[pullquote]Why Neutrinos?
“One of the abiding mysteries of physics is why does the universe seem to consist primarily of matter instead of what might be more natural, which is equal amounts of matter and anti-matter,” says Steve Brice, deputy head of the Neutrino Division at Fermilab. “An asymmetry between matter and anti-matter may well be connected to an asymmetry, if there is one or a difference, between the way neutrinos behave and anti-neutrinos behave. DUNE is looking to see if neutrinos and anti-neutrinos behave differently—it’s something that we technically call CP violation—and to quantify what that difference is, if indeed there is one.” Scientists have already determined that neutrino interactions have a powerful effect on the universe—from facilitating the burning of hydrogen into helium in the sun or reigniting stalled supernova explosions—and furthermore suspect the particles may help explain other cosmic mysteries such as Dark Matter. Unfortunately, neutrinos rarely interact, and so scientists have had to come up with a clever way to increase the likelihood of witnessing and thereafter studying interactions. The solution is size. DUNE’s proposed detector is gigantic, spread across four membrane cryostats that collectively house some 40 kilotons of liquid argon. But even at that volume, scientists only expect between a thousand and ten thousand neutrino reactions per year.[/pullquote]

Over 150 physicists from around the world collided at The University of Texas at Arlington in early January to further collaboration on DUNE, the Deep Underground Neutrino Experiment. The international project, scheduled for completion in the early 2020s, will shoot a neutrino beam from Fermi National Accelerator Laboratory (CSA CSM) in Illinois towards four 10-kiloton liquid argon detectors staged some 4,800 feet below ground at the Sanford Underground Research Facility in South Dakota. The goal of DUNE is to better understand how the universe is put together, and its success hinges on perfecting a novel membrane cryostat to hold both the liquid argon and the cold electronics that will live and function for some 30 years inside it at 88K.

Membrane cryostats are typically used to transport liquefied natural gas. DUNE scientists chose a membrane cryostat because it’s cost effective relative to typical vessels and also more portable, as the pieces come prefabricated and can be welded on-site. The latter is a necessity for DUNE, where scientists will have to lower the pieces down mine shafts before assembly can occur. A special paint is used to detect potential leaks, changing color when exposed to ammonia gas.

Researchers at Fermilab sealed a 35-ton membrane cryostat prototype (Figure 1) in November and are now using it to test the modular detector design and the new cold electronics. “It’s a relatively thin-skinned stainless steel membrane on the inside supported by foam on the outside and finally supported by another steel sheet,” says Alan Hahn, a Fermilab scientist and the coordinator for the 35-ton prototype. It has a concrete vault outer supporting shell. Inside that is a carbon steel vapor barrier, followed by layers of foam that house the stainless steel membrane. The flexibility of the membrane can withstand cryogenic shocks generated by offloading large volumes of cold gas better than a simple cryostat, according to Hahn.

Inside the cryostat is a model of the modular detector scientists plan to put into DUNE, a complicated array of electronics that creates an electric field to make imaging possible. Hahn and other scientists call it a “time projection chamber.” The DUNE detector will work by drawing the atomic electrons freed by the products of neutrino interactions towards a collection of charged wire frames set up within 150 individual modules. Neutrino interactions in the liquid argon create a release of particles and energy, a pulse the wire arrays can record in 3-D and that scientists can later interpret.

Scientists hope that studying these interactions can help them create a “periodic table of particles” that will help explain the universe. “If you look at astrophysical objects you see galaxies have a particular structure,” says Jennifer Raaf, a Fermilab scientist and the assembly manager for MicroBooNE, a short baseline neutrino experiment at Fermilab to study neutrino oscillation. “And then if you back out even further there’s a structure of the entire universe—how the galaxies are clustered—and all of those things can be answered by looking at the underlying particles and how [they are] distributed throughout the universe. And dark matter is another big question that we have… It plays a big role in how the universe structure goes together. Neutrinos are just one part of that big picture in trying to probe into these huge questions.”

Collisions in a small detector can be recorded with electronics housed outside of the cryostat. In a large detector, however, the length of wire necessary to transmit a raw signal outside the cryostat can corrupt data because of degradation of the signal in transit. DUNE scientists are therefore developing new cold electronics to amplify and digitize weak signals before transmission out of the cryostat.

Brookhaven National Laboratory, in partnership with scientists and engineers from other institutions, is undertaking the development. The 35-ton prototype will test Brookhaven’s cold amplifier and digitizer. A multiplexor is still forthcoming. When working together, the digitizer will take amplified signals, basically current pulses that would appear as a curve on an oscilloscope, and turn that pulse into a series of numbers that the multiplexor then buffers and sends out of the cryostat.

Brookhaven’s amplifier is already being tested in MicroBooNE. The experiment first started recording data in October 2015. “The noise that we measured in MicroBooNE is a factor of three smaller than we observe with similar detectors that use warm electronics,” says Hucheng Chen, a Brookhaven physicist who coordinated the integration of the sensors and the cold electronics.

One of the first images (Figure 2) to emerge from the experiment shows a showering event. “If I had to guess what kind of event this was, I would say a neutrino came in and had what we call a deep inelastic scatter and broke up the nucleus that it hit and caused all these other particles to be created instead,” says Raaf. The little red blip at the vertex is probably a proton, according to Raaf. A proton is a heavy particle and so it deposits significant energy, but not a track, as it travels in the detector. In this case the proton didn’t travel very far after impact but left a blip that allowed scientists to observe the collision. A photon created in neutrino interactions converts to an electron/positron pair. The pair does leave a track as it loses energy and is identifiable as a less well-defined track that begins a short distance away from the vertex.

A challenge for Brookhaven engineers was preventing the extreme cold (88K) from impacting electronic performance. As an electronic circuit is cooled its resistance to electron “flow” drops. Lowering the resistance of a circuit changes its operating current and also makes its components more susceptible to damage from fast-moving, wayward electrons. “All integrated circuits have a limited lifetime,” says Gianluigi De Geronimo, the electrical engineer in Instrumentation who leads the team designing the electronics inside MicroBooNE. “For instance, the ones we use in our cell phones typically have a lifetime on the order of 10 years. When you work in cryogenic environments, the lifetime can be greatly reduced. Since these detectors have to work for 15, 20 years, and there is no way that you can go in and replace the electronics, we developed design guidelines for the integrated circuits that allow them to achieve a lifetime in excess of 20 years.”

In the 35-ton prototype, the Brookhaven electronics sit on wire chambers in the detector modules. It will also examine procedures for purifying liquid argon, test new small light detectors and serve as a test bed for the continuous collection of data. Part of the reason DUNE is so big is because scientists want to track neutrinos originating from other sources, such as supernova bursts or proton decay. Moving from a triggered readout to a continuous readout means scientists have to determine everything from how quickly information can be stored to how data can be filtered thereafter to look for specific types of events.

The 35-ton prototype is exposed to air at Fermilab and will be used to record collisions from charged particles originating from cosmic ray interactions in the atmosphere and not from a particle beam. Cosmic rays are copious, so scientists expect to run the cryostat for only three months. Analyzing the data from those three months, however, will take much longer. “The simple thing of knowing a particle went through and taking a picture and making sure you actually see it—that’s easy,” says Michele Stancari, a scientist at Fermilab who works on the 35-ton prototype. “But characterizing how sensitive things are… takes time. And you have to crunch through the numbers real carefully. So that’ll probably take us a year just to process all the data and get to the point where we’re willing to summarize our results and numbers in a few pages.”

Fermilab scientists hope to publish results in time to help a team at CERN that is currently planning ProtoDUNE, the next step in testing DUNE principles. The membrane cryostat in ProtoDUNE will be larger and scientists hope it will include all of the cold electronics planned for DUNE. ProtoDUNE is expected to go online in 2018.