On the krypton background of the Xenon100 and Xenon1T dark matter search experiments

The currently running Xenon100 experiment and its successor, Xenon1T, use liquid xenon as target and detection material in the search for weakly interacting massive particles, a well motivated candidate for dark matter in our universe. As the expected signal rate is less than a couple of events per year, it is absolutely mandatory to understand and reduce the possible background contributions. The man-made and almost pure beta-emitter 85Kr is a very dangerous background candidate, as krypton is intrinsically present on the ppb (parts per billion level b= 10−9) in commercially available xenon. Both further purification and the corresponding analytics are therefore equally important for these kind of experiments. This thesis describes two krypton in xenon measurement procedures and their impact on the understanding of the krypton background in the Xenon experiments. First, a mass spectroscopic set-up using gas-chromatographic pre-separation is introduced, and the improvements in terms of stability and sensitivity down to the ppq (parts per quadrillion b= 10−15) regime are highlighted. Subsequently several xenon assay results are presented: the evolution of the krypton concentration in Xenon100 over a time period of more than a year is reconstructed and linked to the observed radon decay rates. Furthermore, several distillation procedures are examined, showing the high potential of cryogenic distillation for xenon purification. Thereby, a measurement of ultra pure xenon with an so far unprecedented purity is presented. Finally, a second analysis method is investigated, applying a delayed coincidence analysis to the Xenon100 dark matter search data. This in-situ method is limited to the ppt (parts per trillion b= 10−12) regime, but achieves very good agreements with the mass spectroscopic results and confirms its absolute calibration

Charakterisierung von Plastikszintillatoren für das Myonveto eines neuen Gamma-Spektrometers (GIOVE)

Für hochsensible Experimente, wie die Suche nach dem neutrinolosen Doppelbetazerfall oder die Suche nach Dunkler Materie, wird ein effektives Material-Screening benötigt. Dabei müssen verschiedene Materialien auf ihre intrinsische Radioaktivität untersucht werden. Das neue Gamma-Spektrometer GIOVE (Germanium Inner and Outer Veto) soll auch in geringer Labortiefe konkurrenzfähig bleiben und geringste Radioaktivitäten auf dem mBq/kg-Level nachweisen können. Dazu ist eine effiziente Untergrundabschirmung unabdingbar. Hierfür wird ein neues Myonvetosystem eingesetzt, um den durch kosmische Myonen induzierten Untergrund zu reduzieren. Im Rahmen dieser Arbeit werden die Plastikszintillatoren, die für das innere Veto vorgesehen sind, auf ihre Funktionalität getestet. Die optimale Betriebsspannung der Photomultiplier wird ermittelt und der optische Kontakt zwischen Szintillatormaterial und PMT durch Ankleben der PMTs erhöht. Dies ermöglicht für alle Szintillatoren die Bestimmung eines Schwellwerts für die Auslösung des Vetos. Diese Werte können als Obergrenze für den späteren Aufbau von GIOVE verstanden werden, liefern jedoch schon jetzt gute Ergebnisse in Bezug auf die zu erwartende Totzeit des Detektors und den Prozentsatz an nicht registrierten Myonen. Durch die Untersuchung der Szintillatoren auf ihre ortsabhängige Lichtausbeute kann zusätzlich sichergestellt werden, dass Randeffekte keinen entscheidenden Einfluß auf die Effizienz des Vetosystems haben werden. Abschließend wurde mit Hilfe zweier Plastikszintillatoren der integrale Myonenfluss im Low- Level Labor am MPIK bestimmt

A DMA-train for precision measurement of sub-10 nm aerosol dynamics

Measurements of aerosol dynamics in the sub-10 nm size range are crucially important for quantifying the impact of new particle formation onto the global budget of cloud condensation nuclei. Here we present the development and characterization of a differential mobility analyzer train (DMA-train), operating six DMAs in parallel for high-time-resolution particle-size-distribution measurements below 10 nm. The DMAs are operated at six different but fixed voltages and hence sizes, together with six state-of-the-art condensation particle counters (CPCs). Two Airmodus A10 particle size magnifiers (PSM) are used for channels below 2.5 nm while sizes above 2.5 nm are detected by TSI 3776 butanol-based or TSI 3788 water-based CPCs. We report the transfer functions and characteristics of six identical Grimm S-DMAs as well as the calibration of a butanol-based TSI model 3776 CPC, a water-based TSI model 3788 CPC and an Airmodus A10 PSM. We find cutoff diameters similar to those reported in the literature. The performance of the DMA-train is tested with a rapidly changing aerosol of a tungsten oxide particle generator during warmup. Additionally we report a measurement of new particle formation taken during a nucleation event in the CLOUD chamber experiment at CERN. We find that the DMA-train is able to bridge the gap between currently well-established measurement techniques in the cluster–particle transition regime, providing high time resolution and accurate size information of neutral and charged particles even at atmospheric particle concentrations

Emissions from Ethanol-Gasoline Blends: A Single Particle Perspective

Due to its agricultural origin and function as a fuel oxygenate, ethanol is being promoted as an alternative biomass-based fuel for use in spark ignition engines, with mandates for its use at state and regional levels. While it has been established that the addition of ethanol to a fuel reduces the particulate mass concentration in the exhaust, little attention has been paid to changes in the physicochemical properties of the emitted particles. In this work, a dynamometer-mounted GM Quad-4 spark ignition engine run without aftertreatment at 1,500 RPM and 100% load was used with four different fuel blends, containing 0, 20, 40 and 85 percent ethanol in gasoline. This allowed the effects of the fuel composition to be isolated from other effects. Instrumentation employed included two Aerosol Time-of-Flight Mass Spectrometers covering different size ranges for analysis of single particle composition, an Aethalometer for black carbon, a Scanning Mobility Particle Sizer for particle size distributions, a Photoelectric Aerosol Sensor for particle-bound polycyclic aromatic hydrocarbon (PAH) species and gravimetric filter measurements for particulate mass concentrations. It was found that, under the conditions investigated here, additional ethanol content in the fuel changes the particle size distribution, especially in the accumulation mode, and decreases the black carbon and total particulate mass concentrations. The molecular weight distribution of the PAHs was found to decrease with added ethanol. However, PAHs produced from higher ethanol-content fuels are associated with NO2 − (m/z—46) in the single-particle mass spectra, indicating the presence of nitro-PAHs. Compounds associated with the gasoline (e.g., sulfur-containing species) are diminished due to dilution as ethanol is added to the fuel relative to those associated with the lubricating oil (e.g., calcium, zinc, phosphate) in the single particle spectra. These changes have potential implications for the health effect impacts of particulate emissions from biofuel blends

Large-scale manipulation of promoter DNA methylation reveals context-specific transcriptional responses and stability

BACKGROUND: Cytosine DNA methylation is widely described as a transcriptional repressive mark with the capacity to silence promoters. Epigenome engineering techniques enable direct testing of the effect of induced DNA methylation on endogenous promoters; however, the downstream effects have not yet been comprehensively assessed. RESULTS: Here, we simultaneously induce methylation at thousands of promoters in human cells using an engineered zinc finger-DNMT3A fusion protein, enabling us to test the effect of forced DNA methylation upon transcription, chromatin accessibility, histone modifications, and DNA methylation persistence after the removal of the fusion protein. We find that transcriptional responses to DNA methylation are highly context-specific, including lack of repression, as well as cases of increased gene expression, which appears to be driven by the eviction of methyl-sensitive transcriptional repressors. Furthermore, we find that some regulatory networks can override DNA methylation and that promoter methylation can cause alternative promoter usage. DNA methylation deposited at promoter and distal regulatory regions is rapidly erased after removal of the zinc finger-DNMT3A fusion protein, in a process combining passive and TET-mediated demethylation. Finally, we demonstrate that induced DNA methylation can exist simultaneously on promoter nucleosomes that possess the active histone modification H3K4me3, or DNA bound by the initiated form of RNA polymerase II. CONCLUSIONS: These findings have important implications for epigenome engineering and demonstrate that the response of promoters to DNA methylation is more complex than previously appreciated. SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.1186/s13059-022-02728-5