Low-temperature Luminescence of Organic Materials for use in Particle Detectors

Abstract

The fluorescent and scintillation properties of materials used in particle detectors is an important property to understand for the optimal use of the detector. For scintillation detectors, fluorescence of materials used in the construction of the detector needs to be characterized in order to properly understand their backgrounds. For components that directly scintillate, the scintillation should be maximized to obtain the best signal. Both scintillation and fluorescence exhibit temperature dependent properties, thus it is important to study how these materials behave as a function of temperature. This thesis looks at characterizing the temperature dependent fluorescence of PEDOT:PSS, and scintillation of plastic scintillators. PEDOT:PSS (Clevios) is a conductive polymer that is transparent in thin films. This makes it an ideal choice for use as electrodes in noble-liquid rare-event search dual-phase time projection chambers (TPCs). Special care is needed when designing a rare-event search detector to ensure that none of the materials produce any significant fluorescence that will be a background for the detector. Clevios has not been used in rare-event search TPCs before, and the fluorescence has not been quantified, but previous studies suggest that Clevios may fluoresce when excited by UV light at 260 nm. An experiment was designed to quantify the fluorescence of Clevios-coated acrylic using time-resolved, and spectral techniques, between 300 K to 4 K. It was determined that the fluorescence from Clevios is negligible when compared to acrylic, which itself is negligible compared to common wavelength shifters used in TPCs. Plastic scintillators (PSC) are a common organic scintillator that are often used. Many scintillators have an increase in their light yield when they are cooled to cryogenic temperatures, but this has not yet been studied for PSC. An experiment was designed to test the scintillation light yield as a function of temperature for α particles between 300 K and 4 K. This study found that the LY of PSC decreased by ∼25% as it cooled from 300 K to 4 K. Initial tests for γ detection using Compton scattering are also included but further studies are needed to fully characterize the LY of γ interactions.