Qualification Tests of the R11410-21 Photomultiplier Tubes for the XENON1T Detector

The Hamamatsu R11410-21 photomultiplier tube is the photodetector of choice for the XENON1T dual-phase time projection chamber. The device has been optimized for a very low intrinsic radioactivity, a high quantum efficiency and a high sensitivity to single photon detection. A total of 248 tubes are currently operated in XENON1T, selected out of 321 tested units. In this article the procedures implemented to evaluate the large number of tubes prior to their installation in XENON1T are described. The parameter distributions for all tested tubes are shown, with an emphasis on those selected for XENON1T, of which the impact on the detector performance is discussed. All photomultipliers have been tested in a nitrogen atmosphere at cryogenic temperatures, with a subset of the tubes being tested in gaseous and liquid xenon, simulating their operating conditions in the dark matter detector. The performance and evaluation of the tubes in the different environments is reported and the criteria for rejection of PMTs are outlined and quantified.Comment: 24 pages, 16 figure

Identifying ²²²Rn decay chain events in liquid xenon detectors

In the search for dark matter, detectors have been built over the last years which use liquid xenon as a detection medium. For them, 222Rn and its daughter nuclides pose a dangerous background source, as they can cause signals which look like those expected from dark matter particles. Consequently, it is important to understand, how 222Rn decay chain signals can be identified. As a result, this thesis is dedicated to looking for such signals in two detectors. One of them is the XENON100 detector, which belongs to the leading dark matter direct detection experiments. 222Rn decay chain event rates are used here for quantifying the effectiveness of using distillation techniques to separate radon from xenon. The other detector is called HeidelbergXenon (HeXe), and has been recently built at the Max-Planck-Institut für Kernphysik in order to study the decays of 222Rn and its daughter nuclides in liquid xenon. To achieve this task, a data processor is developed and tested over the course of this thesis. Also, the electronics equipment as well as the photosensors used for measuring with HeXe are characterized. Finally, HeXe data, taken with radon-enriched xenon, is analyzed to look for 222Rn decay chain events

Removing krypton from xenon by cryogenic distillation to the ppq level

The XENON1T experiment aims for the direct detection of dark matter in a cryostat filled with 3.3 tons of liquid xenon. In order to achieve the desired sensitivity, the background induced by radioactive decays inside the detector has to be sufficiently low. One major contributor is the $\beta$-emitter $^{85}$Kr which is an intrinsic contamination of the xenon. For the XENON1T experiment a concentration of natural krypton in xenon $\rm{^{nat}}$Kr/Xe < 200 ppq (parts per quadrillion, 1 ppq = 10$^{-15}$ mol/mol) is required. In this work, the design of a novel cryogenic distillation column using the common McCabe-Thiele approach is described. The system demonstrated a krypton reduction factor of 6.4$\cdot$10$^5$ with thermodynamic stability at process speeds above 3 kg/h. The resulting concentration of $\rm{^{nat}}$Kr/Xe < 26 ppq is the lowest ever achieved, almost one order of magnitude below the requirements for XENON1T and even sufficient for future dark matter experiments using liquid xenon, such as XENONnT and DARWIN

Genome-wide study of association and interaction with maternal cytomegalovirus infection suggests new schizophrenia loci.

Genetic and environmental components as well as their interaction contribute to the risk of schizophrenia, making it highly relevant to include environmental factors in genetic studies of schizophrenia. This study comprises genome-wide association (GWA) and follow-up analyses of all individuals born in Denmark since 1981 and diagnosed with schizophrenia as well as controls from the same birth cohort. Furthermore, we present the first genome-wide interaction survey of single nucleotide polymorphisms (SNPs) and maternal cytomegalovirus (CMV) infection. The GWA analysis included 888 cases and 882 controls, and the follow-up investigation of the top GWA results was performed in independent Danish (1396 cases and 1803 controls) and German-Dutch (1169 cases, 3714 controls) samples. The SNPs most strongly associated in the single-marker analysis of the combined Danish samples were rs4757144 in ARNTL (P=3.78 × 10(-6)) and rs8057927 in CDH13 (P=1.39 × 10(-5)). Both genes have previously been linked to schizophrenia or other psychiatric disorders. The strongest associated SNP in the combined analysis, including Danish and German-Dutch samples, was rs12922317 in RUNDC2A (P=9.04 × 10(-7)). A region-based analysis summarizing independent signals in segments of 100 kb identified a new region-based genome-wide significant locus overlapping the gene ZEB1 (P=7.0 × 10(-7)). This signal was replicated in the follow-up analysis (P=2.3 × 10(-2)). Significant interaction with maternal CMV infection was found for rs7902091 (P(SNP × CMV)=7.3 × 10(-7)) in CTNNA3, a gene not previously implicated in schizophrenia, stressing the importance of including environmental factors in genetic studies

Examining Hamamatsu R11410-21 photomultipliers for XENON1T at room and liquid Xenon temperatures

Photomultipliers (PMTs) play a critical role in detecting liquid xenon scintillation light for the planned XENON1T experiment, while also having to meet high standards, especially regarding quantum efficiency and intrinsic radioactivity levels. To meet those standards, the R11410-21 PMT series has been developed by Hamamatsu Photonics in cooperation with the XENON collaboration. During the course of this bachelor thesis, various tests of PMTs of this series have been made at the Max-Planck-Institut für Kernphysik, Heidelberg, in order to determine parameters relevant for the experiment, like gain, dark count rate, transit time spread and afterpulse-to-main-pulse ratio and to verify specifications given by the producer. Because the PMTs will operate in XENON1T at approximately −100 ℃, a specifically designed cool down setup has been used along with a room temperature setup, which has previously been used to test PMTs for the Double Chooz experiment. This allowed testing at room temperature as well as at cryogenic temperatures. Results have verified most of the values given by Hamamatsu and give a positive outlook for usage of the PMT series in XENON1T

Search for Two-Neutrino Double Electron Capture of 124^{124}Xe with XENON100

Two-neutrino double electron capture is a rare nuclear decay where two electrons are simultaneously captured from the atomic shell. For $^{124}$Xe this process has not yet been observed and its detection would provide a new reference for nuclear matrix element calculations. We have conducted a search for two-neutrino double electron capture from the K-shell of $^{124}$Xe using 7636 kg$\cdot$d of data from the XENON100 dark matter detector. Using a Bayesian analysis we observed no significant excess above background, leading to a lower 90 % credibility limit on the half-life $T_{1/2}>6.5\times10^{20}$ yr. We also evaluated the sensitivity of the XENON1T experiment, which is currently being commissioned, and find a sensitivity of $T_{1/2}>6.1\times10^{22}$ yr after an exposure of 2 t$\cdot$yr.Comment: 6 pages, 4 figure