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Water-based fine-grained detectors for the T2K experiment Coulombe, Jérôme

Abstract

T2K is a next-generation neutrino oscillation experiment that will use an off-axis beamline, produced at J-PARC and directed towards the Super-Kamiokande detector, to precisely measure muon neutrino disappearance as well as electron neutrino appearance in a Vμ beam. A near detector, built 280 m downstream of the proton target, is needed in order to monitor the beam as well as to measure neutrino interaction cross-sections. As part of the near detector, two fine-grained detectors (FGDs) combined with three time-projection chambers (TPCs) will form the target mass for neutrino interaction. One FGD will consist of plastic scintillator while the other will contain a significant admixture of water in order to estimate nuclear effects as well as to match Super-K’s target material. The liquid fraction of the water-rich FGD will either consist of pure water (passive configuration) or be made of an active water/liquid scintillator cocktail. This thesis presents the latest design and construction developments relating to the water FGD, looking specifically at important physics issues that need to be solved, such as the mechanical design of the structure, fluid sealing, light yield and stability of aged materials. Extruded sheets of polypropylene with a hollow profile have been considered as a structural element to contain the liquid due to their mechanical strength and low cost. In the active configuration, the individual cells of such an extrusion will become readout channels once threaded with wavelength shifting fibers. Fluid sealing has been studied for both the case of a passive as well as an active water detector, and successful seal prototypes have been built: a simple epoxy seal for the passive water case and a more elaborate two-piece thermal welded endcap for the active detector option. Single-cell prototypes have been filled with the scintillator cocktail, and light yield tests have been performed using a 400-MeV/c proton beam. More specifically, the attenuation length of the fiber has been measured and long term stability of the liquid mixture has been investigated. Results of aging for all considered materials when put in contact with the chemically reactive liquid scintillator are also presented.

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