Optical calibration system for SNO+ and sensitivity to neutrinoless double-beta decay

The SNO+ experiment is primarily looking for neutrinoless double-beta decay, an unobserved, lepton number violating radioactive decay. This is achieved by loading liquid scintillator with tellurium whose isotope 130Te decays via double beta decay with a Q-value of 2527 keV. An optical calibration system, located outside the scintillator, has been developed to help meet the radiopurity requirements of the experiment. This thesis describes the hardware component of the optical calibration system which calibrates the timing and charge response of the photomultiplier tube array of SNO+. A set of quality assurance tests showed that the system was at the required standard for installation. Data taken with SNO+ and the optical calibration system showed that the system was stable enough for photomultiplier tube calibration, identified resolvable issues with the SNO+ data acquisition system and allowed measurement of single photoelectron spectra. Data quality checks have been developed to ensure data is of calibration standard. The sensitivity of SNO+ to neutrinoless double-beta decay with nearly 800 kg of 130Te and five years data taking is investigated with a comprehensive evaluation of systematic uncertainties. Two new methods for acquiring a greater sensitivity to neutrinoless double-beta decay were developed; a one dimensional fit in event energy and a multidimensional fit in event energy and position. A simple event counting analysis, developed previously by the collaboration, was shown to be sensitive to systematic uncertainties. A fit in an extended energy range was shown to constrain the systematics and achieve a half-life sensitivity of 9.30x1025 yr corresponding to a 5.6% improvement over the counting analysis which neglected systematic uncertainties. The multidimensional analysis with systematics included achieved a 20% improvement over the counting analysis with a half-life sensitivity of 1:06 x 1026 yr, corresponding to an effective Majorana mass between 52 to 125 meV.