| Summary: | <p>There is substantiated astrophysical evidence for a substance known as Dark Matter within the universe, at both galactic and cosmological scales. Despite this evidence, the true nature of Dark Matter remains one of the most profound questions in modern physics. One viable candidate is the Weakly Interacting Massive Particle (WIMP), which could nicely explain the observed measurements. The LUX-ZEPLIN (LZ) detector is searching for WIMP interactions with xenon nuclei, through both scintillation and ionisation signals. It comprises a time-projection chamber filled with 7 tonnes of liquid xenon, situated at the Sanford Underground Research Facility in South Dakota.</p>
<p>The experiment operates at low temperatures (about $175\;$Kelvin). During the cool-down of the experiment from room temperature and to ensure stability during running, the temperature needs to be monitored closely to ensure the detector is not damaged and that high-quality data can be collected. The unique implementation of the thermometry system within LZ necessitates bespoke solutions to overcome challenges related to the radiopurity and cleanliness. This thesis examines how these problems are resolved through the careful material procurement and development of a successful calibration procedure.</p>
<p>The ionisation signal plays an important role in the reconstruction of event energies and in interaction type discrimination. However, detector-specific phenomena cause an inherent position dependence. The origins of this position dependence are examined, along with the development of a method to correct signals using dispersed calibration sources. A consideration of the detector contraction is made along with a calibration of the level sensors. Together the position dependence in the ionisation signal, along with these level sensors enables the detector to be precisely levelled.</p>
<p>This work concludes by examining techniques to improve the sensitivity of the experiment below nominal capabilities. Specifically, this includes an examination of realistic pulse shape characteristics to improve the energy threshold of the experiment. Crucially, this provides a series of techniques which can be utilised within LZ analyses.</p>
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