A Modular Neutrino Detector Years in the Making

Newswise — Researchers at the U.S. Department of Energy's SLAC National Accelerator Laboratory have joined collaborators from around the world to build a prototype neutrino detector that has now captured its first neutrino interactions at Fermi National Accelerator Laboratory (Fermilab). 

The prototype detector will help fine-tune a full-size version of the DUNE Near Detector Liquid Argon (ND-LAr) detector in the coming years for the international Deep Underground Neutrino Experiment (DUNE), led by Fermilab, and in the meantime help illuminate some specific neutrino properties.

Researchers will also use the detector to test advanced machine learning techniques, developed at SLAC, that are expected to play a key role in processing the vast amount of data generated by DUNE.

“The prototype is going to be very important because it’s the only source of neutrino beam data at energies comparable to the DUNE beam that will be available before DUNE is running,” said James Sinclair, a SLAC scientist working on the project. “We are excited to be completing this critical step in the experiment and are now ready to study the data that’s coming in.”

A modular design for an unusual problem

Neutrinos are fundamental particles unlike any other. They can pass through almost all matter largely unseen and can change forms along the way – a phenomenon called neutrino oscillation. Scientists think a better understanding of their unusual properties could help answer some of the most challenging questions about the origin of matter in the universe and the pattern of neutrino masses. 

To detect neutrinos, physicists use what's called a time projection chamber (TPC) – a vast tank of liquified noble gases such as argon. When a particle enters the chamber from outside, two things happen. First, interactions between the particle and argon atoms create flashes of light called scintillation. Second, the particle can knock electrons free from argon atoms, ionizing them. TPCs typically include photosensitive equipment to detect scintillation and an electric field that guides free electrons to one end of the detector, where – traditionally – a wire mesh picks them up as an electrical current. By comparing details of the flash with the time it takes electrons to arrive at the mesh, researchers can identify key details including what kinds of particles they're picking up and how fast those particles are moving.

The idea is to capture as many neutrino interactions as possible with a large volume of argon and a relatively small amount of detector equipment, almost all of which stays on the periphery of that volume. 

But something more is needed for DUNE, said SLAC scientist Hiro Tanaka, the technical director for the DUNE near detector and head of SLAC's efforts on the DUNE project. Unlike many other neutrino experiments, DUNE will produce a very large number of neutrinos and beam them in bunches toward DUNE's near detector outside Chicago. Over the course of just a few microseconds, scientists expect to see multiple neutrino interactions in the near detector. The trouble is, all those interactions make it hard to tell which flash of light belongs to which neutrino, in part because large tanks of liquid argon scatter and diffuse each individual flash. It also makes it hard to tell which electron comes from which ionization event, since any one electron takes milliseconds to reach the edge of a TPC, during which time many interactions may have occurred. 

It was out of these concerns that the newly minted prototype, called the 2x2 detector, was born. On one level, the idea is simple: Rather than use one giant TPC, break the device into a set of four TPC modules arranged in a two-by-two grid – hence the name. Each module actually contains two separate volumes of argon with an opaque wall in the center. That wall effectively creates eight optically separate TPC tanks, so that it's less likely to mistake one neutrino flash for another. It also serves as the source of the electric field that draws ionization electrons to the sides of the detector module.  

In addition, each module contains a new system for detecting ionization electrons developed at DOE's Lawrence Berkeley National Laboratory that picks up not just when the electrons arrive, but also precisely where, in contrast to the traditional wire-based designs, where the information provided by each plane of wires can be difficult to reconcile in the high interaction-rate environment of the DUNE Near Detector. Combined with the light flashes, this will help researchers determine where neutrino interactions occurred for the first time without ambiguity in three dimensions.