Abstract
Liquid argon time projection chamber technology is an attractive choice for large neutrino detectors, as it provides a high-resolution active target and it is expected to be scalable to very large masses. Consequently, it has been chosen as the technology for the first module of the DUNE far detector. However, the fiducial mass required for “far detectors” of the next generation of neutrino oscillation experiments far exceeds what has been demonstrated so far. Scaling to this larger mass, as well as the requirement for underground construction places a number of additional constraints on the design. A prototype 35-ton cryostat was built at Fermi National Acccelerator Laboratory to test the functionality of the components foreseen to be used in a very large far detector. The Phase I run, completed in early 2014, demonstrated that liquid argon could be maintained at sufficient purity in a membrane cryostat. A time projection chamber was installed for the Phase II run, which collected data in February and March of 2016. The Phase II run was a test of the modular anode plane assemblies with wrapped wires, cold readout electronics, and integrated photon detection systems. While the details of the design do not match exactly those chosen for the DUNE far detector, the 35-ton TPC prototype is a demonstration of the functionality of the basic components. Measurements are performed using the Phase II data to extract signal and noise characteristics and to align the detector components. A measurement of the electron lifetime is presented, and a novel technique for measuring a track's position based on pulse properties is described.
Highlights
In order to minimise the effects of electron lifetime and diffusion, as well as to reduce the required high voltage (HV), the drift length in the Deep Underground Neutrino Experiment (DUNE) far detector (FD) is limited to 3.6 m
The cosmic-ray counters (CRCs) are used to search the time projection chamber (TPC) data for signals corresponding to cosmic rays that pass through pairs of counters, as well as to determine the event time t0
The 35-ton prototype successfully demonstrated in Phase I that liquid argon of sufficient purity could be maintained in a membrane cryostat with adequate filtering and circulation
Summary
The critical design choices for the DUNE FD are described below, as well as the elements of the 35-ton prototype’s design that test these choices. In order to minimise the effects of electron lifetime and diffusion, as well as to reduce the required high voltage (HV), the drift length in the DUNE FD is limited to 3.6 m This requires the APAs to be placed within the active volume and to be read out on both sides. The short middle APA (APA 1 in figure 1) in the 35-ton prototype was built without the grounded meshes between the collection planes in order to test the impact on operations and measurements. Installed in the vertical gap between the short middle APA and one of the long APAs is an electrostatic deflector, which is designed to control the electric field in this difficultto-model region and make the charge collection on the neighbouring wires easier to understand. Low-voltage electrical power to the detector elements and signals from the FEMBs, the photon detectors and the cameras pass through a custom board called the flange board, which penetrates a flange on the top of the cryostat
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