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

Göldi, Damian (2018). A Novel Liquid Argon Time Projection Chamber Detector: The ArgonCube Concept. (Dissertation, Universität Bern, Philosophisch-naturwissenschaftliche Fakultät)

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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 (2MW) 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 (~ 1mmμs−1). 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 kVcm−1 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 π0-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.

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