Scintillation light detection techniques in a 750-ton liquid argon TPC for the Deep Underground Neutrino Experiment

The experimental confirmation of the neutrino oscillations, and therefore the non-zero neutrino mass, opened a new era of physics beyond the Standard Model of particle physics. Neutrinos do have mass, and their flavour change when propagating. Thanks to the huge effort of the neutrino physics community, many neutrino properties have been measured in the last decades, including most of the neutrino oscillation parameters. However, other properties remain unknown, such as the absolute scale of the neutrino masses, or the nature itself of their masses. The Deep Underground Neutrino Experiment (DUNE) will bring neutrino physics to a new era, measuring the oscillation parameters with unprecedented precision that will allow determining the CP violation phase in the leptonic sector and the neutrino mass ordering. It also aims at detecting neutrinos from a core-collapsing supernova within the galaxy if it occurs during the running of the experiment and performing beyond standard model searches. DUNE is a long-baseline neutrino oscillation experiment designed by an international collaboration of around 1,300 scientists. It will consist of the most powerful neutrino beam installed at Fermilab (US), a Near Detector placed downstream the beam, and a Far Detector located 1.5 km underground at the Sandford Underground Research Facility (SURF), around 1,300 km away from the beam. The Far Detector of DUNE will consist of four liquid-argon time projection chambers (LAr-TPCs) of 17 kton each. DUNE will bring the LAr-TPC technology to an unprecedented large scale. A LAr-TPC is a kind of particle detector that allows the particle-track reconstruction with millimetre precision. In a LAr-TPC, the interacting particles ionize the liquid argon along their tracks. This charge is drifted by a uniform electric field and readout. Additionally, scintillation light is produced, providing a fast signal used as a trigger and also to reconstruct the particle interaction time. The excellent imaging capabilities of the LAr-TPC technology, and the advantages of using liquid argon as a detector medium for neutrino physics, will allow DUNE to fulfil its physics goals. A full prototyping effort is being carried out by the DUNE collaboration to validate the LAr-TPC technology at the kiloton scale. In particular, two prototypes have been assembled and operated at the CERN neutrino platform: ProtoDUNE Single-Phase and ProtoDUNE Dual-Phase, of 300 ton of active mass each, and which took data from 2018 to 2020. While ProtoDUNE Single-Phase is based on a more traditional approach of a LAr-TPC based on wire readout planes, the innovative Dual-Phase approach includes a gas layer at the top, with an extraction layer and Large Electron Multipliers (LEMs) to amplify the signal. The work presented in this thesis has been carried out in the framework of this prototyping work, specifically it has been focused on the operation and performance measurement of the photon detection system of ProtoDUNE Dual-Phase. The obtained results are of interest for any liquid argon experiment. The first chapters of this thesis introduce the status of neutrino physics, the operating principles of the LAr-TPC technology and the DUNE experiment. Then, ProtoDUNE Dual-Phase and its different systems are explained with a particular focus on the photon detection system. The photon detection system is a crucial part of any LAr-TPC, since it provides the interaction time, which is needed to reconstruct the event in three dimensions and contributes to the particle calorimetric recontruction. The photon detection system can also provide a trigger for non-beam events. The ProtoDUNE Dual-Phase photon detection system consists of 36 photomultiplier tubes (PMTs) placed below the TPC active volume. Due to the monolithic design of the detector, the photon detection system must detect scintillation light from more than six meters away, being the longest light detection distance in any liquid argon TPC up to date. A good characterization of the photon detection system is crucial in order to demonstrate its capabilities. The ProtoDUNE Dual-Phase photon detection system performance is studied in detail in chapter 5. Additionally, a good understanding of the light production, propagation and detection processes is key in order to extrapolate the performance of the detector at larger scales. In this sense, the development of a Monte Carlo simulation allows identifying the main parameters affecting the detector performance. A dedicated Monte Carlo simulation of the scintillation light production, propagation and detection has been developed, including the light production from cosmic muons and low energy radiological backgrounds, and compared with data. The understanding of the radiological backgrounds is critical to correctly evaluate the physics performance of DUNE at low energies. The simulation is explained in chapter 6 and the comparison with ProtoDUNE Dual-Phase data can be found in chapter 7. Since scintillation light in liquid argon is produced at 127 nm, at which most photosensors are not sensitive, fluorescent materials are introduced in order to shift the light wavelength towards the visible range, where PMTs have maximal detection efficiency. ProtoDUNE Dual-Phase has carried out an innovative photon detection program, by testing several wavelength shifting techniques at the multi-ton scale which are the object of analysis in this thesis. ProtoDUNE Dual-Phase has been the first detector deploying the novel polyethylene naphthalate (PEN) together with the more traditional tetraphenyl butadiene (TPB) as a wavelength-shifter. Although TPB has proven its good performance in many liquid argon experiments, its deployment requires sophisticated coating setups that are difficult to scale to large surfaces. TPB coatings are also very delicate, making its handling difficult. In this sense, PEN appears as a promising alternative, since it is a thermoplastic similar to PET, very stable and easy to handle. However its wavelength-shifting efficiency is not well known, and it has been never studied in real operating conditions in large scale LAr-TPC. The PEN wavelength efficiency is measured and its performance is evaluated and compared with TPB in chapter 8. This study is critical to understand if PEN represents an effective alternative to TPB for DUNE. Xenon doping is another alternative light detection technique for large scale LAr-TPCs. The presence of xenon at the level of a few ppm (parts-per-million) in the liquid argon acts as a wavelength shifter, shifting part of the scintillation light to longer wavelengths and reducing the light attenuation with the propagation distance. The simplicity of just adding a small quantity of xenon makes it an attractive alternative for large-scale LAr-TPC that is being considered for one of the DUNE Far Detector modules. However, the light production mechanisms in xenon-doped liquid argon have not been characterized yet, and their effects in the detected light in a several-meter-drift LAr-TPC have never been measured. In this sense, a deep study is needed in order to validate the xenon-doping technique for DUNE. ProtoDUNE Dual-Phase took data with xenon-doped liquid argon contaminated with a small amount of nitrogen. The performance of the photon detection system using xenon-doped liquid argon and its comparison with pure liquid argon using ProtoDUNE Dual-Phase data is explained in chapter 9. This analysis represents a unique opportunity to study the performance of a LAr-TPC using xenon-doped liquid argon, as the ProtoDUNE Dual-Phase is the largest monolithic LAr-TPC ever operated. The observation of the proton decay is a process beyond the Standard Model that it has never been observed. Proton decay is one of the requirements of many Gran Unification Theories (GUT), and it would also help to explain the matter-antimatter asymmetry in the universe. Placed deeply underground to mitigate backgrounds and thanks to the good imaging capabilities that allow to identify the interacting particles, DUNE has a promising potential to perform proton decay searches. However, the imaging capabilities rely strongly on having a good and efficient light detection system providing the interaction time of the event. In chapter 10, the performance of a light detection system based on TPB-coated PMTs, as in ProtoDUNE Dual-Phase, is studied. The goal is to validate its capability to provide the event time of proton decay events in the presence of background events in a 10 kton Far Detector of DUNE. This is a key study since the capability to perform proton decay searches is one of the primary physics goals of DUNE.