The majority of matter in the Universe is dark. World wide efforts to understand this dark component of the Universe are underway and the current evidence suggests the existence of a non-relativistic, non-baryonic and weakly interacting massive particle (WIMP). This weakly interacting dark matter should occasionally couple to baryonic matter, primarily through nuclear interactions. The predicted event rates are low (< O(1) events/kg/day). The energy deposited is also expected to be low (<~ 50 keV), and so distinguishing a WIMP signal above the radioactive and cosmic backgrounds is a difficult challenge. The ZEPLIN{III device was designed to meet this challenge and achieve a competitive sensitivity to WIMP-nucleon interactions. The ZEPLIN{III detector is a two phase time projection chamber using liquid xenon as a target. The instrument was designed to detect dark matter by measuring scintillation and ionisation. Measuring two signals produced by incident radiation allows for discrimination between event types. This allows separation of the main component of the background radiation (primarily electron recoils) from any population of WIMP events that may be present in the data. The ZEPLIN-III detector completed its first science run in 2008, achieving a discrimination power of 1:7800 between nuclear and electron recoils, the highest of any liquid xenon detector. This result limited the WIMP-nucleon cross section to less than 8:4 X 10-8 pb at 90% confidence level (double sided) for a WIMP mass of 55 GeV/c2. The ZEPLIN-III detector then entered an upgrade phase. The two main improvements included the installation of a new ultra-low background PMT array, significantly reducing the main source of background events, and the addition of a veto detector. The veto detector significantly increased the detector's ability to reject WIMP-like background signals, which may be produced by background neutron events. The veto detector also aided background discrimination by detecting 28% of γ-ray events from the fiducial volume of ZEPLIN-III. The second science run of ZEPLIN-III began in June 2010 and continued until May 2011. During the second science run the discrimination power was 1:280 between nuclear and electron recoils. A total of 8 events were observed in the WIMP search region, which is consistent with background expectations. Assuming a null detection allowed the exclusion of the scalar cross-section above 4:8 10-8 pb near a WIMP mass of 51 GeV/c2. This result was combined with the result from a re-analysis of the first science run using more recent results for the relative scintillation yield, Leff, to give a total limit on the spin independent cross-section of 3:9 X 10-8 pb at 90% confidence near 52 GeV/c2 WIMP mass for the ZEPLIN-III experiment. The WIMP-neutron spin-dependent cross-section limit is 8:0 X 10-3 pb at 50 GeV/c2 at 90% confidence for the combined first and second science runs. At the time of publication, these were the world's second best, and best results, respectively.

The existence of a significant non-baryonic component to the Universe is widely accepted, with worldwide efforts underway trying to detect this so-called dark matter. The ZEPLIN-III detector utilises liquid xenon (Xe) as a target medium in the search for the expected rare interactions of Weakly Interacting Massive Particles, or WIMPs, with ordinary baryonic matter. The neutralino, arising in supersymmetric extensions to the standard model of particle physics, provides a particularly well-motivated candidate. The ZEPLIN-III experiment, operating in two-phase (liquid/gas) mode, measures both the scintillation and ionisation signatures produced during an interaction. The first science run (FSR) of ZEPLIN-III was performed during three months in 2008. The run culminated in a published result which excluded a WIMP-nucleon interaction cross-section above 8:1 x 10-8 pb for a 60 GeVc-2 WIMP at the 90% confidence level. ZEPLIN-III then entered an upgrade period where the photomultiplier tube (PMT) array, previously the dominant source of background, was replaced with new, ultra-low background, PMTs. The radio-contamination of components used to make these PMTs has been thoroughly studied and their impact on the background rates in ZEPLIN-III characterised. Additionally, a new 1.5 tonne plastic scintillator veto detector was constructed, increasing the ability to reject WIMPlike signals caused by neutron induced nuclear recoil events and improving the γ-ray discrimination capability of ZEPLIN-III. The second science run (SSR) of ZEPLIN-III began in June 2010 and continued for 6 months, with a projected upper limit for the interaction cross-section of 1:52 x 10-8 pb for a 55 GeVc-2 WIMP at the 90% confidence level.

Cold dark matter in the form of weakly interacting massive particles (WIMPs) is a favoured explanation to the galactic dark matter puzzle and could account for a large proportion of the missing mass of the Universe. There are currently numerous detectors around the world attempting to observe a WIMP signal. The ZEPLIN-III detector is one such device. Utilising liquid xenon as a target medium, identification is based on extraction of scintillation and electroluminescence signals from the two-phase xenon target caused when WIMPs scatter and has recently completed its first science run (FSR). With no WIMP signal observed, ZEPLIN-III has excluded a WIMP-nucleon spin-independent cross section above 8.1 × 10−8 pb (90% confidence limit) for a WIMP mass of 60 GeV/c2 and also set a 90% confidence upper limit of a pure WIMP-neutron spin-dependent cross section of 1.9 × 10−2 pb for a 55 GeV/c2 WIMP mass. However, the focus of this thesis is the future of the ZEPLIN-III detector with regards to the second science run (SSR). As with all dark matter detectors, background reduction from neutrons and gamma-rays plays a significant part in obtaining competitive WIMP detection sensitivities. The author has contributed significantly to the design, development and testing of a low radioactivity veto for the ZEPLIN-III detector, to be retrofitted in time for the SSR. It will detect neutrons and gamma-rays in coincidence with the ZEPLIN-III target allowing these events to be removed as candidate WIMP events. This thesis describes the author’s contribution to the design, construction, testing and evaluation of the veto. Also discussed is the development of a comprehensive Monte Carlo simulation, utilised to aid in the design process, to determine the background rates emanating from the veto components (and therefore possible impact on the low sensitivity running of ZEPLIN-III), and to provide an accurate estimation of the overall veto efficiency to reject coincident neutrons and gamma-rays. The veto will have a neutron rejection factor of 67%, reducing the expected neutron background in ZEPLIN-III from 0.4 neutrons/year to 0.14 neutrons/year, a significant factor in the event of a possible WIMP observation. In addition to the work performed on the ZEPLIN-III veto, the author has also contributed to the first science run analysis program by profiling the historical evolution of the electron lifetime throughout the FSR, and implementing consideration of this to improve the data quality.

Galaxies and clusters of galaxies are believed to be dominated by non-luminous non-baryonic dark matter. A favored candidate is a new type of Weakly Interacting Massive Particle (WIMP) with a mass of order 100 GeV/c^2. The ZEPLIN II experiment is a WIMP search experiment that attempts to directly detect WIMP interactions using the two-phase xenon approach. The detector measures both scintillation and ionization generated by interactions in a 31 kg liquid xenon target. This approach provides a powerful discrimination between nuclear recoils, as expected from WIMPs, and background electron recoils. In this work, we develop a new X^2 approach to determine the three dimensional event positions in an attempt to improve the background rejection. The optical properties of the PTFE reflectors and the grids of the detector were determined using the Geant4 simulation, and event positions were obtained by finding the best match to the amount of light in each photomultiplier. This was found to greatly improve the position resolution. The approach was then applied to the WIMP search data. It was found that one of the dominating background sources was events from the gas above the anode grid and not from the PTFE walls caused by the small signals as previously thought. WIMP search results were then obtained from the first 31 days of stable ZEPLIN II data using two methods. Although the X^2 method greatly improved position resolution, the number of background events was not significantly altered and the new limit agreed well with the limit published by the collaboration.

Astrophysical observations give convincing evidence for a vast non-baryonic component, the so-called dark matter, accounting for over 20% of the overall content of our Universe. Direct dark matter search experiments explore the possibility of interactions of these dark matter particles with ordinary baryonic matter via elastic scattering resulting in single nuclear recoils. The ZEPLIN-III detector operated on the basis of a dualphase (liquid/gas) xenon target, recording events in two separate response channels { scintillation and ionisation. These allow discrimination between electron recoils (from background radiation) and the signal expected from Weakly Interacting Massive Particle (WIMP) elastic scatters. Following a productive first exposure, the detector was upgraded with a new array of ultra-low background photomultiplier tubes, reducing the electron recoil background by over an order of magnitude. A second major upgrade to the detector was the incorporation of a tonne-scale active veto detector system, surrounding the WIMP target. Calibration and science data taken in coincidence with ZEPLIN-III showed rejection of up to 30% of the dominant electron recoil background and over 60% of neutron induced nuclear recoils. Data taking for the second science run finished in May 2011 with a total accrued raw fiducial exposure of 1,344 kg days. With this extensive data set, from over 300 days of run time, a limit on the spin-independent WIMP-nucleon cross-section of 4.8 10-8 pb near 50 GeV/c2 WIMP mass with 90% confidence was set. This result combined with the first science run of ZEPLIN-III excludes the scalar cross-section above 3.9 10-8 pb. Studying the background data taken by the veto detector allowed a calculation of the neutron yield induced by high energy cosmic-ray muons in lead of (5.8 0.2) 10-3 neutrons/muon/(g/cm2) for a mean muon energy of 260 GeV. Measurements of this kind are of great importance for large scale direct dark matter search experiments and future rare event searches in general. Finally, this work includes a comprehensive measurement of the energy dependent quenching factor for low energy nuclear recoils in a plastic scintillator, such as from the ZEPLIN-III veto detector, increasing accuracy for future simulation packages featuring large scale plastic scintillator detector systems.