Sasso. In this work, we predict the experimental background and project the sensitivity of XENONnT to the detection of weakly interacting massive particles (WIMPs). The expected average differential background rate in the energy region of interest, corresponding to (1, 13) keV and (4, 50) keV for electronic and nuclear recoils, amounts to $12.3 pm 0.6$ (keV t y)$^{-1}$ and $(2.2pm 0.5)times 10^{-3}$ (keV t y)$^{-1}$, respectively, in a 4 t fiducial mass. We compute unified confidence intervals using the profile construction method, in order to ensure proper coverage. With the exposure goal of 20 t$,$y, the expected sensitivity to spin-independent WIMP-nucleon interactions reaches a cross-section of $1.4times10^{-48}$ cm$^2$ for a 50 GeV/c$^2$ mass WIMP at 90% confidence level, more than one order of magnitude beyond the current best limit, set by XENON1T. In addition, we show that for a 50 GeV/c$^2$ WIMP with cross-sections above $2.6times10^{-48}$ cm$^2$ ($5.0times10^{-48}$ cm$^2$) the median XENONnT discovery significance exceeds 3$sigma$ (5$sigma$). The expected sensitivity to the spin-dependent WIMP coupling to neutrons (protons) reaches $2.2times10^{-43}$ cm$^2$ ($6.0times10^{-42}$ cm$^2$).
XMASS-I is a single-phase liquid xenon detector whose purpose is direct detection of dark matter. To achieve the low background requirements necessary in the detector, a new model of photomultiplier tubes (PMTs), R10789, with a hexagonal window was developed based on the R8778 PMT used in the XMASS prototype detector. We screened the numerous component materials for their radioactivity. During development, the largest contributions to the reduction of radioactivity came from the stem and the dynode support. The glass stem was exchanged to the Kovar alloy one and the ceramic support were changed to the quartz one. R10789 is the first model of Hamamatsu Photonics K. K. that adopted these materials for low background purposes and provided a groundbreaking step for further reductions of radioactivity in PMTs. Measurements with germanium detectors showed 1.2$pm$0.3 mBq/PMT of $^{226}$Ra, less than 0.78 mBq/PMT of $^{228}$Ra, 9.1$pm$2.2 mBq/PMT of $^{40}$K, and 2.8$pm$0.2 mBq/PMT of $^{60}$Co. In this paper, the radioactive details of the developed R10789 are described together with our screening methods and the components of the PMT.
The Large High-Altitude Air Shower Observatory (LHAASO) is being built at Haizi Mountain, Sichuan province of China at an altitude of 4410 meters. One of its main goals is to survey the northern sky for very-high-energy gamma ray sources via its ground-based water Cherenkov detector array (WCDA). 900 8-inch photomultiplier tubes (PMTs) CR365-02-1 from Beijing Hamamatsu Photon Techniques INC. (BHP) are installed in the WCDA, collecting Cherenkov photons produced by air shower particles crossing water. The design of the PMT base with a high dynamic range for CR365-02-1, the PMT batch test system, and the test results of 997 PMTs are presented in this paper.
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