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Development of the source calibration system of the STEREO experiment and search for sterile neutrinos at the ILL

Abstract : The STEREO experiment has been proposed to give an unambiguous response to the hypothesis of a light sterile neutrino state (∆m² ∼ 1eV²) as the origin of the reactor antineutrino anomaly. Its goal is to confirm or reject this hypothesis by searching at short distance (9-11 m) for a neutrino oscillation pattern in the energy spectrum of the ν¯e’s emitted by the research nuclear reactor of the Laue-Langevin Institute in Grenoble (France). To this end, the detector is composed of 2 tons of Gd-loaded liquid scintillator read out by an array of PMTs, and is segmented in 6 cells in the direction of the ν¯e’s propagation. Antineutrinosare detected via the IBD process by observing a time correlated signal composed of a prompt energy deposit from a positron and a delayed signal produced by the neutron capture. Measuring small oscillations superimposed on the reactor antineutrino energy spectrum requires a good energy resolution and an excellent knowledge of the detector response. This manuscript presents a dedicated Geant 4 simulation study of a calibration system based on radioactive sources. This system has been conceived to fulfill all the STEREO physics requirements: calibrating the energy scale and the neutron capture efficiency at the 2 % level, knowing the energy response in the reactor antineutrino energy spectrum (0-8 MeV),and characterizing the detector response in a broader sense (non-uniformities, non-linearity, particle identification, etc). To this end, we propose three calibration subsystems: one automated subsystem to move radioactive sources around the detector, whose main role is to calibrate the energy scale in each cell independently; a second subsystem to inter-calibrate the neutron capture efficiency between cells by moving an AmBe source under the detector; and finally, a third subsystem consisting in three manual calibration tubes inside the liquid scintillator, necessary to assess the absolute neutron capture efficiency inside three different cells. The final part of this manuscript is devoted to the study of the selection criteria, and the proposal of methods to reject the expected gamma background.
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Luis Manzanillas. Development of the source calibration system of the STEREO experiment and search for sterile neutrinos at the ILL. High Energy Physics - Experiment [hep-ex]. Université Grenoble Alpes, 2016. English. ⟨NNT : 2016GREAY079⟩. ⟨tel-01413454v2⟩

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