Ph.d. degree
University of Bern, Switzerland, 2018

Damian Goeldi

Supervisor:Antonio Ereditato

A Novel Liquid Argon Time Projection Chamber Detector: The ArgonCube Concept

Keywords:

• Neutrino detectors
• Particle tracking detectors
• Time projection chambers
• Electronic detector readout concepts (gas, liquid)

Abstract:

The Standard Model (SM) of particle physics has proven to be remarkably consistent in its explanation of experimental observations. An exception is the intriguing nature of neutrinos.
Particularly, neutrino flavour eigenstates do not coincide with their mass eigenstates.
The flavour eigenstates are a mixture of the mass eigenstates, resulting in oscillations for non-zero neutrino masses.
Neutrino mixing and oscillations have been extensively studied during the last few decades probing the parameters of the three flavour model.
Nevertheless, unanswered questions remain: the possible existence of a Charge conjugation Parity symmetry (CP) violating phase in the mixing matrix and the ordering of the neutrino mass eigenstates.
The Deep Underground Neutrino Experiment (DUNE) is being built to answer these questions via a detailed study of long-baseline neutrino oscillations.
Like any beam experiment, DUNE requires two detectors: one near the source to characterise the unoscillated beam, and one far away to measure the oscillations.
Achieving sensitivity to CP violation and mass ordering will require a data sample of unprecedented size and precision.
A high-intensity beam (2 MW) and massive detectors (40 kt at the far site) are required.
The detectors need to provide excellent tracking and calorimetry.
Liquid Argon Time Projection Chambers (LArTPCs) were chosen as Far Detectors (FDs) because they fulfil these requirements.
A LArTPC component is also necessary in the Near Detector (ND) complex to bring systematic uncertainties down to the required level of a few percent.
A drawback of LArTPCs is their comparatively low speed due to the finite charge drift velocity (∼ 1 mm/us).
Coupled with the high beam intensity this results in event rates of 0.2 piled-up events per tonne in the ND.
Such a rate poses significant challenges to traditional LArTPCs: Their 3D tracking capabilities are limited by wire charge readouts providing only 2D projections. To address this problem a pixelated charge readout was developed and successfully tested as part of this thesis.
This is the first time pixels were deployed in a single-phase LArTPC, representing the single largest advancement in the sensitivity of LArTPCs---enabling true 3D tracking.
A software framework was established to reconstruct cosmic muon tracks recorded with the pixels.
Another problem with traditional LArTPCs is the large volume required by their monolithic design resulting in long drift distances.
Consequentially, high drift voltages are required.
Current LArTPCs are operating at the limit beyond which electric breakdowns readily occur.
This prompted world-leading studies of breakdowns in LAr including high-speed footage, current-voltage characteristics, and optical spectrometry.
A breakdown-mitigation method was developed which allows LArTPCs to operate at electric fields an order of magnitude higher than previously achieved.
It was found however that a safe and prolonged operation can be achieved more effectively by keeping fields below 40 kV/cm at all points in the detector.
Therefore, high inactive clearance volumes are required for traditional monolithic LArTPCs.
Avoiding dead LAr volume intrinsically motivates a segmented TPC design with lower cathode voltages.
The comprehensive conclusion of the HV and charge readout studies is the development of a novel fully modular and pixelated LArTPC concept---ArgonCube.
Splitting the detector volume into independent self-contained TPCs sharing a common LAr bath reduces the required drift voltages to a manageable level and minimises inactive material.
ArgonCube is incompatible with traditional PMT-based light readouts occupying large volumes.
A novel cold SiPM-based light collection system utilised in the pixel demonstrator TPC enabled the development of the compact ArgonCube Light readout system (ArCLight).
ArgonCube's pixelated charge readout will exploit true 3D tracking, thereby reducing event pile-up and improving background rejection.
Results of the pixel demonstration were used in simulations of the impact of pile-up for ArgonCube in the DUNE ND.
The influence piled-up pi0-induced EM showers have on neutrino energy reconstruction was investigated.
Misidentified neutrino energy in ArgonCube is conservatively below 0.1 % for more than 50 % of the neutrino events, well within the DUNE error budget.
The work described in this thesis has made ArgonCube the top candidate for the LAr component in the DUNE ND complex.