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Voyage of the ArgoNeuT

The team running the ArgoNeuT detector is ready to send its vessel to sea--or at least into a stream of neutrinos at Fermilab.

ArgoNeuT prepares to embark down the tunnel.

ArgoNeuT prepares to embark down the tunnel.

ArgoNeuT, which stands for the Argon Neutrino Test project, is a liquid-argon detector used to study neutrino interactions. (To read an article about ArgoNeuT from symmetry magazine, click here.)

On Monday, the ArgoNeuT team lowered the detector in pieces into a kilometer-long tunnel where it will be exposed to a high-intensity neutrino beam.

The team tested the detector in August by collecting data from cosmic rays. The members have since made improvements such as replacing damaged readout cables to prepare the detector for the move.

"The detector is going to be 350 feet underground, so you're not near a toolbox," says Mitch Soderberg, a Yale post-doctoral assistant who works at Fermilab. "It's crucial for us to prove it works above ground before moving it underground."

The ArgoNeuT team plans to use the detector as a practice run for MicroBooNE, a liquid-argon neutrino detector to be about 200 times the size of ArgoNeuT.

"ArgoNeuT is a small detector, but it's big step forward for those of us in the US as far as getting the technology to work," Soderberg says.

Neutrinos are abundant in the universe, but they rarely interact with other particles, so physicists prefer large detectors that capture more of those interactions. No one in the United States has ever built a large-scale liquid-argon neutrino detector.

Currently, the largest neutrino detector is Super-Kamiokande in Japan, which uses a tank of about 50,000 tons of purified water to capture neutrinos and their interactions with matter.

Technicians lower ArgoNeuT into the tunnel.

Technicians lower ArgoNeuT into the tunnel.

When a particle produced in a neutrino interaction ionizes a particle of liquid argon, it knocks an electron free. Because liquid argon is more inert than water, the electron can drift through it without being absorbed. The ArgoNeuT detector features a set of wires that create an electric field to attract electrons. The wires register when an electron passes through.

Liquid-argon detectors can differentiate between particles better than water detectors can, Soderberg says, but they do have their drawbacks. Only ultra-clean argon allows electrons to drift without being absorbed by impurities.

"Keeping a big tank full of liquid argon and detector components completely free of tiny amounts of impurities is very hard," Soderberg says. "It's the big challenge we have to solve before building very large detectors."

ArgoNeuT is too small to contain some types of particle interactions, such as muons produced when a neutrino and an argon atom interact. The team will rely on the nearby MINOS neutrino detector, located in the same tunnel as ArgoNeuT, to catch any outgoing particles and create a more complete picture.

Soderberg said he and other physicists hope to eventually build a 50,000-ton version of a liquid-argon detector to place in a proposed Deep Underground Science and Engineering Laboratory.