Development of VUV-reflective PTFE coatings

Experimental searches for neutrinoless double beta decay are listed as the highest priority in the 2015 Long Range Plan for Nuclear Science- The observation of neutrinoless double beta decay would have a fundamental impact on the understanding of neutrinos, lepton number violation, and the origin of the matter-antimatter asymmetry in the universe- At present neutrinoless double beta decay detection experiments use xenon as their active target, which, however, is strongly absorbed by most materials and difficult to detect efficiently- Experiments therefore typically use UV reflectors to reflect the xenon light until it falls on a photosensor; the most commonly used reflector in xenon detectors is polytetrafluoroethylene (PTFE)- In nearly all past and currently running xenon detectors, bulk PTFE panels with thicknesses ranging from 1 to 10 mm are used- As the field of rare event searches pushes towards higher sensitivities the required size of xenon detectors grows larger- Due to the larger size of the detector, the large mass of PTFE required for high reflectivity poses challenges as even small fractions of impurities in the material make significant contributions to the radioactive backgrounds- Additionally, large masses of PTFE slowly outgas air that is trapped in the porous bulk of the material, reducing the purity of the liquid xenon, and negatively impacting signal detection-