PhDSNO+ is a multi-purpose liquid scintillator detector with the main aim of investigating neutrinoless double beta decay (0 2 ) of 130Te. The sensitivity of such rare decay searches is limited by backgrounds, therefore it is crucial to reduce and constrain backgrounds in the energy region of interest (ROI). This thesis presents the comprehensive study of particularly dangerous pileup backgrounds occurring in the scintillator with 0.3% of Tellurium-130 loading. The thesis determines which pileup backgrounds are the most problematic, estimates their event levels, describes their properties and methods to reject them from the ROI of 0 2 . To show the vital importance of this analysis, the thesis demonstrates the improvement of the sensitivity to the 130Te(0 2 ) half-life by 32%. This thesis presents the Channel Software Status (CSS) framework which has been developed to check the performance of each photomultiplier tube (PMT). It is one component of a processing pipeline that is crucial to ensuring the SNO+ experiment takes quality data. The framework has been tested on the air-fill data and is ready for further tuning using the stable water data. SNO+ has an extensive calibration program, including a proposed 90Y -emitting calibration source. This thesis outlines the benefits of using such a source to test the position and energy resolutions across the energy region spread up to 2.28 MeV. The thesis describes the design of the source and the manufacturing procedure. The performed tests demonstrated the promising potential for using 90Y to study properties of scintillators
