Adapted from the Symmetry Magazine article, 7 September 2018.
The dual-phase detector is a concept that’s relatively new for neutrino science but familiar to researchers from other parts of particle physics — for example, for dark matter experiments it is well established and has helped produce leading results for the past decade.
All dual-phase particle detectors to date use a combination of liquid and gas phases. The DUNE far detector module of liquid and gaseous argon will be the largest dual-phase detector ever created when it comes online in the mid-2020s.
Dual-phase detectors can record a particle interaction twice: first when the collision occurs in the liquid, creating a flash of light, and again when the resulting spray of particles enters the area filled with gas and produces even more signals. Having these two indicators allows for an especially precise and clear reconstruction of the original interaction.
At CERN’s neutrino platform, led by Marzio Nessi, scientists are now completing two 800-ton prototype detectors to give the technologies for DUNE a final test.
“The goal of these devices is to develop the technology and be sure we do things the right way,” says Filippo Resnati, technical coordinator of the neutrino facility at CERN. “But it’s also very nice to know that despite these being tests, these are the two biggest liquid-argon time projection chambers that have ever been built, and by far in the shortest time.”
The single-phase prototype should finish filling with liquid argon and be commissioned by the end of summer, seeing first particle tracks in the fall. The dual-phase prototype recently finished the first test of a key component called the charge readout plane, or CRP, which will amplify the electrons into the gas and collect their signals. The CRPs and other components should be installed this fall.
“We have to make sure we are working together as a team and that each of these technologies can work,” says Jae Yu, a physicist at the University of Texas at Arlington who works on the dual-phase ProtoDUNE. “It’s always better to have different technologies so we can cross check each other.”
Some advantages of the dual-phase technology, compared to the single-phase setup, are stronger and cleaner signals and a lower energy threshold, meaning the detector can see lower-energy neutrinos. Amplifying the electrons in the gas makes the signal stand out from background noise. Single-phase detectors try to collect the signal as soon as possible, meaning the electronics are usually inside the detector, within the liquid argon at a cryogenic temperature. In contrast, the dual-phase detector’s electronics will be housed in special chimneys that are accessible from the outside.
“You can access the electronics at any moment needed, without contaminating the liquid argon,” says Dario Autiero, DUNE project leader for the French National Institute of Nuclear and Particle Physics (IN2P3) groups. “This concept and the design of the electronics are innovative, and it took a long time to develop them.”
Another bonus: almost all of the liquid argon inside the dual-phase detector is in one large, signal-producing region, simplifying the data analysis. In contrast, single-phase detectors are segmented into chunks, meaning the different sections later have to be combined together for data analysis, and gaps accounted for.
But with those benefits come challenges.
The cathode of the dual-phase field cage—the electrical component that draws the electrons towards the signal-recording pieces—must be operated at a mind-boggling voltage of around 600,000 volts. In addition, the CRPs must lie perfectly level at the border of the liquid and gas phases of argon and function stably, without sparking.
“We are pioneers, in a sense,” says Inés Gil Botella, leader of the CIEMAT group in Spain that is working on the elements that will capture the light within the dual-phase detector. “This is a technology challenge at these scales because it has never been done before. It’s a very exciting time, but also a very critical time. We are advancing the technology.”
The overlap between dual-phase technologies for dark matter and neutrino experiments will continue for the foreseeable future. The Global Argon Dark Matter Collaboration already is looking at the design of the ProtoDUNE cryostat as a potential casing for their 20-ton experiment, and ProtoDUNE collaborators are looking at how the dual-phase prototype detector could be used to look for a particular kind of dark matter.
“The more I think about it, the more I fall in love with this technology,” Yu says. “It’s beautiful. It’s mesmerizing. It’s a piece of art. It’s elegant. And it’s just the beginning. There’s a lot more work to be done.”
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