A setup for measuring the reflectivity of cold teflon in the VUV for the XENON project

The XENON experiment is searching for dark matter by looking for the nuclear recoil signal induced by a WIMP in a 2 phase xenon time projection chamber (TPC). Interactions in the liquid phase will cause scintillation and ionization signals. The electrons are drifted from the liquid into the gaseous phase and accelerated there, causing fluorescence light. Both light signals are detected by photomultipliers. Their ratio and the 3-dimenstional position reconstruction allows to discriminate WIMPs from background (e.g. γs) from surrounding materials. The active volume of the TPC is confined horizontally by a cylindrical wall of Teflon (PTFE). It holds the field shaping wires and should reflect the scintillation light towards the photomultipliers, thus optimizing the light yield. The reflection properties depend on surface conditions and temperature. To determine the specular and diffuse reflectivity at the scintillating wavelength of xenon in the VUV (178 nm) and at liquid xenon temperatures (165 K) a chamber was constructed in which the incident wavelength selected VUV light and the detector position relative to the sample surface can be fully varied.