Investigation of coating methods for radon background reduction in liquid xenon experiments

According to astrophysical and cosmological observations a large fraction of the energy density of today's universe exists in the form of non-luminous matter. This so-called dark matter may consist of particles like WIMPs (weakly interacting massive particle) which could be detected by liquid xenon detectors like DARWIN. Because expected interactions between WIMPs and baryonic matter are rare a very low background is necessary. A big contribution to the experimental background is caused by radon emanation, which can be reduced by application of surface coatings. In this work several coating techniques as well as a subsequent heat treatment were investigated. These techniques include electrodeposition, epoxy coatings as well as a diamond-like carbon coating. Coating tests were done on thoriated tungsten welding rods containing 232Th, which _- nally decays to 220Rn. A systematic study of coating parameters for electrodeposition was done including the material of the counter electrode and the applied voltage. Two coatings were found with reduction factors of 40.6 and 28.7 for 220Rn. Moreover, the studies of a diamond-like carbon coated sample, showed a reduction of about 3 for 220Rn and good adhesion of the coating. For an epoxy coating using Stycast vacuum glue, good radon mitigation properties were found. Finally, a thermal treatment of electro deposited copper layers hinted to a further reduction of activity