The first prototype Anode Plane Assemblies (APAs) for ProtoDUNE have arrived at CERN, where two large liquid argon detectors are under construction to test the engineering specifications for the US-based Deep Underground Neutrino Experiment (DUNE). The APAs are large rectangular steel frames covered with approximately 4,000 wires that will be used to read the signal from particle tracks generated inside the experiment’s liquid-argon detectors. At 2.3m by 6.3m, the frames are roughly as large as five full-size pool tables placed side-by-side.

Engineers at UK-based Science and Technology Facilities Council (STFC) laboratories designed the APAs, the first such anode planes ever built in the UK, and will deliver 150 units in total to support the project over the next few years. “This shipment marks the culmination of a year of very hard work by the team,” says Dr. Justin Evans of the University of Manchester, who is leading the protoDUNE APA-construction project in the UK. “Constructing this anode plane has required relentless attention to detail, and huge dedication to addressing the challenges of building something for the first time. This is a major milestone on our way to doing exciting physics with the protoDUNE and DUNE detectors.”

DUNE is a flagship international experiment run by the US Department of Energy’s Fermi National Accelerator Laboratory (CSA CSM) that involves over 1,000 scientists from 31 countries. Various elements of the experiment are under construction around the world. Using a particle accelerator, an intense beam of neutrinos will be fired 800 miles through the earth from Fermilab’s location outside Chicago to the detectors in South Dakota.

The DUNE project aims to advance scientific understanding of the origin and structure of the universe. One aspect of study is the behavior of neutrino particles and their antimatter counterparts, antineutrinos. Scientists say this research could provide insight as to why we live in a matter-dominated universe and also contribute to the debate on why the universe survived the Big Bang.

“Each one of the four final DUNE modules will contain 17,000 tons of liquid argon,” says Söldner-Rembold, a professor at the University of Manchester and leader of the DUNE APA consortium. “For a single module, 150 APAs will need to be built, which represents a major construction challenge. We are working with UK industry to prepare this large construction project. The wires are kept under tension and we need to ensure that none of the wires will break during several decades of detector operation as the inside of the detector will not be accessible. The planes will now undergo rigorous testing to make sure they are up for the job.”

Engineers at CERN are building two large liquid argon detectors for ProtoDune. Researchers will use them to investigate two ways to use argon in the future experiment, either in single-phase with only liquid argon or in dual-phase with argon as both a liquid and a gas. “They’re the largest liquid-argon particle detectors that have ever been built,” says Ed Blucher, DUNE co-spokesperson and a physicist at the University of Chicago.

As DUNE’s test bed, the ProtoDUNE detectors also have to offer researchers a realistic picture of how the liquid-argon detection technology will work in DUNE, so the instrumentation inside the detectors is also at full, giant scale.

“If you’re going to build a huge underground detector and invest all of this time and all of these resources into it, that prototype has to work properly and be well-understood,” says Bob Paulos, director of the University of Wisconsin–Madison Physical Sciences Lab and a DUNE engineer. “You need to understand all the engineering problems before you proceed to build literally hundreds of these components and try to transport them all underground.”

Most of the space inside the detectors serves as the arena of particle interaction, where neutrinos can smash into an argon atom and create secondary particles. Surrounding this interaction space is the instrumentation that records these rare collisions, like a camera committing the scene to film.

One signal is ionization charge, where a neutrino interaction generates other particles that propagate through the detector’s vast argon pool, kicking electrons—called ionization electrons—off atoms as they go. The second signal is light.

The first signal emerges as a streak of ionization electrons. To record it, scientists will use the APA screens, created using 24 kilometers of precisely tensioned, closely spaced, continuously wound wire. The wire screen is positively charged, so it attracts the negatively charged electrons.

The anode planes attract the electrons. Pushing away the electrons will be a complementary set of panels, called the cathode plane. Together, the anode and cathode planes behave like battery terminals, with one repelling electron tracks and the other drawing them in. A group at CERN designed and is building the cathode plane.

APAs for the single-phase ProtoDUNE are under construction at the STFC facilities in the UK. The dual-phase detector will operate on the same principle but with a different configuration of wire arrays. A special layer of electronics near the cathode will allow for the amplification of faint electron tracks in a layer of gaseous argon. Groups at institutions in France, Germany and Switzerland are designing those instruments. Once complete, they will also send these arrays to be tested at CERN.