Development and optimization of novel cryogenic calorimeters for COSINUS

Raghunath Bharadwaj, Mukund (2025): Development and optimization of novel cryogenic calorimeters for COSINUS. Dissertation, LMU München: Fakultät für Physik

Abstract

Understanding the particle nature of dark matter remains one of the central challenges in modern physics. Despite extensive astrophysical evidence supporting its existence, direct experimental detection of such particles remains elusive. This thesis presents the systematic development, optimization, and characterization of cryogenic sodium iodide (NaI) calorimeters tailored for the COSINUS experiment, a direct dark matter detection experiment designed to independently verify the DAMA/LIBRA findings of a potential dark matter-induced modulation signal. Central to this research is the remoTES design - a novel approach for cryogenic detectors, in which an absorber crystal is connected to a Transition Edge Sensor (TES) via a dedicated thermal gold link. This decoupling allows mass fabrication of TES devices on separate wafers, preserving the radiopurity of the absorber by eliminating additional surface treatments. The first part of the thesis addresses some of the crucial challenges faced by scintillation-only NaI-based readout experiments currently operational or under planning and presents a detailed study of the energy-dependent quenching factors of NaI crystals - essential for accurately interpreting experimental results as electron recoils and nuclear recoils have dissimilar scintillation light yields. COSINUS utilizes a two-channel readout system based on TESs that allows for particle discrimination and in-situ determination of these quenching factors. It consists of ultrapure scintillating NaI crystals, read out using the remoTES design to measure the phonon signal of a particle interaction. A silicon detector surrounding the crystal is used to measure the light signal from the same particle interaction. Subsequent chapters document the construction and commissioning of the low-background experimental facility at Laboratori Nazionali del Gran Sasso (LNGS), Italy, featuring a muon veto system, dedicated clean room, and a specialized dry dilution refrigerator tailored for COSINUS commissioned in the context of this work. Initial demonstrations with standard absorbers, namely silicon and tellurium dioxide, validated the feasibility of remoTES technology. Performance bottlenecks introduced due to the introduction of the gold link were addressed in subsequent optimization studies. In parallel to these optimizations, experimental tests on NaI were performed, beginning with above-ground measurements on NaI crystals, demonstrating successful event-by-event particle discrimination. These results were followed up with an underground measurement at LNGS, achieving dark matter exclusion limits close to the benchmark set by the DAMA/LIBRA experiment with only a limited exposure (11.6 g d). Additional refinements included dedicated studies on the silicon-based light detector, feasibility investigations into evaporated gold films on NaI crystals, and systematic standardization of fabrication techniques, culminating in the optimized 4pi module design prepared for COSINUS Run 1.

• Dokumententyp: Dissertationen (Dissertation, LMU München)
• Keywords: Dark Matter, Direct Detection, Cryogenics, Calorimeters
• Themengebiete: 500 Naturwissenschaften und Mathematik ; 500 Naturwissenschaften und Mathematik > 530 Physik
• Fakultäten: Fakultät für Physik
• Sprache der Hochschulschrift: Englisch
• Datum der mündlichen Prüfung: 1. Juli 2025
• 1. Bericht­erstatter:in: Biebel, Otmar