Laboratory link : http://irfu.cea.fr/dphn/Phocea/Vie_des_labos/Ast/ast_sstheme.php?id_ast=31&voir=technique
The central topic of this thesis is the NUCLEUS experiment, whose motivation is to measure the coherent scattering of neutrinos emitted by the reactors of the EDF power plant at Chooz, in the Ardennes. Although, in the MeV energy range that concerns us, coherent scattering on nuclei is the most probable mode of interaction of neutrinos with matter, it is extremely difficult to detect because its only signature is the tiny recoil of the target nucleus. Thus the first observation of this process dates from 2017 only, with neutrinos of a few 10 MeV from the Oak Ridge spallation source. Measurements at the reactors have yet to be made, and NUCLEUS aims to carry out a precise study of this as yet unexplored neutrino-matter coupling, with a unique sensitivity to potential new physics in the electroweak sector of the standard model. Coherent scattering differs from the beta-inverse reaction used up to now by an interaction cross section several orders of magnitude higher allowing a miniaturization of the detectors: only 10g of target for the first phase of NUCLEUS. Finally, the absence of a reaction threshold (instead of 1.8 MeV for the beta-inverse reaction) could also allow direct monitoring of the accumulation of plutonium in the nuclear reactor cores.
NUCLEUS will use sapphire (Al2O3) and calcium tungstate (CaWO4) bolometers in the form of 5 mm edge cubic crystals. A detection threshold of 20 eV has already been reached with this technology. The thesis work proposed here will focus on two central aspects of the experiment: the calibration of the detectors and the rejection of cosmic rays, the main source of background. An accurate calibration is indeed crucial to study coherent scattering and to reach the best sensitivity on a potential new physics. Although the energy range of the expected nuclear recoils, of the order of 100 eV, is above the achieved detection thresholds, no absolute calibration method for bolometers currently exists for this new region of interest. The extrapolation of the available measurements from the keV scale is problematic due to a rapid and non-trivial evolution of the contribution of the different excitation modes: phonons, ionization and scintillation. A new method proposed by the Department of Nuclear Physics of CEA-Saclay (DPhN) would give access for the first time to calibrated nuclear recoils, in the 100 eV range and uniformly distributed in the volume of the bolometer. The validation of this method and a first measurement with a NUCLEUS bolometer will be developed during the thesis, in collaboration with the IJCLab d'Orsay, the University of Munich (TUM) and the University of Vienna (TU Wien). Applicable to different types of bolometers, this method has potentially a strong scientific impact towards coherent neutrino scattering programs, light dark matter research but also solid state physics.
DPhN is also heavily involved in the development of the NUCLEUS muon veto. This active shielding surrounds as hermetically as possible the central detectors with plastic scintillator panels whose light is extracted by optical fibers connected to Silicon-Photomultipliers (SiPM). Its purpose is to sign the passage of cosmic rays near the bolometers in order to reject any event (potentially background) during the next ~100 microseconds. Data from this detector is a natural input to the NUCLEUS analysis. The start of the data collection on EDF site is planned for the end of 2022 - beginning of 2023.
Finally, the DPhN is also at the origin of the STEREO experiment which is motivated by the search for sterile neutrinos and the precise measurement of the neutrino spectrum resulting from the fission of 235U. It is installed at the ILL research reactor and is completing its data collection this year. Part of the thesis work could be oriented towards combining the final results of STEREO with those of other neutrino experiments, an effort already started with the PROSPECT collaboration. Some of the techniques involved in spectrum unfolding and global fit could be transferable to NUCLEUS.
Organization of the work:
The priority at the beginning of the thesis will be put on the development of the calibration method for 100 eV bolometers with a first step of proof of concept at CEA and Orsay in 2021-22, then a measurement with the a NUCLEUS bolometer in Germany in 2022-23. This work should lead to several publications.
Involvement in the analysis of NUCLEUS data will be stepped up in the second part of the thesis. The entry point will be the exploitation of data from the muon veto, installed on the EDF site from the end of 2022. The first work will be the optimization of gains and thresholds for each SiPM in order to ensure a high rejection of ambient gamma rays, a high muon detection efficiency and a controlled acquisition dead time. An automatic monitoring of the evolution in time of the performances will be set up. Then further analysis will focus on a specific source of background generated by cosmic rays.
In connection with the work on the calibration of bolometers, sensitivity studies could be carried out within the framework of low energy tests of the standard model accessible by NUCLEUS: evolution of sin2_theta_W, magnetic moment of the neutrino ... A synergy with some developments of the final analysis of STEREO would then be exploitable.
Through this work the student will have a complete training as an experimental physicist with aspects of simulation, detector development and data analysis. The physics topics addressed, coherent neutrino scattering and bolometer calibration, are very active in the community and will offer many research perspectives at the end of the thesis. The student will evolve in international collaborations. Within the CEA he (she) will benefit from the "transverse" character of the neutrino and will be in regular interaction with the nuclear physics, particle physics and reactor physics communities.