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Abstract

The existence of dark matter has been supported by a significant amount of evidence, yet its nature remains an open question. DAMA/LIBRA experiment claims to observe a signal that can be interpreted as evidence for dark matter in our galaxy. The Cryogenic Observatory for SIgnals seen in Next generation Underground Searches (COSINUS) experiment, located at the Laboratori Nazionali del Gran Sasso (LNGS) in Italy, aims to provide a model-independent cross-check of this observed signal by utilizing the same target material (NaI). COSINUS operates NaI at very low temperature of O(mK). The phonon signal resulting from a particle interaction in the NaI crystal is measured by a Transition-Edge Sensor (TES). TESs are thermometers made of superconducting thin films usually deposited onto the absorber surface. The deposition of TES on NaI is particularly challenging due to the hygroscopic nature and low melting point of this crystal. To overcome this limitation, COSINUS was the first experiment to implement the remoTES readout design to measure the phonon signal; a proof-of-principle was successfully accomplished. The novelty of the remoTES design consists in depositing the TES on a separate wafer and remotely connecting it to the absorber via a gold link. This approach ensures that the crystal is not subjected to the fabrication process of the TES. The remoTES readout design is a flexible and easy-to-fabricate alternative that could be beneficial for other rare-event searches, enabling mass fabrication and better reproducibility among detectors. The optimization of the remoTES readout design is crucial to meet the sensitivity goal of the COSINUS phonon detector threshold of 1 keV (200 eV baseline resolution). The primary objective of this thesis is to systematically study and understand the impact of different detector components on the overall detector performance with the aim to achieve the best possible sensitivity. Since handling NaI is challenging the studies were performed on silicon (Si) which is a well-studied material. They involved varying the gold link properties, which includes the gold pad thickness and bonding technique, and TES geometry. It was observed that the above-mentioned parameters influence the performance of the detector prototypes. By optimizing the design a baseline resolution of 89 eV was reached.