Last year has been particularly busy in CERN’s EHN1 test facility in the north area of the Prévessin site, as work is ongoing for the construction of the two ProtoDUNE detectors. The two prototypes test different concepts of the future Deep Underground Neutrino Experiment (DUNE) detector, planned to start operations by 2024 as part of Fermilab’s Long Baseline Neutrino Facility (LBNF).  Each of them is a 11x11x11-metre Liquid Argon Time Projection Chamber, with a single- (SP) or dual-phase (DP) configuration that will soon be filled with 800 tons of liquid argon (LAr). Despite their large dimensions, they are mini models of the four DUNE far detectors.

Following a well-coordinated international effort the detectors were assembled in the two cryostats at CERN and the first tests will start this August. The team plans to start with test beam data for the characterisation of the performance of the detector, followed by months of cosmic data taking to establish the long term stability of the complete system.

A crucial step for ProtoDUNE was welding together the cryostat, or cold vessel, that will house the detector components and liquid argon. (Credits:CERN). 

Success of the ProtoDUNE is key in developing and testing the required technologies of the TPC detector of the future DUNE detector and demonstrate the production and integration schemes. Professor Christos Touramanis, University of Liverpool, one of the coordinators of the single-phase prototype explains: “The results will allow validating the technologies used and provide invaluable feedback in the technical design review of DUNE in spring 2019. Therefore the team faced a rather tight timeline to provide the required data for the DUNE TDR review that will consequently allow onset of DUNE construction by 2020.”

A major challenge for ProtoDUNE, given its large dimensions, is the cryostat. The ProtoDUNE cryostat - the largest ever constructed for a particle physics experiment - has been built at CERN. A major difficulty stems from the fact that the cryostat contains the liquid argon as well as the detectors and read-out electronics while very good insulation and high purity is required. To meet this challenge, the collaboration explored a novel technological solution inspired by the liquified-natural-gas (LNG) shipping industry. The patent is based on a membrane-type containment system with two cryogenic liners that support and insulate the liquid cargo. CERN established a fruitful collaboration with Gaztransport & Technigaz (GTT), a firm that deploys LNG in about 80% of all transport ships worldwide, thanks to which the same patent was available for the ProtoDUNE cryostat. Following the success of this project, the same cryostat will serve as a prototype for the cryostat of the DUNE far detector.

The ProtoDUNE-SP TPC. The beam enters from the left into the nearside (Saleve-side) drift volume through the Beam Plug.

A Greek engineer, a Turkish, an American and an Italian physicist and a Spanish worker in the completed ProtoDUNE-SP cryostat. International cooperation in science, what CERN does best in the last 63 years (Image Credits: Prof. Christos Touramanis). 

The ProtoDUNE detector uses the state-of-the-art time projection technology of Liquid Argon TPC (LAr TPC), to capture 3D images of particle tracks created in neutrino interactions. The technology, originally proposed by Carlo Rubbia in 1977, was conceived as a tool for a uniform imaging with high accuracy of massive volumes. The operational principle is based on the fact that in highly purified liquid argon ionization tracks can be transported, practically undistorted, by a uniform electric field over macroscopic distances. The ionization electrons are drifted with a constant electric field away from the cathode plane and towards the segmented anode plane.

The DUNE collaboration plans to use the Liquid Argon TPC technology for the massive and extremely sensitive DUNE Far Detector. The reference design is based on a single-phase readout, similarly to the one applied in ICARUS and MicroBooNE, where the readout anode is composed of wire planes in the liquid argon volume. A second design adopts a dual-phase approach, in which the ionization charges are extracted, amplified and detected in gaseous argon (GAr) lying above the volume of liquid argon. Clearly, one of the goals of the project is to validate how LAr TPC can scale up in the dimensions of DUNE as well as other alternative technologies for the DUNE far detector.

“As Liquid argon technology is sufficiently new it is highly desirable to perform a large-scale test of single phase ProtoDUNE, to reduce risks associated with the operation of the DUNE far detector” explains Prof. Touramanis . “ProtoDUNE-SP is pushing the limits of LAr TPC technology with key dimensions and technological solutions crucial for de-risking the scaling of the ICARUS technology” and adds “measuring and understanding the TPC performance will help to remove any risks related to the project and to move forward with the submission of the Technical Design Report (TDR) of the DUNE far detector”. Moreover, these tests will demonstrate the ability to construct this detector in a technically and financially feasible way. Although there are several single-phase LAr TPCs in operation or under construction (MicroBooNE, SBND, ICARUS), the design for the DUNE far detector includes various specific features that have not been field-tested.

The single-phase ProtoDUNE has the longest drift distance between cathode and anode planes of 3.6 m compared to 1.5 m in ICARUS and 2.6 m in MicroBooNE. Moreover, its drift volumes of 155.5 m3, exceeding the 86.4 m3 ones of ICARUS and the 62.2 m3 of the single drift volume of MicroBooNE, thus paving the way to the 10−kt LArTPCs foreseen in the final DUNE far detector. For the TPC of the ProtoDUNE-single phase (protoDUNE-SP), a critical component was the delivery of the Anode Plane Assembly (APA) modules at CERN and their successful integration in the TPC/Cryostat.

The ProtoDUNE SP detector consists of six APA modules (6 m high and 2.3 m wide) each of which uses approximately 24 kilometers of precisely tensioned, closely spaced, continuously wound wire.  Four of the six ProtoDUNE-SP APAs were constructed at PSL (Wisconsin) and two at Daresbury (UK) using identical semi-automatic wiring machines and “process carts”. The wire screen, receive an induced signal, recorded by the electronics and then send to computer farms, allowing to study the neutrino interactions. This is why precision in positioning and tension of the wires is critical, as well as electrical continuity and isolation.

The DUNE APAs installed inside the cold-box. APAs is one of the key novelties of proto-DUNE. The detector consists of six APA modules (6 m high and 2.3 m wide) each of which uses approximately 24 kilometers of precisely tensioned (Image credits: Prof. Christos Touramanis). 

The photo-detector system, developed by a collaboration of US and Brazilian institutes, is another important part of the protoDUNE TPC. The scintillation light from the liquid argon allows defining the reference time, i.e the time at which the particle crossed the detector volume. However, though liquid argon is a good scintillator, it emits photons in the far ultraviolet (~128 nm), which doesn’t match the sensitivity of most photodetectors.  Therefore, the photon detection in ProtoDUNE is done using lightguide, installed between the wire planes of APA, coated with a wavelength shifter, which absorbs the UV light and re-emits in the visible. This secondary emission is then detected by "silicon photomultipliers" (SiPM) and the analog signals are transferred outside the cryostat and read by dedicated electronics operating at room temperature.

Single-phase liquid argon TPCs don't allow for charge multiplication to achieve high signal-to-noise ratio. This means that the protoDUNE electronics have to be as close as possible to the actual wires electrodes and inside the cryostat. Therefore the team had to develop electronics able to achieve high performances and very low noise levels, while coping with the very low temperatures of the liquid argon.

Finally, an important development has been the design of the DAQ system in collaboration between CERN, UK and US institutes. Presently, two readout technologies are employed for the cold electronics: the CERN’s Front-End Link EXchange (FELIX) system and the SLAC RCE system. One out of six ProtoDUNE TPC Anode Plane Assemblies is read out using FELIX, a project initially developed within the ATLAS Collaboration. Its purpose is to facilitate the development of high-bandwidth readout, needed for the High-Luminosity LHC, presently planned to start in 2026. DAQ is designed for an average record rate of 34 Gb/sec and is located next to the detector. The DAQ system is connected to the CERN IT infrastructure, with a dedicated line of 20Gb/s installed and running since August 2017.

Today, all the elements of the TPC and the cold electronics have been produced in parallel to the integration of the detector at CERN achieving a just-in-time delivery mode of operation. Within only a year, the production of all components was completed and their integration in the proto-DUNE SP started early this year. Furthermore, power, signal and data connections to the outside are done with cables running through one dedicated feedthrough on the cryostat roof for each APA.

"In the last months, the progress in the construction and test of the various components of the second DUNE prototype detector at CERN has changed gear,” said Filippo Resnati, the technical coordinator of the Neutrino Platform at CERN. “The field cage is fully installed and has been tested at 150,000 volts. The first 3-meter-by-3-meter signal-amplification system is fully assembled and ready for tests in realistic thermodynamic conditions. Light readout, electronics, data acquisition and detector control systems are in very good shape too."

The installation of the detector will be completed in fall of this year. The team aims to have a complete, fully equipped tested detector filled with Liquid Argon and ready for commissioning by August 2018, to start the first tests with the dedicated H4 line, spanning a range of particle types and energies. At the same time, a second beam line (H2) can provide beam to the ProtoDUNE dual phase prototype. These tests will allow measuring charged particle cross sections in argon, at energies relevant to DUNE neutrino interactions, while also advancing the state of the art LAr event reconstruction algorithms and software suites. The test-beam data will involve strong feedback between reconstruction, detector simulation, and hadronic modelling. Touramanis notes: “The beam tests will give us precise measurements of how sub-GeV particles interact with Argon which will then feed the Monte Carlo for DUNE far detector thus allowing for improved modelling and simulations and consequently a more rigorous physics analysis“. The beam tests will run until mid-November and after that, the team will start a cosmic data taking campaign while trying to extract the performance characterisation results that are required for the DUNE TDR in early 2019. Due to some delays for the dual phase prototype, that detector is now aiming to be ready for tests with cosmic rays by the end of the year.“

Closing of the ProtoDUNE cryostat. The detector system is ready for the first test beam in August.

DUNE marks the evolution of neutrino physics as a global enterprise. Almost five years ago the leaderships of Fermilab, CERN and KEK realised that a long baseline neutrino facility offers a good physics case that could attract senior researchers and early-career scientists from all over the world. This was also reflected in the last European Strategy update where it was clearly stated that CERN should contribute to neutrino physics done at any place in the planet. ProtoDune helped to bring CERN’s expertise in the game while CERN also offered the platform for all its member states to work together and integrate in DUNE. For Touramanis, ProtoDUNE already reflects a sociological evolution of the neutrino community: “a paradigm change in the way neutrino science is done; we saw the transition from the previously fragmented communities to a 1000-strong, truly global collaboration where people thought globally and were able to interact beyond borders and any form of boundaries”.

The unprecedented event reconstruction capability offered by the ProtoDUNE LAr TPC combined with the lessons from the first beam tests of the detector at CERN will open the way to a truly rich programme of new physics investigations into particle interaction processes.