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Search for sterile neutrino oscillations with the SoLid experiment at BR2 reactor : Energy calibration of the detector and antineutrino signal extraction

Abstract : SoLid (Search for oscillations with a Lithium-6 detector) is a very short baseline reactor antineutrino experiment based at the Belgian BR2 reactor. It is a 1.6 ton detector covering a distance from 6.3 to 8.9 m from the reactor core. Its main purpose is the search for electron antineutrino oscillation to a light sterile state in order to probe the “reactor antineutrino anomaly”. Thanks to the highly enriched in ²³⁵U BR2 reactor core, the detector also aims to provide a precise measurement of the antineutrino spectrum from ²³⁵U. The measurement of this energy spectrum could help to understand the spectral anomaly at 5 MeV in the reactor antineutrino energy spectrum observed by precedent experiments. The SoLid detector uses a novel hybrid scintillation technology. It is based on the combination of plastic scintillator and ⁶LiF:ZnS(Ag) screens. The use of plastic scintillators allows a fine segmentation of the detector with 12800 detection cells in the form of 5×5×5 cm³ PVT cubes with ⁶LiF:ZnS(Ag) screens on two sides. The PVT cubes are used as targets for the antineutrinos via inverse beta decay (IBD) interactions. The energy of the resulting positron is measured with the PVT scintillation, while the resulting neutron is captured by the ⁶Li, inducing the scintillation of the ZnS. The scintillation photons are then transmitted via wavelength shifting fibres to MPPCs that read out the light signals. After a review of the neutrino oscillation with the latest experimental results and a complete description of the SoLid experiment, the two main tasks of this thesis are presented. The first main contribution of this thesis to the SoLid experiment is the energy calibration of the detector. A great focus has been put on the calibration of each detection cell with light yields measurement, linearity assessment and the test of the homogeneity of the detector response. The contribution of this work to the energy calibration of the detector allowed a precise knowledge of the energy response of the detector. The evolution through time of the detector energy response has also been studied with an analysis of multiple calibration campaigns. The second task presented in this thesis is the extraction of the antineutrino signal. As it is a detector located at a few meters from a nuclear reactor, with close to no overburden, SoLid faces a large proportion of various backgrounds. The extraction of the antineutrino signal is thus real a challenge. The backgrounds faced by the experiment are described and analysed in this work, as well as the methods developed to reject them. The final analysis using boosted decision trees is then presented with a discussion on the systematic uncertainties related to it. The stability of the analysis has been tested and gives a robust measurement of the antineutrino rates. The conclusion of this work is an analysis of 21 days of reactor data, with the extraction of an antineutrino energy spectrum and distance travelled before interacting in the detector. This last measurements give the necessary inputs to determine the sensitivity to sterile neutrino of the experiment and to perform the search for a sterile neutrino signal.
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Submitted on : Monday, January 31, 2022 - 10:32:08 AM
Last modification on : Wednesday, February 2, 2022 - 3:33:31 AM
Long-term archiving on: : Sunday, May 1, 2022 - 6:59:53 PM


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  • HAL Id : tel-03548942, version 1


Noë Roy. Search for sterile neutrino oscillations with the SoLid experiment at BR2 reactor : Energy calibration of the detector and antineutrino signal extraction. High Energy Physics - Experiment [hep-ex]. Université Paris-Saclay, 2021. English. ⟨NNT : 2021UPASP142⟩. ⟨tel-03548942⟩



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