Design, construction and first commissioning of the ArDM detector

Abstract

With today's understanding of the Universe, a large quantity of non luminous matter is necessary to describe several cosmological eects. This so called Dark Matter (DM) is hunted, among others, with the Argon Dark Matter experiment (ArDM). ArDM aims at a direct detection of the nuclear recoil from a collision of a (hypothetical) DM particle with an argon nucleus. ArDM is a liquid argon double phase time projection chamber (TPC) with calorimetric capabilities. The detector has a ducial volume of about 800 l of highly puried liquid argon, combined with a charge readout system in the argon vapor above the liquid surface. Besides the detection of the ionization charge, ArDM also has a highly sensitive light readout system. It is capable of reading out single photoelectrons and, therefore, to quantify the emitted argon scintillation light from the interaction between an entering particle and the argon atoms. This allows to measure the recoil energy and to discriminate between electron and nuclear recoils, yielding a clue to the nature of the interacting particle and allowing to suppress background events. The topic of this thesis is the design, construction and commissioning of the ArDM detector at CERN. It describes the development of the dierent sub-detectors and the technical and physics challenges faced in their construction. Having a good knowledge of the electric drift eld is important for the operation of a TPC. It has to be uniform and constant, with a known eld strength, in order to be able to evaluate the drift distance from the primary interaction to the readout of the ionization electrons. The high voltage needed in ArDM to maintain the electric eld over a drift distance of more than one meter is rather big and providing the source for this HV is a major challenge. Instead of a standard power supply, a Greinacher high voltage generator was developed. First results of a 210 stages Greinacher circuit, operated in liquid argon, are discussed. Also essential for the operation of a cryogenic experiment are the facilities to keep the liquid gas at stable pressure and temperature. Dierent topics like the insulation vacuum and cooling of the experiment are described. For safety reasons, the experiment is built as a zero loss experiment, which means there is no argon nor other gas escaping from the closed system. For this purpose, two cryocoolers are installed and a constant liquefaction of the argon vapor has to be maintained. An ecient operation of the TPC can only be achieved if the contamination of the liquid argon with electronegative molecules, like oxygen or water, is very well controlled and kept below one part per billion (1 ppb). For this reason, the detector material has to be cleaned of the above mentioned molecules. This calls for an initial evacuation of the setup for several months to reduce the outgassing of these molecules to an acceptable level. A detailed look on the outgassing properties of dierent materials is given in this work. Also, the active recirculation and purication of the liquid, and the argon vapor are discussed. Because ArDM is aiming for the detection of very rare events, the control of the background is essential. To reduce the unwanted interactions of cosmic muons with the argon atoms, the experiment has to be installed in a low background environment. This was found in the underground laboratory of Canfranc, Spain (LSC). After several commissioning runs on the surface at CERN, the experiment was dismantled and the cryogenic facilities were moved to this underground laboratory. In this thesis, the installation and rst results from the commissioning of the insulation vacuums and the cryogenic facilities are discussed.