Surface coatings as xenon diffusion barriers on plastic scintillators Improving Nuclear-Test-Ban Treaty verification

Sammanfattning: This thesis investigates the ability of transparent surface coatings to reduce xenon diffusion into plastic scintillators. The motivation for the work is improved radioxenon monitoring equipment, used with in the framework of the verification regime of the Comprehensive Nuclear-Test-Ban Treaty.A large part of the equipment used in this context incorporates plastic scintillators which are in direct contact with the radioactive gas to be detected. One problem with such setup is that radioxenon diffuses into the plastic scintillator material during the measurement, resulting in an unwanted memory effect consisting of residual activity left in the detector.In this work coatings of Al2O3 and SiO2, with thicknesses between 20 and 400 nm have been deposited onto flat plastic scintillator samples, and tested with respect to their Xe diffusion barrier capabilities. All tested coatings were found to reduce the memory effect, and 425 nm of Al2O3 showed the most promise.This coating was deposited onto a complete detector. Compared to uncoated detectors, the coated one presented a memory effect reduction of a factor of 1000. Simulations and measurements of the expected light collection efficiency of a coated detector were also performed, since it is important that this property is not degraded by the coating. It was shown that a smooth coating, with a similar refractive index as the one of the plastic, should not significantly affect the light collection and resolution. The resolution of the complete coated detector was also measured, showing a resolution comparable to uncoated detectors. The work conducted in this thesis proved that this coating approach is a viable solution to the memory effect problem, given that the results are reproducible, and that the quality of the coating is maintained over time.