A light injection system using LEDs and optical fibres was designed for the calibration and monitoring of the photomultiplier array of the SNO+ experiment at SNOLAB. Large volume, non-segmented, low-background detectors for rare event physics, such as the multi-purpose SNO+ experiment, need a calibration system that allow an accurate and regular measurement of the performance parameters of their photomultiplier arrays, while minimising the risk of radioactivity ingress. The design implemented for SNO+ uses a set of optical fibres to inject light pulses from external LEDs into the detector. The design, fabrication and installation of this light injection system, as well as the first commissioning tests, are described in this paper. Monte Carlo simulations were compared with the commissioning test results, confirming that the system meets the performance requirements. yield and attenuation length -, a high flashpoint and its chemical compatibility with acrylic. With the new target material, several changes to the detector were needed, including new scintillator processing and purification systems, new trigger and readout electronics and new calibration systems. Structural improvements were required since the relative density of LAB is 0.86, causing a significant buoyant force on the scintillator-filled AV. This force is countered by means of a rope net [4] that is anchored to the floor of the detector cavity.The use of liquid scintillator opens up the range of physics goals, making SNO+ a multipurpose experiment. The main goal of SNO+ is the search for neutrinoless double-beta decay (0νβ β ), by loading a large mass of a suitable isotope into the liquid scintillator. When compared to experiments based on solid state detectors, the strategy of SNO+ is to compensate the lower energy resolution with a large mass of isotope and low background. The chosen isotope for SNO+ is 130 Te, with high natural abundance and favourable (0νβ β ) nuclear matrix elements and phase space, and a relatively small two-neutrino double seta decay rate. The optical absorption of Te in the loaded scintillator impacts the detector performance, limiting the initial concentration to 0.3%. Without the Tellurium loading, several solar neutrino measurements can be carried out at SNO+, including a precision measurement of the pep neutrino flux, observation of the low energy survival probability rise in 8 B neutrinos and, possibly, direct measurements of the CNO neutrinos and the pp neutrinos. The observation of anti-neutrinos from nuclear reactors in Ontario and from the natural radioactivity chains of Uranium and Thorium present in the Earth's crust and mantle are additional goals. Throughout all the data-taking phases, the detector will also be part of the SNEWS [5] network monitoring for supernova neutrinos.The physics goals of SNO+ require a low energy threshold, for measurements of solar neutrino elastic scattering signals, and for the tagging of decays of radioactive isotopes needed for the reduction of backgrounds in the 130 Te 0νβ β region-...
