Thesis

Optical Calibration of a large volume liquid scintillator detector for Neutrino Physics

• Call:

  IDPASC Portugal - PHD Programme 2014

• Academic Year:

  2014 /2015

• Domain:

  Experimental Particle Physics

• Supervisor:

  José Maneira

• Co-Supervisor:

• Institution:

  Faculdade de Ciências - Universidade de Lisboa

• Host Institution:

  Laboratório de Instrumentação e Física Experimental de Partículas

• Abstract:

  The Sudbury Neutrino Observatory (SNO) is a large volume neutrino detector located in the SNOLAB underground laboratory in Canada. It consists of a 12 m diameter transparent acrylic vessel surrounded by 9500 very sensitive light detectors (photomultipliers, or PMTs). The large water volume around the acrylic vessel (several thousand tons) and the underground location (2 km depth), make it a very low-background facility, ideal for Neutrino Physics. SNO has demonstrated that solar neutrinos do change flavor and thus have a small, but non-zero, mass. The SNO+ experiment will replace SNO's heavy water target by liquid scintillator, that will provide sensitivity to several new low energy neutrino physics measurements. If it exists, the rare process of neutrino-less double-beta decay would prove that neutrinos are Majorana particles – their own antiparticles – and would allow the measurement of the absolute scale of neutrino masses. SNO+ will use the advantages of a large mass and very-low background detector to search for this process by loading large quantities of Tellurium in the liquid scintillator. During all the phases, SNO+ will also detect anti-neutrinos from nuclear reactors and from the Earth's natural radioactivity, as well as galactic Supernova neutrinos. The detector upgrade of the SNO+ experiment (scintillator purification system, new acrylic vessel support, new calibration systems, etc..) is currently being completed at SNOLAB, in Canada. Data taking is expected to start in 2015. A short commissioning run period with pure liquid scintillator will be followed by the Tellurium-loaded phase. In later phases – with pure liquid scintillator again – SNO+ will also be able to measure several components of the solar neutrino spectrum. The LIP group is responsible for several aspects of the calibration system – PMT and scintillator optical calibration, source insertion mechanism – as well as Anti-neutrino analysis. The broad scope project's goals are to obtain the first neutrinoless double-beta decay limits (or positive measurement) with SNO+. The quality of the measurement is crucially dependent on the detector's energy resolution, since the signal is a narrow energy peak, and on achieving the lowest possible backgrounds. The work plan for this thesis project will target the first of these aspects, by means of the energy calibration with different sources. Being a homogenous unsegmented scintillation detector, the uniformity of the energy response of SNO+ depends crucially on an accurate knowledge of how the scintillation light is produced, propagated and detected. Several effects impact that knowledge, mainly the processes of absorption, re-emission and scattering of light, as well as refraction and reflection on the detector elements. Due to the large dimensions of the detector (diameter of 17 m), the in-situ measurement of these properties is essential, and makes use of several optical sources, from uniform diffusers to narrow beams of laser light. The goals of the project are to obtain the full optical characterization of the SNO+ detector, in different detector configurations, namely the isotope loading. For that, the existing multi-parameter analysis methods, inherited from SNO, need to be adapted to liquid scintillator. This adaptation will need to take into account several differences between water and scintillator, from the point of view of optics: - optimization of the used wavelengths, in order to cover the appropriate spectra, and to measure the effect of absorption/reemission with/without the use of wavelength shifter, - optimization of the chosen source positions, in order to take into account the effect of total internal reflection at high angles of incidence in the acrylic sphere; - developmente of a new optical fit algorithm that can use both direct and non-direct light in a simultaneous measurement of the absorption and scattering. The validation of the optical calibration is also a goal, and it can be carried out either with deployed radioactive sources or with naturally present background sources. Finally, the outcome of the work is expected to lead into an improved measurement of neutrino-less double beta decay. Initially, the focus will be on commissioning the calibration systems, especially the optical sources (the main one is a diffuser for N2-dye laser pulses) needed for energy response calibration, taking calibration data, and developing dedicated software for automated quality control. A second step is to develop methods to analyse the data and model as accurately as possible the light propagation in the detector and the collection efficiency of the array of PMTs, that is known to degrade over time. To conclude, the analysis of events from natural radioactivity contaminants still present in the detector, will be used to validate and fine tune the calibrated energy response, since it has a uniform distribution more similar to the neutrino data. The work will include also participation in in-situ activities in SNOLAB including, in the early stages, the commissioning of the calibration systems, and later on, data-taking and calibration data analysis.