Enhancing light collection in scintillation detectors

Abstract

Advances in particle-physics experiments increasingly depend on highly efficient scintillation-based detectors, as the search for rare processes and weakly interacting particles pushes detection systems to their performance limits. Improving photon collection and photon-sensing technologies is therefore essential for experiments that aim to probe hidden-sector particles, long-lived states, and other signatures of physics beyond the Standard Model. The first part of this thesis examines a BaSO4-based diffuse reflective coating (OPRC) intended to increase photon collection in the Surrounding Background Tagger (SBT), a liquid-scintillator veto detector used in prototype configurations of the SHiP experiment. Initial laboratory tests demonstrated that OPRC achieved a diffuse reflectivity exceeding 95% in the wavelength range 250nm < λ < 800nm, corresponding to an increase of up to 300% in the UV region compared to stainless steel. When implemented in a detector prototype, an approximate reflectivity of 65% was achieved during beam operation due to the presence of rust-induced staining. Despite these conditions, the prototype reached a detection efficiency of ≥ 99.5% at a confidence level of 68% during a test beam campaign. The coating was evaluated through systematic reflectivity measurements, compatibility tests with detector materials, and accelerated aging studies. Although the initial results demonstrated enhanced light collection, long-term exposure to liquid scintillator revealed several degradation mechanisms, including partial dissolution of the coating, fluor deposition, and reduced reflectivity caused by swelling and increased translucency. Additional complications arose when OPRC was applied directly to corten steel, where corrosion-induced discoloration further diminished optical performance. While clearly not suitable for long-term applications as in the SHiP-SBT, OPRC lining on stainless steel could still be used over short periods of time for significantly enhancing light collection. The second part of the thesis focuses on the development of polystyrene-based plastic scintillators for use as active light guides in the SiALG photo sensor concept. A controlled polymerization procedure was established to produce high-quality scintillators with adjustable fluor loading. Optical characterization confirmed successful incorporation of fluors, though transparency and light yield remained below commercial standards, reaching approximately 57% of the SP32 reference performance. These results outline clear directions for improving polymer quality and fluor integration. Together, the two studies contribute to the advancement of photon-collection and photon-sensing technologies for future scintillation-based detectors.