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Abstract

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.