Long before the Deep Underground Neutrino Experiment begins its first measurements to expand our understanding of the universe, a prototype of one of the experiment’s detectors is broadcasting new neutrino detection techniques.

Currently under construction, DUNE will be a massive experiment spanning over 800 miles. The neutrino beam, originating at the U.S. Department of Energy’s Fermi National Accelerator Laboratory, will pass through a particle detector on the Fermilab site before traveling through the ground to a massive detector at the Sanford Underground Research Center in South Dakota.

A proximity detector consists of particle detection systems. One of them, known as ND-LAr, will have a liquid-argon time-projection camera to record particle tracks; it will be placed in a container full of liquid argon. When a neutrino collides with one of the particles that make up the argon atoms, more particles are produced during the collision. As each particle from the collision exits the nucleus, it interacts with nearby atoms, removing some of their electrons, producing detectable signals in the form of light and charge. The ND-LAr is optimized to see both of these types of signals. DUNE researchers chose liquid argon for one of the near-field detector systems for direct comparisons, analyzing the results of both the ND-LAr and the far-field detector, which also rely on liquid argon for particle detection.

The ND-LAr prototype received the name 2×2 prototype because its four modules are arranged in a square. The final version of the ND-LAr will have 35 modules, each slightly larger than those used in the prototype. The 2×2 prototype will soon be installed underground in Fermilab’s NuMI neutrino beam path for testing.

“We’re going to include this in what is currently the world’s most intense neutrino beam,” said UC Irvine professor Juan Pedro Ochoa-Ricoux, who is leading the 2×2 data analysis effort. prototype. “We will be able to test our prototype under real conditions.

Sort the neutrino flood

The 2 × 2 prototype, and eventually the ND-LAr itself, will detect the neutrino beam close to its most intense point.

When a beam of protons from an accelerator collides with a target, it creates a spray of other charged particles that rapidly decay into other particles, including neutrinos. The beam of charged particles used to generate neutrinos is tightly focused, but once this beam of neutrinos is created, they can no longer be directed or focused because they have no charge. As the beam travels through space, the neutrinos scatter and the beam becomes less dense.

“It’s a bit like a flashlight: when you point the flashlight at a wall, if you’re close to the wall, you see a small circle, but if you move away from the wall, the circle gets bigger and bigger,” Ochoa-Ricoux said.

Because the near detector will be close to the source of the neutrino beam, it will capture more neutrino interactions in a smaller space than the far detector. This powerful influx of neutrinos poses some challenges for efficient recording of neutrino interactions in ND-LAr. While a distant detector can only pick up one neutrino at a time, a nearby detector will see many more interacting neutrinos.

“All of these interactions happen almost simultaneously,” Ochoa-Ricoux said. “We have to be able to separate all these interactions.

Fortunately, researchers at the University of Bern and DOE’s Lawrence Berkeley National Laboratory are developing new liquid argon detector designs and technologies better suited to this high neutrino density.

A team from the University of Bern has developed a new design for liquid argon neutrino detectors. Instead of one large volume of liquid argon, this design divides the detector into modules.

The new design not only results in a shorter distance for the stripped electrons to drift toward the detection surface, but also provides a better understanding of where the neutrino interactions take place. By reducing the modules, the visible light that is produced by the interaction of neutrinos in one particular block, narrowing its location.

The modular design also means that there are fewer interactions within each module. This makes it easier to couple the detection of light and charged particles to understand neutrino interactions. This type of detector can more efficiently handle a large number of interactions occurring in a short period of time.

These two consequences of the split detector make it ideal for ND-LAr, as the design allows for a more precise three-dimensional image of where the neutrino interaction occurred, said Michele Weber, a professor at the University of Bern who is working on the detector prototype. develop and lead ND-LAr efforts.

“It’s great to see a concept developed at our university find application at DUNE in collaboration with Fermilab,” said Weber. “One challenge we had to solve in order to know which signal belongs to which interaction is to improve the 3D representation of each interaction.

A clearer picture

Meanwhile, another team at Berkeley Lab has developed a new type of signal-scanning system that can resolve the massive amount of data expected at the nearest detector.

Traditionally, liquid argon time projection cameras, or LArTPCs, used a series of layered wires on the side of the detector to capture the signal from the ejected electrons that are released by the interaction between the neutrino and the argon. Combining the signals collected in wire layers that provide a series of two-dimensional projections provides enough information to reconstruct a three-dimensional image of the interaction.

But when there’s a lot of neutrino-argon interaction in the detector (a phenomenon called neutrino bunching), this scanning system doesn’t provide such a clear picture, said Brooke Russell, a Chamberlain collaborator at Berkeley Lab who works with 2×. Prototype 2.

Instead, the readout system developed at Berkeley Lab uses full-pixel readout, meaning that each physical channel of the detector corresponds to one digital readout channel. Using this pixel array, the three-dimensional location of the interaction is directly represented and all the many neutrino interactions occurring almost simultaneously can be resolved.

“This has a big impact on the types of signals we create and the intensity of activity we can be tolerant to,” Russell said. “With DUNE next to the detector, we are for the first time in a regime where we have a bunch of neutrinos. Such a reading is necessary to reconstruct neutrino events.

2×2 test

Prototype modules were developed and tested at the University of Bern, then sent to Fermilab and tested again before installation. Preparations are now underway to deploy the prototype by the end of the year to test neutrino detection when the NuMI beam comes back on this winter.

The experiment deployment team will place the prototype detector in a cryogenically cooled container and place it between two reused detector parts from the retired MINERvA neutrino experiment at Fermilab. MINERvA measured neutrino interactions from 2010 to 2019.

Because the ND-LAr prototype detector is not very large, it cannot measure the entire path of some of the particles produced when neutrinos interact with argon. Prominent examples are muons, which usually travel long distances before stopping. This is where the old MINERvA detector components come into play. By using these components to track muons leaving the prototype detector, scientists can distinguish muons from charged pions, another type of subatomic particle.

Placing the prototype between MINERvA segments also helps to identify muons that pass the detector but did not originate from the detector, distinguishing them from muons that originate from the detector as a product of neutrino interactions.

“We can use the MINERvA planes to help us track the neutrinos that interacted in the rocks in front of the detector and formed muons that entered the detector,” said Jen Raaf, director of Fermilab’s Neutrino Division, who is coordinating the 2×2 prototype project. . “We’ll be able to match the tracks to identify them.” [that didn’t originate in the detector] and get rid of them, because that’s not what we’re interested in.”

The MINERvA planes also allow scientists to track particles produced by neutrino interactions at LArTPC, but which exit the argon volume before stopping. “MINERvA will allow us to track these outgoing particles and measure their energy,” said Raaf, “so that we can accurately estimate the energy of the neutrino when it interacted with LArTPC.”

Once the 2×2 prototype is tested in the neutrino beam, it will not only ensure that the prototype works properly, but also allow scientists to perform neutrino physics experiments, Ochoa-Ricoux said.

Although the full DUNE experiment won’t be operational for several years, he said, “we’re already producing some important physics results with this prototype.”

Some of these experiments before DUNE in the 2×2 prototype include studying the reactions between neutrinos and argon and measuring cross sections or the probability of particle interactions.

Between its modular design and pixel readout, the ND-LAr will be unique among liquid argon neutrino detectors. This means that prototyping and testing is critical to ensure that an innovative design works as expected. When a new technology is developed, scientists must test every step of the construction to demonstrate its capabilities, Weber said.

“The ND-LAr has an unusual design,” Russell said. “We want to confirm that some of the design principles that we think will work will actually work.”

It’s also important to make the prototype large enough to build and install the final piece of equipment, Raaf said.

“Doing something on a smaller scale, but big enough that you can identify the difficulties in construction and assembly, is a really important step in all particle physics experiments,” she said. “You want something that’s big enough that you can experience the different things you have to do, like using a crane to pick it up and being able to move it in certain ways.”

The DUNE collaboration is divided into consortia that focus on different aspects of the project. The development of the 2×2 prototype is part of the ND-LAr consortium, of which the University of Bern and Berkeley Lab are just two of dozens of institutions.

“All of those people are involved in this prototype at some level to make sure that what they envisioned for the full-size thing actually works on a smaller scale and we don’t have to change anything,” Raaf said. “Maybe we will, which is good – that’s why we’re building prototypes.” We meet weekly and discuss how things are going? What should we do next? What went well? What can we improve?”

Such a large task requires multi-institutional collaboration, said Weber, who leads the ND-LAr consortium. Between Fermilab’s neutrino beam, the University of Bern’s modular detector concept, Berkeley Lab’s scanning technology, and data processing and analysis across multiple institutions, each collaborator in the ND-LAr consortium brings its unique capabilities to bear on this project.

“This effort is too large for one institution,” Weber said. “You talk to different people and share the burdens. It’s a challenge to work with so many people, but it’s the only way, and it’s great to see different ideas come together successfully.

Fermi National Accelerator Laboratory is supported by the US Department of Energy’s Office of Science. The Office of Science is the single largest supporter of basic research in the physical sciences in the United States and works to address some of the most pressing challenges of our time. For more information, please visit science.energy.gov.