Laboratory link : http://irfu-i.cea.fr/Phocea/Vie_des_labos/Ast/ast_technique.php?id_ast=424
More : http://irfu-i.cea.fr/Phocea/Vie_des_labos/Ast/ast_technique.php?id_ast=4248
This PhD topic is about the NUCLEUS experiment, which aims at precisely measuring coherent elastic neutrino-nucleus scattering (CEvNS) at the Chooz nuclear power plant (France). Although at ~MeV energies, CEvNS is the predominant interaction process of neutrinos with matter, it has remained unobserved for a very long time because of the difficulty to measure the very low energy nuclear recoils it induces. It was only 40 years after its first prediction that this process was observed in 2017 with neutrinos of a few tens of MeV at the Oak Ridge laboratory (Tennessee). The first detection of CEvNS at a nuclear reactor remains to be achieved, especially because the corresponding nuclear recoils lie in an energy regime (~100 eV) which is difficult to measure with conventional detection technologies, and also because of the unfavorable background conditions nuclear power plant environments generally offer. The NUCLEUS collaboration is therefore working on the design of an innovative detection system using two cryogenic calorimeter arrays capable of reaching ~10 eV energy thresholds, and surrounded by a twofold system of instrumented cryogenic vetoes. This set of cryogenic detectors will be protected by an external passive shielding and by a muon veto to improve the identification and discrimination of backgrounds. With this system, NUCLEUS aims at a precise measurement of CEvNS in order to push the study of the fundamental properties of the neutrino as well as the search for beyond standard model physics towards the low energy frontier. Interestingly, CEvNS also exhibits a cross-section 10 to 1000 times larger than the usual ~MeV neutrino detection channels (inverse beta decay reaction, neutrino-electron scattering process), making it possible to miniaturize future long-range neutrino detection setups. The first phase of the NUCLEUS experiment will for instance deploy an array of cryogenic calorimeters made of sapphire (Al2O3) and calcium tungstate (CaWO4) crystals, totaling 10 g of detector.
In addition to the characterization and preparation of the experimental site at Chooz, our team at Irfu is taking a leading role in the project through several hardware and software developments. In particular, the DPhP is strongly involved in the realization of one of the instrumented cryogenic shielding of the experiment, here called the cryogenic outer veto. This detector consists of an arrangement of high-purity Germanium crystals, erected around the two cryogenic calorimeter arrays, and operated in ionization mode. This detection system will play a central role in the identification and discrimination of external backgrounds, such as ambient gamma radioactivity or atmospheric muons resulting from the interaction of primary cosmic rays in the atmosphere. The exploitation of the data delivered by this detector is then a natural entry in the global analysis effort to extract a first CEvNS signal at a reactor, with first background data collected in 2021/2022 during the blank assembly phase at the Technical University of Munich, and with data collected during the first physics run planned in 2023 at Chooz.
The work proposed in this PhD thesis is focused on the external cryogenic veto of the experiment, with the ultimate goal of achieving a comprehensive understanding of the backgrounds in the CEvNS region of interest, between 0.01 and 1 keV. The priority at the beginning will be given to the realization and commissioning of the external cryogenic veto system during the blank assembly phase in Munich. This work includes the assembly of the different detector elements (crystals, support mechanics, readout electronics, etc.) in the cryostat of the experiment, and includes all the necessary tests to validate and quantify the performances of this detector. In a second step, the student will ramp up in the collaboration analysis effort by contributing to the development of analysis and simulation tools. These tools will be used to interpret the background and detector calibration data acquired during the blank assembly phase and during the first physics run. He (she) will focus on the study of a specific source of external background, and quantify its impact on the physics potential of the experiment. This work will require a good understanding of the processes governing radiation interactions in matter and of the solid-state physics driving the behavior of cryogenic detectors (e.g. phonon propagation). Finally, the student will use the first data from the physics run at Chooz to conduct a search for new physics beyond the standard model (measurement of the weak mixing angle at low energies, search for new neutrino couplings, constraints on the electromagnetic properties of the neutrino, etc.). This work will require the implementation of advanced statistical methods for interpreting the data, in order on the one hand to understand the impact of the various sources of uncertainty on the constraints obtained, and on the other hand to guarantee the reliability of the results.