Characterization of pyrene films for background rejection in liquid argon dark matter experiments

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Benmansour, Hicham
Dark Matter , Liquid Argon , Wavelength Shifters , Background Rejection , Pyrene , TPB
About a hundred years after the first references to dark matter, the mystery around the nature of that elusive type of matter has still not been solved. Direct detection dark matter experiments are getting more and more sensitive but still face a lot of various backgrounds. We focus here on DEAP-3600, a liquid argon (LAr) target WIMP detector for which pulse shape discrimination (PSD) techniques are particularly efficient to distinguish nuclear recoils (NR), events characteristic of WIMP-argon interaction, from electronic recoil (ER) background events. However, α-events occurring in the neck of the DEAP-3600 detector lead to shadowed nuclear recoil-like events which mimic WIMP low energy scintillation pulse shape. Applying a wavelength shifter coating with high light yield and slow decay in the neck of the detector is expected to lead to a discrimination power of O(10^5) assuming a 100% signal acceptance. This thesis investigates the fluorescence properties of the favoured coating candidate, a pyrene-polystyrene thin film that could alter the pulse shape of these α-events in order to take them out of the WIMP region of interest. Emission spectra, fluorescence yield and fluorescence time constants were studied at 18 temperatures between 4 K and 300 K, including room temperature and LAr temperature (87 K), for several pyrene-polystyrene samples with varied pyrene concentrations (12% and 15%) and purities (98%, 99% and 99.9%). The highest pyrene concentration (15% pyrene) and the highest pyrene purity (99.9%) were found to have the highest light yield at all temperatures. At LAr temperature, the light yield of the 99.9% purity 15% pyrene sample relative to 1,1,4,4-tetraphenyl- 1,3-butadiene (TPB), a wavelength shifter commonly used in dark matter detectors, was found to be 45%. The highest pyrene purity samples (99.9% purity) also showed the slowest decay. The fluorescence profiles were modelled first using an adhoc model with a bi-exponential decay. The majority of the light at LAr temperature for both 12% and 15% concentration samples (99.9% purity) was found to be dominated by a decay with time constant higher than 250 ns. Overall, the 15% pyrene (99.9% purity) sample showed the highest potential for discrimination of DEAP-3600 neck α-events. For this sample, pyrene monomer and excimer fluorescence profiles were separated and modelled separately. At LAr temperature, the decay was dominated by the monomer fluorescence profile, a 293 ± 7 ns decay.
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