Optimizing liquid Xenon TPCs

Kaminsky, Basho (2017). Optimizing liquid Xenon TPCs. (Dissertation, Universität Bern, Philosophisch-naturwissenschaftliche Fakultät)

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Detecting dark matter is one of the biggest challenges in modern physics. Many astrophysical observations indicate its existence, however a confirmed direct detection of dark matter is still missing. Weakly interacting massive particles (WIMPs) are possible candidates, which are predicted by extensions of the standard model of particle physics. The most sensitive WIMP searches employ dual-phase time projection chambers (TPC) filled with the liquefied noble gas Xenon to search for the expected extremely rare interactions of WIMPs with ordinary matter. These interactions yield to the emission of faint light at 178 nm. This demands for a huge number of possible scattering targets to increase the sensitivity, low backgrounds and a high efficiency to detect the scintillation light emitted by the Xenon-WIMP interaction. Current detectors, like XENON1T, reached ton-scale target masses. Their sensitivity can be optimized for example by reducing the loss of the few photons that emerge from the interaction of a WIMP with the target nuclei. That includes reducing the absorption of these photons on the detector walls by making them highly reflective. Since WIMPs were not detected yet, future detectors are required to have an even higher sensitivity compared to the current ones. It requires
further optimization and novel technologies to reach this goal. This work presents contributions to the optimization of one of the currently most sensitive detectors, XENON1T, by optimizing the reflectivity of the inner TPC walls (chapter 2). The development of a cryogenic test platform for the development and research towards future detector is shown in chapter 3, including the first operation of a small TPC.

The optimization of the reflectivity of the PTFE reflectors of the XENON1T TPC (sec.2.1) is done by a surface treatment reducing the surface roughness to less than 0.1 μm (sec.2.3). The increased reflectivity was approved with a reflectivity measurement apparatus in the VUV range in LXe (sec.2) and the findings were confirmed in the optical wavelength range (sec.2.3.3). This effort results in a light yield of (8.02 ± 0.06) PE/keV at a drift field of 125 V/cm for the XENON1T TPC. Beyond the optimization of current detectors, the development of a cryogenic test platform is demonstrated, which will be used to develop future detector technologies. A small TPC (sec.3.5) was successfully installed and operated: first results of the characterization in terms of charge 10.8 PE/keV and light yield 3.7 PE/keV are presented in sec.3.10.

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